PEDOLOGIE Edité avec Ie concours de la Fondation Universitaire

et du Ministère de l'Education nationale et de la Culture

Uitgegeven met de steun van de Universitaire Stichting en van het Ministerie van Nationale Opvoeding en Cultuur

Guy D. Smith .

Lectures on

Soil Classification

Comité de rédaction

1 9 6 5

n° spéc. 4 spec. nr.

Redactiecomité J. AMERYCKX, L. DE LEENHEER, C. DONIS, J. FRIPIAT,

H. LAUDELOUT, G. MANIL, A. NOIRFALISE, G. SCHEYS,

D. STENUIT, R. TAVERNIER, A . VAN DEN HENDE

PREFACE

The purpose of the Francqui Foundation is to advance both the

development of university education and scientific research in

Belgium. To this end the Foundation has among other things

established « Francqui chairs» so that distinguished scientists from

abroad may be invited to lecture during an academic year at one

of the Belgian Univer'sities.

For the Academic Vear 1964-1965 Dr. Guy D. Smith of the U.S.A.

was appointed to this chair to lecture on soil survey and soil

classification at the International Training Centre for Post-graduate

Soil Scientists of the State University of Ghent.

Dr. Smith, being director of the S.C.S.'s Soil Survey Investigations

of the U.S.O.A., had given staft leadership and had contributed

personally to the development of a system of Pedotaxonomy designed

for use in soil survey programs. The Soil Survey Staft in Belgium

had worked with him since 1952 in the development of the system.

It had been used as a tooi to correlate the various classification

systems of Europe during the preparation of a new soil map of

Europe on a 1: 2,500 000 scale, recently published by F.A.O.

However, published materials that defined the taxa of the system

did not elaborate on the reasons for selecting most difterentiae that

were used. So we asked Dr. Smith to explain as much as was

possible in these few lectures what the system is, and why it had

developed as it i~.

Because many of the post-graduate students come from tropical

and subtropical countries, special attention was requested to

problems in those parts of the world. Furthermore, many Belgian stu­

dents follow the courses in this specific branch in order to be able to

cope with soils problems in developing countries, many of which are

situated in the tropics.

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While portions of some of the lectures have only transient interest,

and other portions are not new to the literature, a large part is

original and should have interest to the wide audience concerned

with soil classification and soil survey.

A great many Belgian and foreign soil scientists have given

evidence of interest in Dr. Smith's lectures. In order to meet this

general interest and with the approval of the Francqui Foundation,

it has been decided to make these lectures available in printing.

We seize this opportunity to convey our most sincere thanks to

the board of the Francqui Foundation and also to Dr. Guy D. Smith

who graciously prepared the texts of his lectures for publication. We

also wish to thank the Academic authorities of Ghent University for

having assigned the 1964-1965 Francqui chair to soil science.

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Prof. Dr. R. TAVERNIER

State University of Ghent

THE PURPOSES OF SOll SURVEYS

A soil survey includes the examination and classification of soils in the field, location of soil boundaries in the field, plotting of the soil boundaries on a map, description of the soils shown by the map including the morphology and statements about the important properties and qualities, and finaIly, interpretation of the map units especially for the purposes for making the soil survey.

Kinds of soil surveys

Soil surveys can be made at various levels of intensity depend­ing on our purposes and on the conditions in the area. It obvious­ly would be foolish to make the same kinds of soil surveys in the uninhabited Amazon rain forest and the areas adjacent to the suburbs of Chicago where land is going out of use for farming as new houses are built. We must have several levels of in tensi ty.

Exploratory soil surveys are the most genera!. They are made largely by inferences dvawn from general knowledge of the soil forming factors - the dim.ate, the vegetation and the geology including the nature of the rocks, the relief and the age of the land surfaces and any published or unpublished studies of the soils, and reports of travelers. The inferences drawn from the genera;l knoWledge ave checked if possible by traverses through the area. Gartographic units are normally associations of phases of great groups.

Such a survey would normally be the first step in a soil survey of a country, or in an uninhabited part of a country that needed new land for settlement.

Exploratory surveys are useful to locate areas of soils that differ in properties. They teIl us wh ere to make more detailed

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surveys to develop and test a legend for the soils of a country, or to guide land settIement.

Reconnaissance soil surveys are ifue next more intensive surveys. These are made by traverses through an area with observations of the soils along the traverses. Lines connecting the boundaries of soils between the traverses are drawn fr om general information without actual observation between the traverses. Cartographic units are generally associations of phases of soil series or of classes of higher categories in the classification.

Detailed soil surveys are the most intensive. Boundaries between the units shown on the map are seen throughout their course in detailed surveys.

The amount of detail that is shown depends upon the us es that can be foreseen for the soil survey. In arid regions where grazing is the principal use of the soil, the cartog~aphic units are of ten phases of associations of soil series. In mountainous forest land we of ten have similar cartographic units. These are low intensity soil surveys.

Detailed soil surveys of medium intensity are the most com­mon surveys made in the United States. In these the cartograph­ic units are generally phases of soil series. Examinations of the soil generally end at a depth of a meter or a meter and a half, and are adequate for most of our farm land.

High intensity detailed soil surveys are restricted to areas of very intensive use. They may be made for development of irrigation projects, or for areas of rapid population growth where a substantial part of the land is to be used for housing. Examinations of the soil and of the geologic materials below the soil to depths of a few meters are commonly required in our high intensity soil surveys.

Modern high intensity soil surveys are of ten expensive. The costs of some of these approach $2.00 per hectare. Medium intensity surveys are now costing, on the average, about $1.25 per hectare. These costs are inclusive of printing and all over­head costs, but no government or institution is so wealthy that it can support an extensive program of such surveys unIe ss it has been shown that the soil surveys are useful enough to repay their costs many times over.

A soil survey therefore is expected to be of practical use. Many have been extremely useful and have repaid their costs many times over every year since they were made. But some have failed.

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The failures were mostly the result either of failure to identify the purposes for making the soil survey or failure to maintain strict scientific control over the identification, description and classification of the soils. To be useful, the soil survey must be both practical in purpose and scientific in its construction. It should not be either purely scientific, or purely practical. It must be both to be useful for more than the purposes of the moment.

I should like to emphasize this point with some examples because it is the most important statement I can make about the soil survey.

Some years ago a soil survey was started in a region where wheat is the principal erop. The map was intended to be a practical map that could be used as a basis for tax assessments. It was decided, administratively, that it would be simplest to map the yields of wheat that the soils would produce. This was the interpretation desired. So, the legend was set up in terms of wheat yields rather than soil properties. Soils with rather unlike properties were grouped if the wheat yields were approxi­mately the same. About the time the map was completed the plant breeders introduced improved varieties of wheat that pushed the margin of wheat production some hundred miles from the earlier limit. These varieties made it possible to grow wheat on soils that had not previously been suitable for wheat production, but they had little effect on yields of the soils that had previously produced wheat. The entire survey was wasted because the map showed only the interpretations, and these change when the technology changes.

A second example could be taken from one of our southern States. Here the principal crops were cotton and tobacco, with some maize. The prime purpose of the soil survey was to serve as the basis for planning farm operations and erosion control. The soil units shown on the maps were homogeneous in a num­ber of properties. Great attention was paid to soil colors, to slope, to erosion, and to the soil parent materiaIs. But, there were vast areas of gray, wet, acid soils that were of little agri­cultural importanee. The soil map units here were defined rather broadly in terms of soil texture because the texture had little effect on the then current use for forestry. Then the technology changed. The cotton pieker made it more economie to produce cotton in the south-western States and in the Missis­sippi delta where the smooth topography permitted the use of large machines. The enormous population growth furnished a market for winter vegetables, and low cost fertilizers made their production very profitable.

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We learned how to use pine for paper pulp. 80 the soils that had been in cultivation were largely planted to pine, and those that had been in forest were cleared for vegetable production. But the soil maps that had been made for the old technology were inadequate to guide the development of the land for the new uses. N ew soil surveys had to be made. The failure of the soil survey to be useful under a new technology was the result of failure to maintain reasonable scientifie standards. Taxonomie units had been narrowly defined for the cultivated soils with respect to a few properties that were important to the uses of the moment. Properties of soils that were not important to the uses of the moment were permitted to vary widely. 80 wh en the technology changed it was impossible to reinterpret the maps. We could not distinguish soils that could be readily drain­ed from soils impossible to drain. Rather extreme variations in the clay content of the B horizons, perhaps from 28 to 80 per cent, had been permitted within a soil series if the soils were wet, acid, had little organic matter, and were not cultivated. If the soils were being cultivated and were moderately weIl or weIl drained, much narrower ranges had been permitted in the clay content of the B horizon of a soil series. One can hardly claim that reasonable scientific standards are being met if the present land use determines the range in properties that is to be permitted within a soil series. 80, when the technology changed the soil survey became obsolete because of failure to maintain scientific standards.

The soil surveys must have reasonable scientific standards and they must have practical purposes to be useful. And they must be useful to be supported with public funds. The ministers have too many demands on their funds to continue to pay for expensive studies óf no practical use.

Uses made of soil surveys

The historic purpose of soil surveys has been the improvement of agriculture. Every member of parliament who votes for appropriations, and every minister who allots funds has this in mind. In some countries this has meant a search for new lands suitable for settIement. In the United States it has meant mainly the improvement in efficiency of an already established agri­culture, and to a very minor extent the development of new lands through irrigation or drainage. In other countries it has been in part for reaIlotments to consolidate scattered small hold­ings.

We make the basic assumption that experiences with a partic­uI ar kind of soil in one place can be applied to that particular

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kind of soil wherever it exists if consideration is taken of any climatic differences. The soil survey acts as a bridge that let us transfer the knowledge gained by research or by the experience of cultivators from one place to all other places where it is applicabie.

In the United States, the program of the Soil Conservation Service on any one farm is based on the soil map of that farm and the assembied interpretations. The interpretations used by the Service to develop a plan for a fann include, if they are applicable, the following : (a) the need for drainage and methods of providing it, (b) the adaptations of specific crops, (c) the hazards of erosion and methods for preventing it, (d) the need for irrigation and methods for irrigating, (e) the yields of adapted crops under varying systems of

management, (f) the suitability of the soil for construction of farm ponds, (g) the need for special measures of reclamation, (h) special cultural practices required by specific crops on

specific soils, or possible practices that reduce costs, (i) forestry practices applicable to wooded portions of farms, (j) grazing practices and carrying capacities for livestock on

range lands, and (k) needs of grazing animals for cobalt and copper.

For watershed proj ects where prevention of flooding and development of water supplies and recreation are additional purposes we require additional interpretati.ons.

These include : (a) rate of sediment production by kinds of soil under specified

management practices, and (h) rate of runoff of water, and amount of runoff.

For irrigation proj ects, interpretations are required to indi­cate the areas suitable for irrigation by various methods, crops that may he grown and yields that may he expected, prohlems in drainage, special reclamation practices, subsidence, and water delivery and requirements. Almost identical interpretations are required for drainage proj ects that involve hringing new land under cultivation.

For forests we make similar interpretations about the adapt­ability and growth rates of specific species on specific kinds of soils and about problems of management of forests, including regeneration, control of undesirable species, and planting and thinning. In addition, special interpretations are needed for construction of logging roads. These include stability of slopes, erosion hazards and trafficability.

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You must not think that the interpretations are easy to make or that they are always as precise as we might like. The knowl­edge we have about soil-plant relations has come from many decades of research and from former experience, but we never know enough. Too of ten we can answer the questions that were important thirty years ago but have only very imperfect answers for today's questions. This is the price of a changing technology and more efficient farming. Many of our interpre­tations have to be judgements based on limited knowledge of a specific variety of a plant and relatively good knowledge of the properties of the soils.

Advisory agencies of our States also use the soil surveys to give individual advice to farmers on rates and kinds of fertilizers. Soil tests are used in part as a basis for this advice, but the test results are of ten interpreted differently for dif­ferent kinds of soil. General recommandations on kinds and amounts of fertilizers for specific crops are of ten made by kinds of soil.

Uses by local agencies of government of agricultura:1 i·nterpretations

Local agencies of government use the same interpretations that we give the farmers. lncreasingly, tax assessments are based on soil surveys with adjustments for the non-soil factors that contribute to the value of farm land. Rural zoning that prohibits farming or sale of public lands in areas where suitable soils are scarce reduces local costs of maintaining roads and schools for isolated families. Development of non-farm income through recreation and tourism is being increasingly based on special interpretations of soil surveys that supplement the interpretations of soil-plant relations.

Engineering and non-farm interpretations

The soil properties that influence plant growth are also prop­erties that affect the engineering uses of soil and many other non-farm uses. The content of clay and its ability to shrink and swell, the permeability to water, the conductivity, and the depth ~ to rock, for example, are properties that affect the growth of plants and ,a whole host of non-farm uses. The soil maps carry the information ; all that is needed is the ability to make the interpretations. A slowly permeable horizon not only restricts root growth, but limits the use of a soil for septic tanks and makes a very poor subgrade Jor a highway. The effluent from

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the septic tank win not soak down into the soil. The water below a highway produces pumping when a heavy truck passes and causes the highway pavement to faiI. A soil rich in montmoril­lonite has a high capacity to shrink and swell with changes in moisture. This process will break the roots of plants, and it will break large pipelines and reinforced concrete foundations of buildings as easily. It will tilt the trees in an orchard so that spray equipment can not pass and it will also tilt the poles of a power line so that maintenance is very expensive. A high conductivity in the soil can stunt the growth of plants, and it can corrode a pipeline within a few years causing repeated leaks. A high humus content generally is good for plants but bad for highways. Many examples could be given to show that the soil properties that affect plants affect other us es of soils. If appropriate interpretations are made, the soil maps are useful for many non-farm purposes.

Highway engineers make use of engineering interpretations to locate new highways. A shift of a half kilometer in the location of a modern highway can reduce construction costs by millions of dollars by avoiding areas of unstable soiI. I t may be a buried peat, a swelling c1ay, or an unstable slope that should be avoided. Engineering properties of soils are important in planning secondary roads. The feasibility of a soil-cement mixture or bitumen stabilization can be determined from the soil survey and the engineering knowIedge. The presence of seasonal ground water, the suitability of the soil as subgrade material, the location of deposits of sand and gravel close by the highway, and the location of soil materials suitable for use as top soil on cut banks are further examples of interpretations used by highway engineers.

The interpretaions are made pos si bIe by co-operation between the pedologists and the engineers. The pedologists sample the' important soils, the engineers make the engineering tests, and the interpretations are made jointly.

Public health officials and planning agencies are making extensive us es of soil surveys to guide the expansion of suburban developments. In Europe the purpose may be in part to keep the most productive land in farming. In the United States the primary purpose has been to avoid construction of houses that will be unsafe. Houses on soils of low permeability require sewage lines, and the size of the lot must be small. Otherwise the costs of the sewage lines are excessive. Larger lots may be planned on permeable well drained soils where septic tanks are feasible. The largest lots are needed . on soils with moderately slow permeability for large disposal field and for space for

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additional disposal fields should the first one fail. So, health authorities use the soil survey to determine the size of the lots that must be provided, and even whether homes may be built. Areas that flood are of ten zoned for public parks or farming to prevent construction of houses that are certain to be flooded.

Developers, who subdivide the land, and build and sen the houses, use the soil surveys and their interpretations to plan the lay-out of the lots and streets to avoid building on unstable slopes, or on otherwise unsuitable soils, to design the founda­tions of the houses, and to plan the landscaping.

Architects planning large buildings, such a new schools, of ten have used the soil surveys to select locations where foundation costs win not be excessive. The cost of ,a soil survey of an entire county has been saved by moving the site of a high school some 300 meters from the site first proposed by the school officials. Many more examples of uses of soil survey information could be given. These include locations of airports, pipelines, radio trans­mitters, camping sites, playgrounds, and even the selection of colors for wallpaper and rugs.

Soil surveys can be useful and they win be supported if they are properly made and interpreted. I have already emphasized that they must be both practical in purpose and based on scien­tific principles. But this is not enough. They must also be inter­preted. We can not expect the highway engineer, the public health official, or the cultivator to know enough about soil science to make his own interpretations of the map. Soil maps are not apt to be used unless the pedologist takes the responsi­bility for making the important interpretations or for seeing that they are made by conaboration with the engineers, forest­ers, and specialists in other field. And, having made the inter­pretations, he must work enough with the users to show them how the interpretations can be used. Unless he has done these things the soil surveys are apt to sit on a shelf where they do no one any good. And if they do no good it win not be long before the minister finds a more important use for his funds.

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THE lOGIC OF SOll ClASSIFICATION

Cline (1) has said « The purpose of any classification is so to organize our knowledge that the properties of objects may be remembered and their relationships may be understood most easily for a specific obiective. The process involves formation of classes by grouping the objects on the basis of their common properties. In any system of classification, groups about which the greatest number, most precise, and most important state­ments can be made for the obiective s'erve the purpose best. As the things important for one objective are seldom important for another, a single system will rarely serve two obj ectives equaIly weIl. "

We should note that this statement has three important facets :

First, a classification is an organization of the knowledge of the moment. It is not apt to be much better than that knowIedge, and it is certain that changes will be required as knowledge changes.

Second, a classification has an objective. It is made by man to serve some purpose, and the nature of the purpose is im­portant in the design of the classification.

Third, the process of making a classification involves the grouping of objects on the basis of their common properties.

I should like to elaborate on these points, not that I have anything new to say, but because the best of us are inclined to forget them.

Knowledge changes. Once the sun revolved around the earth. This knowledge caused Galileo considerable personal inconven­ience according to our histories. Our knowledge of soils and their properties is still extremely sketchy, and many things we "know" today wiIl be disproven tomorrow. For example, we put great faith in our clay mineralogists, their X-ray machines, and other devices for identifying the kinds of clays in soils. Perhaps

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they are right, but we have only indirect evidences for the most part today. One must wonder why, if they are right, it is possible that, by one method, a soH has none of the kind of clay th at is dominant if a different method is used to prepare the sample.

Our knowledge not only changes with time, but it is dependent on the operations we have used to obtain it. Bridgeman (2), in his Logic of Modern Physics, has dealt at length with a simple concept - that of the length of an object. What, for example, is the length of astreet car, and how do we measure it ? The usual method is to select a convenient rod, say a meter stick. One end is placed at the end of the car, and we mark the position of the other end. Then we move the stick and repeat the oper­ation. When we reach the other end of the car we count the times we have moved the stick, and if we have done so 10 times we say the length is 10 meters. Bridgeman goes on to point out that Einstein, wh en dealing with the length of a rapidly moving obj ect, was unable to use this method. An entirely different set of operations was involved, so that the concept of the length of a rapidly moving object is not the same as that of a stationary object. Hence, he says, it is not surprising that Einstein con­cludes that the length is a function of velocity. His meaning of length is not the familiar meaning of the length of stationary objects.

80, in soH science we must not be surprised that measurements of organic matter, pH, per cent clay, base exchange capacity, kinds of clay, and so on vary with the operations that we use to measure them. If we use any of these properties as differen­tiae in our classification, Bridgeman's logic dictates that we must specify the operations, or the methods, by which we measure them.

Cline (3) has pointed out that classifications are "orderly abstracts of knowIedge, and of concepts derived from knowIedge, both of which are legacies from experience of the past. Part of that legacy, consisting of data from observation and measure­ment, we consider fact, though even it is fact only within the perspective of the operations by which the data we re obtained (Bridgeman, 1927). Part consists of purely empirical relation­ships among those facts, which we caU laws. Part consists of theory that seems to explain real things and their relationships, including laws. Theory is not fact, yet it forms the nucleus of concepts that, in the aggregate, describe and explain the uni­verse of our consciousness to a degree that most nearly provides satisfaction of the moment ...

"Data, laws, theories, and concepts derived from experience of the past are the elements of our current understanding of

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nature. We organize and reorganize them into patterns of consciousness that most nearly satisfy our mental standards of that which is reasonable. We classify the subjects of natural phenomena to reveal relationships we deern to be important within the current pattern of our consciousness for the purposes we find useful. The system of classification under discussion rests on that foundation.

"The elements of knowiedge, our understanding of nature, and the classification systems we devise are, of course, insep­arabie from the capacities and functioning of the h uman mind. The obvious possibility exists that absolute truth is purely a human concept devoid of reality or that absolute truth exists but transcends the capacity of the human mind. Such conjecture is futile and belongs with Bridgeman's (1927, p. 28) meaningless questions. Throughout the history of science, however, con­tinuing experience has revealed new 'facts' whose incorporation into our consciousness has demanded complete reorganization of understanding. We must aecept the provisional nature of current knowledge and of current devices, such as classifications, th at organize knowledge to fit our patterns of consciousness.

"A classification system can be a potent factor limiting the possibilities of new experience... If its criteria are theories without some device for constant and inescapable scrutiny in relation to fact, the . concepts implicit in the system become accepted as facto Such acceptance can mould research into patterns of the past and can limit understanding of even new experience to concepts established in the past. Most soil classi­fication systems have had these effects to some degree. The point is illustrated excellently by the impact of the theories of V.R. Vil'yams (Williams) on classification, thought, investi­gations, and applications of soil science in the Soviet Union as they are discussed by Vilenskii (1960)."

We must constantly bear in mind that new knowledge and new experience are certain to come. Bridgeman has pointed out that the scientist "recognizes no a priori prineiples which determine or limit the possibilities of new experience". If we kno.w that something is impossible our minds are closed and we may refuse even to see the evidences that our knowledge is wrong.

The second facet of Cline's statement was that a classification has an objective; We have emphasized that classifications are not truths that are discovered but are contrivances made by man to suit his purposes. For different purposes we normally require different classes, or groupings of obj ects. If our purpose is narrow, as in a classification of soils according to their poten-

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tials for production of a single crop, only those properties important to the production of that crop need be taken into account in the classification. This would be an example of what J ohn Stewart Mill called a "technical classification". We ' need many technical classifications of soils to meet the practical uses of the soil survey. If our purpose is broad we must take many properties into account. For the broadest purposes a natural classification is needed. Mill (4) has defined the natural c1as­sification as follows :

"To provide that things shall be thought of in such groups, and those groups in such an order, as will best conduce to the remembrance and to the ascertainment of their laws.

"The ends of scientific classification are best answered, when the obj ects are formed into groups respecting which a greater number of general propositions can be made, and those proposi­tions more important, than could be made respecting any other groups into which the same things could be distributed. The properties, therefore, according to which objects are classified, should, if possible, be those which are causes of many other properties ; or, at any ráte, which are sure marks of them. Causes are preferabIe, both as being the surest and most direct of marks, and as being themselves the properties on which it is of most use that our attention should be strongly fixed. But the property which is the cause of the chief peculiarities of a class, is unfortunately seldom fitted to serve also as the diag­nostic of the class. Instead of the cause, we must generally select some of its more prominent effects, which may serve as marks of the other effects and of the cause.

" A classification thus formed is properly scientific or philo­sophical, and is commonly called a N atural, in contradiction to a Technicalor Artificial, classification or arrangement."

We should note that the purposes of the natura! classification cited by Mill are remembrance, and ascertainment of laws. These purposes meet many of the needs of soil science but they are, by themselves, inadequate for the practical needs of the useful soil survey. At least one more step beyond the natural classifica­tion is required to order the "natural" groups into "practical" groups. This can be accomplished by groupings of phases of classes in the natural system, selecting for characteristics to define the phases those characteristics that are important to the purpose of the moment.

The third facet of Cline's statement was that the classes are formed by grouping the objects on the basis of their common properties. This concept, eoming from Mill, has not been strictly

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observed in the construction of most of the earlier systems of soil classification. The zonal soils of Sibirtsev and of the 1938 American classification represent a grouping of soils largely by genetic theory. The definition of zonal soils as those that express the influence of the climate and vegetation is in terms of genesis. The system used in the USSR still uses climate per se to define classes in the highest categories. Otherwise there is now rather general agreement with Mill that a natural classifica­tion should be arranged according to the properties of the objects being classified.

We should note that if we use theories instead of properties of the obj ects to arrange and define our groups the theories, being imperfect but being accepted as fact, will act to limit the possibility of our having new experiences and will tend to tie the classification to the knowledge of the past. The Soviet use of climate to define classes of their system distinguishes all soils of tropical regions from all soils of temperate regions. If one accepts as a fact, that the soils of these two regions are different, and they are given different names, it becomes extremely difficult to see what similarities, if any, exist. The belief is that the soils mU8t be different because their environ­ments are different. This belief prevents the average mind from thinking of the soils of the two regions at the same time and comparing their actual properties. The mind knows th at the soils are different, and there is nothing to be gained by such a comparison.

The soil individual

Our concept of an individual is conditioned by our experience. We can recognize a cat or a dog without difficulty. With coral, the recognition of the individual is more difficult, and to a limited degree is arbitrary. The problem is still more difficult with other objects such as bedrock and soils. These are partS of a continuum in which properties may change gradually over distance. A boundary of one soil is also a boundary of another soil, but it is usually diffuse over some tens of meters and is only rarely obvious. No general agreement exists as to the soil individual. One solution was proposed in Soil Classification. A Comprehensive System, 7th Approximation. For the moment, we must only assume that it is possible to recognize a soil individual. The pedon that you will study later is considered the smallest volume that may be called "a soil". Normally "a soil" consists of many contiguous similar pedons, and is called a polypedon.

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- -- ----- --'

Taxa and categories

The individuals in a population, whether it be of plants, animaIs, or soils, may be grouped according to their common characteristics or properties into a few groups or many groups. A group of individuals so formed is called a class or a taxon (plural taxa). The groups, ordered by the properties selected, are conceptual.

There are too many soil individuals, even in a small area of a few hundred square kilometers, for the mind to understand. So, we arrange the individuals in our minds into a sm all enough number of classes or taxa that we can see their relationship and remember something of their properties.

For small areas of relatively uniform soils we can create taxa that are homogeneous in a very large number of properties and be able to understand the relations between the taxa. If we wish to arrange the soils of a continent for similar purposes we can only devise taxa that are homogeneous with respect to a few properties. Otherwise we have too many taxa to permit us to achieve our purposes. In the U.S., for example, we now recognize about 8,500 soil seri'es and the number is constantly growing as we study new areas. So, we must devise taxa at different levels of abstraction - some that take many properties into account, and some that take account of only a few.

A category is a group of taxa, defined at about the same level of abstraction, and including the entire population. lVlost natural classifications are of necessity multi-categoric systems. In the plant taxonomy family, genus and species are categories. Series, families, great groups and orders are examples of cate­gories in soil taxonomy. Each is composed of a number of taxa, with the smallest number in the highest category and the largest number in the lowest category.

The forming of taxa. - The purpose of a classification controls the nature of its taxa. If it is a natural classification our primary purpose, aecording to Mill (4), is to arrange the classes of objeets so that we ean discover the laws and remember the properties. This is achieved if we can make the greatest number and the most important statements about the taxa. To form our taxa we consider all of the properties of the objects that we knowand try to form those that best meet our purposes. Cain (5) has said, a elassification is a grouping of things that "belong together". They belong together because they have the maximum number of properties or attributes in eommon. It is because of these similarities that we can make the greatest number and the most important statements about a taxon.

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Cline (1) has pointed out that "classes of naturalobjects are not separated by insurmountable barriers ; they grade by small steps into other classes. Within every class is a central core or nucleus to which the individual members are related in varying degrees ... A class is a group of individuals bound from within, not circumscribed from without. The test of proper placement of any marginal individual is its relative degree of similarity to the modal individuals (that is to the nuclei) of different classes."

This is a reasonable statement of principle, but its application is difficult even with a very appreciable body of knowledge about the objects being classified. It seems doubtful to me that such a body of knowledge about soils exists except for taxa in high categories where we consider only a few properties. In low categories, where many similarities are required, we rarely have sufficient data to determine what properties these individuals actually have, let alone what our nuclei or modal individuals should be. In the present state of our knowledge we are only groping for such individuals. We are forced in the lower cate­gories to circumscribe taxa that our present knowIedge, theories, and laws suggest may be homogeneous. We have not yet been able to test our ideas to see how they correspond to reality except by examination of the field descriptions of many pedons, and the laboratory data from a few samples of a few individuals. Only rarely do we have laboratory data from more than 10 individuals in a taxon of the lowest category, the soil series, or more than 20 in a taxon of the next higher category, the family.

The definitions of taxa. - Having decided the grouping in a taxon that serves his purposes best, the classifiér looks for differentiae to distinguish that taxon from all others. He can not use all of the properties in his definition. He must find some one or a few that win serve the purpose. These should be capable of expres sion in terms of operations so that their mean­ing is as unequivocal as possible.

I have pointed out that . properties selected as differentiae should be soil properties. The use of theories acts to limit the possibility of acquiring new knowIedge. The use of non-soil properties, such as the bedrock or the climate, tends to focus our attention on the climate or the geology rather than on the soil, and again limits the possibility or makes difficult the acquisition of new knowledge about the soil.

Insofar as possible the properties selected should be properties that can either be measured in the field or that can be inferred from properties observable in the field. They should be proper-

19

ties that are the result of soil genesis or that affect soil genesis because such properties carry the maximum numbers of acces­sory properties. And as Cline (1) has pointed out, the differen­tiae selected in a given category must not separate like things in a lower category.

Bridgeman has pointed out that there are "meaningless questions", questions for which no operations can be devised to obtain an answer. The concept of meaningless questions can be expanded to one of meaningless definitions if no operations can be devised or if none have been specified that will enable one to determine whether or not an individual is included by the definition. A recent definition of a taxon of soils reads "Soils with an AC profile. The A horizon is thick, dark in color but is relatively low in organic matter content. The upper part of the A horizon is generally granular when dry and sometimes develops a crust. The lower part of the A horizon usually develops a prismatic structure."

No specifications are given for what is thick, for what is dark, or for what is a low organic matter content. The meanings of these terms must therefore depend on the past experiences of the user of the definition. What constitutes A and C horizons is unspecified, and the balance of the definition appears to be only a description of frequent properties that are not diagnostic of the taxon. This definition is rather typical of the definitions of taxa in many earlier systems of soil classification. 1 t is per­haps not quite "meaningless" in Bridgeman's sense, because operations for determining wh at is thick, what is dark colored are readily at hand. But definitions of this sort are to be deplor­ed in science which should be as quantitative as possible.

The definition of zonal soil (6) is "Any one of the great groups of soil having well-developed soil characteristics that reflect the influence of the active factors of soil genesis - climate and living organisms, chiefly vegetation." It is possible that opera­tions could be devised to determine whether or not a great soil group is zonal, but at this moment I do not see how to do it. In the sense of Bridgeman, the definition of zonal soil would seem to be "meaningless".

Application to the soil survey

In my opening lecture I tried to point out why the soil survey needs to be both practical and scientific. The classification sys­tem used by the soil survey must be capable of meeting these needs. The soil survey requires a classification that will facilitate comparisons of soils for both slmilarities and differences. I t

20

uses the classification to determine what experience is applicable to the soils of a specific tract of land. It makes the most intensive use of soil science possible, but for practical ends. A classification that meets the needs of the soil survey should have many other uses, but it is not the only classification possible, nor is it necessarily the best for every use.

We should note that there are certain very specific require­ments imposed on such a classification by its purposes. It is to be used by an individual to apply the results of experience of other men with soils that the user has not seen. Large numbers of individuals with very diverse backgrounds and training are expected to use the classification accurately to transfer expe­rience with soils. Insofar as possible, the classification should have the followirig attributes :

1. The definitions of the taxa should be operational so that they carry the same meaning to each user. We may not say "a soil has a high content of organic matter" because what is high in one place is low in another. 2. The classification should be a multi-categoric system and should have a large number of taxa in the lower categories if we are to use it to transfer experience. Many soil properties are important to soil use and can vary independently. Clay content alone is not closely related to permeability, reaction, cation exchange capacity, organic matter, or capacity to shrink and swell. If we control both kind and amount of clay, the clay content may be very closely related to several of these other properties. The lower taxa of the classification therefore must be as specific as possible about a great many soil properties. Higher categories are essential for comparisons of the soils of large areas, but are of limited value for the transfer of experience. 3. The taxa must be concepts of real bodies of soil, polypedons that have geographic areas. 4. The differentiae should be properties that may be observed in the field or that may be inferred from properties observable in the field. Base saturation is an important soil property that can not be readily measured in the field, but that can be inferred within limits from the pH, and general knowledge of age of the landforms, the climate, and of the kinds of clay. 5. The classification must be capable of providing taxa for all soils in a landscape, not just selected pedons, because the soil map must cover the whole of the landscape. 6. The c1assification should be capable of modification to fit new knowiedge.

21

Cline (3) has reviewed the new classification with particular reference to its suitability for use by the soil survey. I can do no better than to quote at length from his paper which has not had wide circulation. He says "The system has been develop­ed deliberately to place soils thought to have had similar genesis in the same group. The groups, however, have been defined equally deliberately in terms of soil properties without reference to genesis. Clearly, the bases on which the classes have been formed are genetic considerations, and in this sense the system is based on genesis. The criteria by means of which the classes are differentiated, both in the written outline of the system and in practice in the field, however, are soil properties. In this sense, the system is based on soil properties. In this case also, at least one step of reasoning is necessary to develop genetic interpretations from the definitions of the classes.

"If the classes were defined in terms of either direct inter­pretations for applied objectives or in terms of theories of genesis, the definitions would prejudice future experience by directing understanding of them into channels of past experience and, more importantly, by defining reality in terms of theories, which will certainly change. Future experience is prejudiced in the system in spite of factual definitions, because genetic theory has directed the choice of criteria and practical considerations have influenced their application. That prejudice, however, is in terms of mechanics of the system, not in terms of corres­pondence of classes to real things. The system deliberately subordinates both genesis and practical considerations to quan­titative definition in terms of soil properties, which are fact within the limits of operational measurements. These definitions provide a built-in test of theory. This, in the judgement of this author, is the most important single attribute of the system.

"It is obvious that classes of the system must have at least approximate counterparts in mappable bodies of soil if the pur­poses described by Kellogg and Smith are to be served. Such bodies of soil are, however, real physical things within which one must accept pedons and their geographic relationships, one to another, as he finds them. The orderly arrangements of attributes in classes that can be applied to significant areas must, therefore, be conditioned by the variability that exists among pedons within bodies of soil that occupy areas large enough to be delineated on maps.

"Most soil series, however, are not sufficiently homogeneous in all properties to serve adequately the -practical purposes described by Kellogg (1963). For applications to individual farms, fields, and construction sites, the map units are commonly

22

L

subdivisions of series, called phases, based on single properties of the soil or site. Furthermore, phases of series are commonly grouped independently of their parent series to provide meaning­ful interpretive groups for specific objectives. Phases are de­vices that have been defined deliberately as units outside the taxonomie system but related to it. They are devices used to provide identity for map units designed specifically to meet requirements of homogeneity demanded by the applied objectives of the survey. Their identity is provided by the name of their parent series (or class of a higher category) plus a term descrip­tive of the special property on which they are differentiated. Even in the phase, therefore, the name of the parent class of the taxonomie system is the nucleus of identity.

"There are, however, compelling reasons for a device that is not subj eet to the restrictions of definition inherent in taxo­nomie classes. A class of a taxonomie system must be defined in terms that hold wherever and whenever that class is encounter­ed. Otherwise, meaningful definition would be impossible, not only for that class but also for related classes of the same and higher categories. A given soB series, for example, may be an identifying class in several widely separated surveyareas. In all areas its name must convey the same concept of sets of properties. In one survey, interpretations for forestry may be the primary purpose ; in another, for cropping systems ; in still another, for urban planning. Neither the properties impor­tant for these three objectives nor critical limits of them are necessarily the same. Map units should be fashioned to fit their uses to the extent that this can be done and still represent consistently mappable bodies of soiI. Consequently, flexible cri­teria are needed below some level of generalization for fullest satisfaction of practical objectives.

"The (development of the system) would have been impossible in detail without the accumulated facts about the sets of proper­ties of recognizable bodies of soil, especially those identified as soil series, derived from 60 years of soil survey. For many areas, including parts of the United States, that kind of expe­rience is limited or non-existent, and many kinds of soil are either unknown or known only in terms of gross and incomplete sets of attributes. Consequently, the system is incomplete in terms of classes, criteria of classes, and precision of definition of criteria of classes to varying degrees crudely related to the amount, validity, and genetic correlations of data avaiIable. The system is "open-ended", and subject to additions to all three of the elements mentioned. Every class in the system currently is known to have a counterpart in reality, and the

23

criteria by which it is differentiated are known to be properties of that reality and to be definable in terms of operations.

"Probably the most important feature of the system is the fact that it contains a built-in mechanism to destroy itself, in part or in whoIe. Each application of definitive properties to a real soH can be a test of the validity of the choice and definition of those properties. Collectively, they should force the exposure of false hypotheses and concepts that have been elements governing the choice and d~finition of criteria. This is a guaran­tee against prejudice of future experience and a tooI that should force better understanding."

(1) Cline, M.G. 1949. Basic Principles of SoH Classification. SoH Science, 67, page 81.

(2) Bridgeman, P.W. 1927. The Logic of Modern Physics. MacMillan, N.Y.

(3) Cline, M.G. 1963. The New Soil Classification System. Cornell University Agronomy Mimeo No. 62-6.

(4) Mill, J.S. 1891. A System of Logic. Ed. 8, Harper and Bros, N.Y.

(5) Cain A.J., 1954. Animal Species and Their Evolution. Hillary, London.

(6) Soils and Men. 1938. U.S. Dept. Agr. Yearbook, p. 1180.

24

ORDER 1: ENTISOLS

The Entisols are primarily soils that lack diagnostic horizons other than an ochric epipedon or an albic horizon. They may be found in virtually any climate on very recent geomorphic sur­faces, either on steep slopes that are undergoing active erosion or on fans and floodplains where the recently eroded materials are deposited. They mayalso be on older geomorphic surfaces if men have recentIy disturbed the soil to such depths that the horizons have been destroyed or if the parent materials are resistant to alteration, as is quartz.

One may not say that Entisols are always young soils, though most of them are. One may only say that they lack weIl develop­ed horizons.

The Entisols cannot be compared strictly with the common European concept of an (A) C soi!. The (A) is supposed to differ from the C primarily in structure, and should show little visible darkening by humus. We have permitted both (A) and A horizons in Entisols provided the darkened layer is thin enough to be destroyed by plowing.

This seems a small difference, but it reflects a very basic difference in the approaches of countries that make detailed soil maps (large scaIe) and those that make only small scale generalized soil maps. One cannot claim that two soils are exactly the same if one soil has a distinctly darkened Al horizon that is 5 cm. thick and the other has none. But, if the two soils are plowed, they may become indistinguishable ; in any event, the very thin A or (A) horizon that was present is replaced by an Ap that looks like the C.

The American classification is intended to be used to relate the soils of different fields and farms, so that what we have learned about one kind of soil may be used to advise the cul­tivators of that kind of soil wherever it may occur. There is no question but that plowing mixes any thin surface horizons and

25

-changes the soi!. Yet if we change the classification at a high -categoric level because these thin horizons are mixed, we can -not use the classification of a virgin soil to predict its behaviour under cultivation. Such a classification would impede the exten­-sion of our knowledge of soil behaviour.

The Western European soil classifications have not generally been used as a basis for advising cultivators. They have been developed primarily for use in understanding soil genesis and to a limited extent for forest management. Many cultivated soils have no place in the system of Kubiena, for example. Or, because the humus form is changed if the soil has been limed, fertilized and cultivated, the classification is changed. But, if the clas­:sification is not used as a basis for advisory work with cultivators, the changes in classification that come from plowing alone are -not important. One can simply ignore the cultivated soils if there is no soil survey. If there is a soil su'rvey, one must be 'prepared to classify all of the soils. In Western Europe this has been a problem mainly in Belgium, the Netherlands, lreland and England.

As our system now stands there are three suborders of Enti­sols, the Arents, Orthents and Psamments. (Fluvents were added -af ter this lecture was given.)

.Arents (1.6)

The Arents provide a place for soils that have been me­-chanically mixed deeply enough to destroy the diagnostic horizons as horizons, but not thoroughly mixed. Consequently, :fragments of diagnostic horizons may be identified.

The mixing is most commonly from land levelling, deep plow­ing and shaping. It may be the natural effect of mass movement -in slides, but this is not common.

The Arents are normally variabIe from spot to spot. Soil :series cannot be mapped, and normally we do not attempt to -define series. Sometimes families can be mapped, but most of ten ·only the subgroup can be identified. One may find fragments ·of a spodic horizon, an argillic horizon, or even fragments of a .<furipan and an argillic horizon. The subgroup is determined by the nature of the fragments that can be identified. No typic subgroup can exist.

The Arents have not been extensive. U se of the modern earth moving equipment and large plows that can turn the soil to depths of 1.5 to 2 meters is rapidly increasing the importance ·of this suborder.

:26

Orthents (1.5)

The Orthents are extensive and important soils, but extremely variabIe in nearly all properties. The distinction between Udents and Ustents (moist and dry EntisoIs) provided in the 7th Approximation was dropped because we have so far been unable to devise a satisfactory c1assification of moisture regimes that can be applied to the EntisoIs. The moisture regime of Entisols is complicated by the position of many on floodplains. Excess moisture can come from rains that fall in distant mountains. Climatic data for a given locality are not always useful in estimating the moisture regime of the nearby Orthents on flood­plains. (Fluvents were added later to help resolve the problems of characterizing the soil moisture regimes.)

The variability of the Orthents is reduced at the great group level. I hope that we can eventually do something at this level to distinguish between the dry and moist Orthents. You should note that for Orthents, wh ere there is no solum, we use an arbitrary control section that extends to a lithic contact or to one meter, whichever is shallower. If there are important features deeper than a meter, they are recognized by the use of phases. Wh ere texture is concerned, we use the same lower limit of the control section, a lithic contact or one meter, but we treat as a phase the texture of the pI ow layer, or the surface 25 cm., whichever is thicker. This is perhaps only a bow to the tradition of the American soil type which was determined by the texture of the plow layer or its equivalent in virgin soils. The soil type has been dropped as a category but the texture of the p]ow layer still can vary significantly within a soH series and needs to be indicated. So, the texture of the plow layer and features below the solum or the arbitrary control section are treated as phases. They may be indicated if they are relevant to soil behaviour, but they may be disregarded if they are not relevant.

Agrorthents (1.52) This great group has been temporarily provided here, though

it seems likely to me that it should be moved to the order Inceptisols. In the U.S. we have no good basis for a decision.

Cryorthents (1.51) These are the cold Orthents.

H aplorthents ( 1.53 ) The concept of the Typic Haplorthent has heen set on the

soils of very recent erosional surfaces rather than depositional

27

surfaces, although these is no clear dichotonly. A recent erosional surface may now be receiving eolian deposits. This is the situation in parts of Alaska and N ew Zealand where loess is currently being deposited. It is the reason for the limit of 25 per cent slopes. Soils on slopes with this gradient or more are considered Typic even though they are accumulating sediments.

In addition, the concept of the Typic Haplorthent is one of soils in thick or moderately thick regolith. It is the former concept of Regosols rather than Lithosols.

But the concept of Haplorthents still has some very serious defects. It takes no account of moisture regimes beyond the absence of ground water. I would hope that we can find some way to correct this defect. In addition, the concept includes soils on the equator as weIl as soils of the boreal forests. It is quite possible that either a suborder of Tropents or several great groups of tropical Entisols will be added.

Psamments (1.X)

The Psamments include all sands lacking a diagnostic horizon other than an ochric epipedon or an albic horizon, or both.

The surface area of sand, per hundred grams of soil, is small by comparison with other textural classes. Very small amounts of clay have an enormous effect on the surface area.

Table 1

Surface area in 1 gram

Soil separate

Coarse sand Fine sand Silt Clay

Diameter (mm)

1.0 - 0.50 0.25 - 0.10 0.05 - 0.002

< 0.002

Surface area (cm2)

22 90

453 11,300

Consequently, extremely small changes in sands, either altera­tions, translocations or additions are apt to produce visible color changes. It has been said that it takes very little paint to make a barn red. Very small removals may make a sand white. We have used the principle throughout the system that a diagnostic horizon should be the result of a significant altera­tion or addition. Horizons in Psamments that differ significant­ly in color may be indistinguishable in the laboratory, so we concluded that we should not make separations of sands on color alone exêept at a lpw categoric level. Therefore, sands

28

and loamy sands are grouped in the single suborder if they lack argiUic or spodic horizons, or moHic, umbric or pJaggen epi­pedons. This course was als'O suggested by the many important physical and chemical properties common to sands. They have low water holding capacities, they lack structure, they are sub­ject to wind erosion, they have low cation exchange capacities, and so on. The similarities are many, and the differences few and of little significance, although there may be striking dif­ferences in col or.

Four great groups are recognized.

Aquipsamments (l.X1)

These are the wet sands.

Cryopsamments (l.X4)

These are the cald sands.

N ormipsamments (l.X3) These are the sands that contain significant amounts of

mineraIs, such as feldspars, that can weather or dissolve in a warm humid climate. (We are testing the possibilities of re­placing this great group by several great groups defined by their moisture regimes.)

Quartzipsamments (l.X2)

These are the sands that are almost entirely quartz. Because of their chemical nature, they may be on stabIe surfaces of considerable age, Pleistocene or possibly much older.

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ORDER 2: VERTISOLS

The Vertisols are intended to include the cracking clays-soils that . shrink and swell over the seasons and that have wide deep cracks during the seasons of deficient moisture.

They have never fitted comforlably into any classification. In a given locality they may appear to be intrazonal soils if they differ greatly from the nearby zonal soils. Yet they are to some extent zonal because they must be alternately moist or wet and dry or nearly dry. They are easily confused with Mollisols if they occur in the same landscape with them.

Like the Psamments, the Vertisols have many common proper­ties associated with their fine texture. They are plastic, sticky, have moderately high or high cation exchange capacities and a high capaci ty to shrink and swell wi th changes in moisture. Because the lower horizons are 'Subject to pressure when the soils swell, they are compact and very slowly permeable to water. They have low available water holding capacities although they have high total water holding capacities. The soil movement makes the Vertisols troublesome for engineering uses, and seriously affects the growth of trees. Fences, telephone lines, highways and trees are thrown out of line or tilted in various directions, and so on. Certainly they have many common prop­erties.

They mayor they may not have diagnostic horizons. Mollic and umbric epipedons are common. Some have calcic hor izons. A few have argillic horizons by the present definition and an albic horizon, but these are thin enough to be destroyed by the first cultivation and are not considered diagnostic.

The determination of the limits of the Vertisols and their various great groups continu es to be troublesome. Changes seem to be needed but there is no agreement yet as to what the changes should beo Some would broaden the definition of Verti­sols to include all clays with a high capacity to shrink on drying.

30

This seems unwise to me, because the Vertisols not only must shrink but they must shrink as fairly large blocks to form wide cracks. An equal shrinkage into fine prisms or fine blocks might produce no visible cracks. Yet some montmorillonitic clays that crack widely show no slickensides or other subsurface structural features of the Vertisols. At present these are exclud­ed from Vertisols, but the pedologists who work with them and with Vertisols believe they should somehow be grouped in a single order. .

An additional problem in the classification of the Vertisols comes from the distinction between the self mulching and crusty soils. This has been recognized at the Great Group levelt

but the definition we have proposed seems to be inadequate. In California the soils on one side of a fence or road may be crusty, but on the other side they may have a granular surface. Such a split within a series is the result of soil management. Very probably the differences are quickly reversible if the management practices are changed. Such a transient feature as the til th of a soil should not be used at a high categorie level in a classification system. N ormally we would not even consider treating it as a phase.

But other Vertisols always seem to be crusty. These are distinguished as different series. Generally, if they are crusty, they are low in organic matter and have moist color values in the surface horizons of 4 or more. To date, the laboratory studies have shown that the crusty Vertisols are low in organic matter and become very light in color on treatment with H202. The self mulching Vertisols are not only darker in color but remain relatively dark af ter H202 treatment. The ratio of free iron to clay seems to be related to crusting. The less iron we have, the poorer the structure, and the lighter the color af ter H20 2 treatment. (Subsequent to this lecture, the classification of the Vertisols was drastically revised.)

Aquerts (2.1)

The Aquerts have the low chromas or mottling that we normally associate with wetness, or periodic high ground water. But because of their texture, slow permeability, and high density, the norm al concept of ground water is meaningless. Water commonly will not appear in a borehole, through it may if the borehole intercepts a recent large root channel or the burrow of a crayfish. It seems likely, however, that the oxygen pressures (redox potentials) are very low in these soils for

31

long periods. There could be frequent reducing condition:3 in the soil even in the absence of ground water.

A few Aquerts occur in depressions where water may be ponded for several months, but many are on gentIe slopes where water cannot stand. The moisture regimes in the Aquerts are not strictly comparable to those of other Aquic suborders, such as Aquepts, although the redox potentials are probably similar.

The great groups of Aquerts are not yet agreed upon. It has been proposed that the Grumaquerts and Mazaquerts be combined. The majority of these with serious crusting problems are sepa­rated at the subgroup level in the various Ultic, Natric and Entic subgroups. It has also been proposed that a great group of Tropaquerts be recognized to distinguish between the Aquerts of tropical and temperate regions. We will discuss this in more detail in a later lecture.

Usterts (2.2)

The Usterts provide a place for the Vertisols that are weIl enough aerated that the colors are brownish, and that show no mottling except perhaps in the deeper layers. Mostly, the Usterts are restricted to drier climates than the Aquerts, although there is not a perfect correlation. In the transition areas we may find Aquerts in depressions and U sterts on slopes. In desert areas the U sterts may occupy depressions that are only occasionally flooded.

The problems of the classification of the Usterts are parallel to these of Aquerts.

32

ORDER 3: INCEPTISOLS

The Inceptisols include soils with diagnostic surface or sub­face horizons in addition to the ochric epipedon and albic horizons of the Entisols. The Inceptisols lack illuvial horizons unless in a lower sequum, and we may say that they are primarily eluvial soils - soils that are losing materials through- . out, although a few show illuvial horizons of opal. It is not sur­prising that the Inceptisols are restricted to climates in which there is some leaching in most years. Generally, the direction of soil development is not yet evident from the marks left by the various soil forming processes, or the marks are too weak to permit classification in another order.

The order has been criticized because it includes soils in rather widely differing environments in which different process­es are presumed to be active. Presumption is not an adequate basis to classify a soil objectiveIy. Soils should not be classified on the basis of what we think they will become but rather on the basis of what they are.

For example, in the Tropics one may find an Inceptisol on a steep young surface, adjacent to Oxisols, and having some or many of the properties diagnostic of Oxisols. But, the Incep­tisols may have too many weatherable mineraIs, and the cation exchange capacity may be too high for an Oxisol. One can assume that this InceptisoI, in time, win become an Oxisol, but such an assumption is not always realistic. The geomorphic surface may rather safely be assumed to be young if the slope is steep, and there obviously has not been time to destroy the weatherable mineraIs. However, the slope is steep, and the slope win recede as erosion continues. It seems most probable that the retreat of the slope will entirely destroy the present Inceptisol long before an Oxisol can be formed. The soil is now inter­mediate in its properties between an Entisol and an Oxisol, but it is not necessarily going to become an Oxisol. To do that,

33

weathering of the primary minerals must go on more rapidly than removal of weathered materials byerosion. The soil may actually represent an equilibrium between erosion and weather­ing; it may be in the process of reverting from Oxisol to Entisol, or developing from Entisol to Oxisol. The classification of the soil as an Oxic Inceptisol is as much as our present knowledge of the soil's genesis permits.

Another example may be taken from Europe. The "Rendzine typique" (Rendoll) has a dark colored, humus rich surface (a mollic epipedon), that rests on a highly calcareous parent material. A soil that is poorer in humus, but very rieh in chalk, lying on a retreating slope, is called a "Rendzine blanche", presumably on the assumption that it will develop a dark surface in time. Otherwise it would be called a "sol brut d'érosion". But there is no assurance that this soil will ever become a "Rendzine typique". Current erosion may be quite active. 80 long as the slope continues to retreat, little or no additional organic matter can accumulate, and no Rendoll can form. We have considered the "Rendzine blanche" an Entisol, equivalent to a "sol brut d'érosion", and placed it in a carbonatie family.

The Inceptisols therefore include the soils that are dominantly eluvial throughout. They may show a number of the charac­teristics of other orders, but these characteristics are too weakly developed to permit classification in the other orders, and one may not safely assume what kind of soil the Inceptisols will become.

Andepts (3.2)

These are the Inceptisols formed in ash, pumice, or other pyroclastic materiaIs, either fresh or reworked. They are rich either in glass or an amorphous clay generally called allophane. And, they lack the strongly gleyed horizons characteristics of the Aquepts.

Glass is a rather unique pare nt material. It is relatively soluble compared to the crystalline alumino-silicates. Initial weathering products tend to be amorphous. The amorphous clays have a high affinity for water and organic matter, and have very high pH dependent cation exchange capacities. The Andepts could easily have been placed in a separate order. They were made a suborder of Inceptisols because they resembIe the other Inceptisols in many respects. The horizons can form rapidly, but are distinct and are eluvial except for the duripans in which opal accumulates. With time, the allophane is thought to crystallize into layer lattiee clays, because we can find a

34

variety of kinds of clay in old volcanic ash deposits. But the direction of development is obscure. As with other Inceptisols, one cannot safely postulate that a given soil will develop into an Oxisol or an Ultisol. One can only indicate by the subgroups that a given Andept may have several properties in common with the soils of these orders.

There have been some substantial changes in definitions of Andepts given in the June '64 supplement. The revised defini­tions are given later.

The intent of the changes has been to provide better defini­tions of Andepts and its various great groups and subgroups, and to improve the groupings.

Some Andepts, mostly of pumice and with only traces of fine earth, have mollic or umbric epipedons by the definitions. Such soils seem to fit much better with the Vitrandepts. Because there is no reliable method of particle size analysis on Andepts in general, we substituted moisture retention as a measure of surface area. In general, one's fingers are adequate to permit the estimation of particle size in Andepts, and are more reliable than laboratory measurements of particle size distribution.

Aquepts (3.1)

The Aquepts include the strongly gleyed Inceptisols. They include most soils classified as Aquents and as Aquepts in the 1960 classification. Only the Aquipsamments of the former Aquents remain as EntisoIs. The reasons for keeping together all of the sands that lack illuvial horizons were given in the discussion of Psamments.

The Aquents were transferred to Aquepts because we found the former distinction so fine, and of ten theoretical, that it could not be applied consistently by different pedologists, or even by the same pedologist on different days.

The Aquepts do not include all gleyed soils ; these are scat­tered through most of the orders according to other charac­teristics. This, like the classification of the former Solonetz, has not been traditional, has been strongly criticized, and deserves explanation.

Gleying may be considered as a single property resulting from biologic activity in an anaerobic environment. The soils that are strongly gleyed have a great many other properties, and many diagnostic horizons - mollic, umbric, and ochric epi­pedons are all common. Thus, the soils may be rich in orgallic matter and either very strongly acid or even calcareous. The same variability is found in the soils with ochric epipedons.

35

1-

The soils may have argillic horizons that are either very strongly acid or nearly neutral. They may have natric horizons, calcic horizons, salie horizons, plinthite, and even fragipans. They may be rich in weatherable mineraIs, or lack them com­pletely. These are properties that mostly have been used to define orders and great groups. Except for the gley colors and the distribution and valence of the free iron that produces the color, we find as much variability in the gleyed soils as in those that are not gleyed.

In the regions dominated by Ultisols or Inceptisols, the gleyed soils normally have the properties that are diagnostic of Ultisols or Inceptisols. In regions dominated by Mollisols, the gleyed soils normally have the properties that are diagnostic of Mollisols. In the region of Oxisols many of the gleyed soils have the properties diagnostic of Oxisols. In the region dominated by Alfisols many of the gleyed soils have the properties diagnostic of AlfisoIs, but a number have the properties of Mollisols.

If the strongly gleyed soils are grouped into a single order, attention is focused on gleying. Features that are important enough to define orders in other soils get reduced emphasis. But these features are not actually any less important in the gleyed soils then in the freely drained soils. This migh t not be true if men did not commonly drain the swamps and wet places. But men do drain them, and afterwards gleyed soUs begin to behave like the naturaIly weIl drained soils that they re­semble in properties other than col or.

Distributing the gleyed soils among the several orders does not prec1ude regrouping them as Aquic great groups if one wishes to focus attention on the wetness. But grouping the gleyed soils into a single order makes it extremely difficult to regroup them with their appropriate orders. The problem is fundamentally one of nomenc1ature, but I have been unable to see any way to name a regrouped set of soils with the proper­ties that are diagnostic, for example, of UltisoIs. It would not be easy, by a name or phrase, to bring to mind all of these soils if the gleyed soils are set apart in a separate order. 80, we decided to break with tradition, because we thought a more useful classification could be developed if the gleyed soils were divided among the various orders.

Andaquepts (3.16)

These are the Aquepts of temperate and warm regions that are rich in glass and/ or in amorphous c1ays. I should note that there are also Andepts that are quite wet, but for some reason the soils from more or less pure volcanic ash of ten show no

36

signs of gleying. The reason for this is unknown, but the Andaquepts do not seem to include all of the wet soils formed in ash. They include only those that show evidences of strong gleying.

Cryaquepts (3.14)

These are the cold Aquepts. Because they are both wet and cold, cryoturbation is generally active, and ruptic subgroups with Histosols are common. These have not been defined in the J une '64 supplement because we have not been making soH surveys in the regions where these soils occur. It is important to note that mollic epipedons are permitted in this great group if the base saturation in the cam bic horizon drops below 50 per cent. So far as we know, such horizon sequences result from liming soils with umbric epipedons. We certainly do not want to change the classification of a soil because of a single applica­tion of lime, but if the cultivator does lime an umbric epipedon he can easily convert it to a mollic epipedon.

Fragiaquepts (3.13)

These are the Aquepts with a fragipan. It would appear that we have defined this great group on the presence of a single horizon. The same appearance is found in the definitions of many taxa, orders, suborders, great groups, and so on, and we have been criticized for this, particularly by the Soviet soil scientists. They say "such an approach is not genetic but formal. Soils cannot be classified according to the presence or absence in them of a single horizon alone." (1). It would appear that we have done this in most categories. All soils with spodic horizons are grouped in Spodosols. Ochrepts and Umbrepts are distinguished by their ochric and umbric epipedons. Fragi­aquepts have a fragipan, and so on. The fact is that these are diagnostic features that have been defined and selected af ter considering all the properties of the soHs that are grouped by their use. A somewhat comparable situation exists in the tax­onomy of animaIs. All Carnivores have three bones that are fused in the wrist. This fusion is not known to exist in any other class of animaIs, and it is the only known feature that is common to Carnivores and that is not also common to other mammaIs. It is a diagnostic feature, but it was selected only af ter all the characteristics of Carnivores had been considered. The fragipan was selected as diagnostic among the Aquepts for a great group, but the fragipan by itself has a great deal more importance to the soil than do the th ree fused wristbones to the Carnivores. The Fragiaquepts, so far as is now known, have many common

37

properties that are not common to all Aquepts. In addition to the fragipan, they have either loamy or silty textures, low base saturation, low contents of organic matter, low available water holding capacities, perched water tables in some seasons and shallow ground water in others, slow permeability, shallow pene­tration of frost, seasonal absence of saturation with water, and so on. Plants are shallow rooted, even af ter drainage. I have discussed this at some length because I want to make it as clear as I know how that we have tried to group the soils with the greatest number of common properties. We cannot use many properties to define the taxa. We must select a very few prop­erties for the definitions if we expect anyone to be able to use them, or even on occasion, remember them. If taxa are to be defined, they should be defined in this manner - by selection of as few properties as possible to produce the desired groupings.

Halaquepts (3.11)

This great group may be an exception to the rule that Incepti­sols are eluvial soils, but the soils have not been studied. They are the wet sodic soils, soils with high saturation with sodium. Of ten they are saline, but they do not have salic horizons. We have found them mostly in rather dry places where there are pronounced wet and dry seasons, and where the ground water fluctuates. The sodium saturation that decreases with depth presumably reflects capillary rise during the dry season. The absence of salic horizons presumably reflects leaching during the rainy season.

Humaquepts (3.12)

These soils are required to have an umbric, histic or mollic epipedon, and low N values (Note that the June '64 supplement omitted the mollic epipedon in this definition). Mollic epipedons in this great group, insofar as we know, are formed by liming umbric epipedons. Consequently, mollic epipedons are not con­sidered to differ from umbric epipedons in this class, and are not used to distinguish subgroups.

Hydraquepts (3.17)

These are the Aquepts (formerly Hydraquents) that have always been saturated with water. Water tables may fluctuate with the tide, but the soils have never dried. The high water contents and the low bulk densities that produce the high N values can not exist in a horizon that has once been dry. A single drying produces a partially irreversible shrinkage and leaves cracks that are subsequently filled by additional soil material.

38

This is why alluvium deposited by rivers in their flood plains has a higher bulk density. When the floods recede, the soils eventually dry and crack. The next flood deposits new sediments in the cracks. Or, mixing of the surface deposits by animals and wind has the same effect. Thus, deposits accumulate in these positions with too high a bulk density for Hydraquepts. In deltas and in coastal marshes the deposits may never dry, and materials may accumulate with low bulk densities because they have never dried. The Hydraquepts have Iow bearing capacities. Many will not support grazing animaIs, particuIarIy cattIe. Cat c1ays are common in the Hydraquepts with brackish water, and one must be extremely careful in the drainage and reclamation of these soils. They not only subside in drainage, but they are apt to become extremely acid if they contain more sulfides than carbonates.

Normaquepts (3.15)

These are the Aquepts with ochric epipedons, with warmer temperatures than these of Cryaquepts, with low N values, and without fragipans. They are gray, or mottled from the surface down. Mostly they have a high ground water at some season, but the water table fluctuates.

Ochrepts (3.4)

These Inceptisols are formed in parent materials with crys­talline mineraIs. They are found in temperate to cold climates in which there is some annual excess of precipitation over evapo­transpiration. They are characterized by the presence of eluvial horizons and the absence of illuvial horizons except in a lower sequum. Mostly, the colors are shades of brown and yell ow, though those from red pare nt materials have reddish hues. Umbric epipedons are absent or of minimal thickness.

Traditionally, these soils have been grouped with the Udalfs, which they resemble in color, both in Europe and the U.S. Some still feel that this is proper. Apparently some believe that the color is more important than the base status and the presence or absence of an argillic horizon.

We have been unable to find any property that correlates with a brown color but not with the reddish colors. We think that base saturation and the presence or absence of illuvial horizons are far more important then col or alone. The absence of illuvial (argillic) horizons is correlated with the age of the geomorphic surface and with c1imate. They seem absent unless there is a period of saturation deficit. They are absent on the

39

youngest geomorphic surfaces. Both time and some dryness seem important to their formation. Color of a soH may be im­portant if it reflects some other property of a soil, as do the colors of mollie epipedons, but it is the humus and not the color that is important. Throughout the system we have reduced emphasis on color unless the color is associated with some other important properties. So we grouped the Ochrepts with Incep­tisols instead of Alfisols because they had many resemblances to other Inceptisols, and only one, color, to the AlfisoIs.

Cryochrepts (3.41)

These are the cold Ochrepts without fragipans.

Dystrochrepts (3.44)

These are the acid Ochrepts without fragipans.

Eutrochrepts ('3.43)

These are the base rich Ochrepts without fragipans.

Fragiochrepts (3.46)

These are the Ochrepts with fragipans.

U st.ochrepts (3.45)

These 'are the Ochrepts in climates with dry summers and humid winters. They are, so far as we know, restricted to young geomorphic surfaces.

Plaggepts (3.6)

The Plaggepts, formerly Plàggents, could be assigned about equally weIl to either Entisols or Inceptisols. They have a plaggen epipedon that at one extreme is black and rich in humus. The other extreme is brown or yellowish brown and is not always particularly rich in humus. It seems best to keep all the soils with plaggen epipedons together, but we have none in the U.S. and are not in good position to decide the classifi­cation. We received complaints about their inclusion with Enti­sols, and moved them to Inceptisols to let them be tested there. We do not intend to make the final decision on their classifi­cation. Others must do this.

Tropepts (3.5)

The Tropepts include the Inceptisols whose mean summer temperature at 50 cm. depth differs from the mean winter temperature by less than 5°C. and that have mean annual soH

40

temperatures in excess of 8.3°C. (47°F.). In tJhe 7th Approxima­tion we pointed out that classification of tropical Inceptisols in the same great groups with those of temperate regions. might not be desirabIe. We think now that a separate suborder would be best in the Inceptisols. The tropics have wet and dry seasons, but summer and winter of ten are meaningless as we­know the seasons.

Biologic activity is continuous unless there is a pronounced dry season. Cultivated soils cannot remain bare and exposed to erosion for long periods, for if there is rain the plants start to grow. The continuous biologic activity is reflected in the nature of the organic matter in at least two important ways. One is that for any given kind of soil, C/N ratios tend to be' lower in tropical than temperate regions. C/N ratios of virgin soils in the humid tropics compare with those of Aridi­sols in temperate regions ; of ten they are extremely low. The other difference is that the amount of organic matter is not weIl reflected by the color of the soil. Black soils may have little organic matter ; light colored soils may be very rich in organic matter, or very poor. The distinction between Ochrepts and Umbrepts is useless in tropical regions. Tropical great groups. in these suborders would produce irrational groupings with little in common but col or. To permit use of color where it has meaning, in the temperate regions, it seemed essential to set. up a separate suborder of tropical Inceptisols.

At the moment we are proposing four great groups, defined later.

Humitropepts (3.53)

These are found in "cool" humid tropical regions. HumuS', contents are high in virgin soils, and relatively high in cul­tivated and eroded soils. These soils have mean annual tem­peratures of 22°C. or less.

Eutropepts (3.52)

Tropepts with medium or high base status; with short or no dry season.

Dystropepts (3.51)

Tropepts with low or very low base status, and m'ean annuaI temperatures above 22°C.

Ustropepts (3.54)

Tropepts with medium or high base status and/or with long dry seasons.

41

The development of the dassification of Tropepts is far from complete. You should note, however, that we permit mollie epipedons in this suborder as weIl as in Aquepts, Andepts and Umbrepts.

Umbrepts (3.3)

The classifieation problems of Umbrepts parallel those of Ochrepts. The Umbrepts have more organic matter than the Ochrepts, and have only medium low or low base saturation. If base saturation were high throughout, the soils would be Mollisols. So, Umbrepts on the whole are restrieted to some­what more humid climates than are some Ochrepts. However, Iiming may produce a mollie epipedon, so we permit this epipedon in the suborder.

Anthrumbrepts (3.34)

This great group is so scarce in the U.S. that we have no basis for defining subgroups.

Cryumbrepts (3.31)

These are 1fue cold soils again, that lack fragipans.

Fragiumbrepts (3.35)

These are the Umbrepts with a fragipan. They are extremely rare in the U.S.

H aplumbrepts (3.33)

The other Umbrepts.

Andepts (3.2)

These are Ineeptisols that (1) have an exchange complex that gives no x-ray pattern or that is dominated by amorphous materiaIs, and either (2) have bulk densities of less than 0.9 at 1/3 bar tension in the fine earth fraction of the epipedon or the cambic horizon or both, or (3) have more than 60 per cent of vitrie volcanie ash, pumice, or other pyroelastie material in the silt and eoarser fraetions.

Cryandepts (3.21)

The definition is unchanged. These are the Andepts of cold places.

Durandepts (3.22)

The definition is unchanged. These are the Andepts with duripans.

42

Hydrandepts (3.25) The definition is unchanged. The definition of the typic sub­

group is changed by the deletion of item b that prohibited motties with chromas of 2 or leslS within 1 meter.

Eutrandepts (3.26) (repLaces Mollandepts) Andepts that :

1. have a mollie epipedon ; 2. lack a duripan or fragipan ; 3. have temperatures warmer than those of Cryandepts ; 4. do not contain clays that dehydrate irreversibly into gravel­

sized aggregates ; 5. have textures finer than coarse-Ioamy or coarse-sHty, based

on the whole soi!. (Parlicle size analyses are of ten unreliable in these soils, and the 15 bar moisture content is a better guide to texture. The 15 bar moisture, based on the whole soil, should be more than 20 per cent.)

Typie Eutrandepts (3.260) Eutrandepts that :

a. have a mollie epipedon 25 cm. (10 inches) or more thick ; b. have no lithic contact within 50 cm. of the surface ; c. do not have buried soils other than buried Eutrandepts

within the upper meter of the soH ; d. lack mottIes with chromas of 2 or less within 1 meter (40

inches) of the surface ; e. lack a fragipan ; f. have no more than traces of 2 : 1 lattice clays ; g. have a cec of more than 40 meq./100 grams of soH in all hori­

zons.

Entie Eutrandepts (3.26-1) Eutrandepts like the Typic except for a.

Tropie Eutrandepts (3.26-3.5) Eutrandepts like the Typic ex cept for g with a cec of more

than 20 meq./100 grams of soH af ter correcting for organic matter, and with less than 5°C. difference between mean winter and summer soil temperatures at 50 cm. depth.

V itrie Eutrandepts (3.26-3.23)

Eutrandepts like the Typic except for g, and with more than 5°C. difference between mean winter and mean summer soil temperatures at 50 cm. depth.

43

Normandepts (3.24)

Andepts that : 1. have an umbric or ochric epipedon ; 2. do not have clays that dehydrate irreversibly into gravel-

sized aggregates ; 3. have temperatures warmer than those of Cryandepts ; 4. have no duripan ; 5. have textures finer than coarse-Ioamy or coarse-silty based

on whole soil. (Particle size analyses are of ten unreliable in these soils, and the 15 bar moisture content is a better guide to texture ; the 15 bar moisture, based on the whole soil, should be more than 20 per cent).

Typic Normandepts (3.240)

N ormandepts that : a. have an umbric epipedon 25 cm. (10 inches) or more thick ; b. have no buried albic, argillic, spodic or oxic horizon with its

upper boundary within 1 meter of the surface ; c. lack a lithic contact within 50 cm. (20 inches) of the surface ; d. lack mottles with chromas of 2 or less within 1 meter of the

surface ; e. lack a fragipan ; f. have clays that do not dehydrate irreversibly (15 bar water

is not affected by drying) ; g. have no more than traces of layer lattice silicate clays ; h. have cec of more than 50 meq./100 grams of soil in all hori­

zons.

Aquic Normandepts (3.24 Aqu.)

N ormandepts like Typic except for d.

Hydric Normandepts (3.24-3.25) Normandepts like Typic except for f.

Oxic Normandepts (3.24-9)

Normandepts like Typic except for h, or g and h with less than 20 meq./100 grams of soil in some horizon (af ter correction for effect of organic matter).

Thapto-Normandepts (3.24) Normandepts like Typic except for b. (Thapto-Spodic, -Alfic,

-Ultic and -Oxic subgroups may be needed, according to the nature of the buried soil).

44

Tropie Normandepts (3.24-3.5)

N ormandepts like Typic except for h, or g and h. With some hor&ion that has cec between 20 meq./100 grams of soil (af ter correction for organic matter) and 50 meq./100 grams of soil without correction for organic matter ; and with less than 5°C. difference between mean summer and mean winter soi! tempera­tures at 50 cm. depth.

Umbrie Normandepts (3.24-3.3)

N ormandepts like Typic except for g.

Vitrie Normandepts (3.24-3.23)

N ormandepts like Typic except for a.

V itrandepts ('3.23)

Andepts that : 1. have temperatures warmer than those of Cryandepts ; 2. have no duripan ; 3. have textures of coarse-silty, coarse-Ioamy or coarser, based

on the whole soil. (ParticIe size determinations are of ten un­reliable in these soils, and the 15 bar moisture content is a better guide to texture. The 15 bar moisture, based on the whole soil, should be 20 per cent or less).

Typie V itrandepts (3.230)

Vitrandepts that : a. lack a fragipan ; b. lack a lithic contact within 50 cm. of the surface ; c. lack mottIes with chromas of 2 or less within 1 meter of the

surface ; d. have no clays that dehydrate irreversibly on drying (that is,

15 bar moisture is not affected by drying) ; e. no buried albic, argillic, spodic or oxic horizon within 1 meter ; f. have an ochric epipedon.

Aquie Vitrandepts (3.23 Aqu.)

Vitrandepts like Typic except for e.

Lithie Vitrandepts (3.23 L) Vitrandepts like Typic except for b.

Eutrie Vitrandepts (3.23-3.26) Vitrandepts like Typic except for f; and with a mollic

epipedon.

45

Normie V itrandepts (3.23-3.24)

Vitrandepts like Typic except for f ; and with an umbric epipedon.

Thapto-Alfie Vitrandepts (3.23-Thapto 7)

Vitrandepts like Typic except for e ; with a buried albic horizon and with a buried argillic horizon that has chromas of less than 6, and that has base saturation in excess of 35 per cent within 125 cm. of its upper boundary.

Tropepts (3.5)

Inceptisols other than Aquepts and Andepts that : 1. have less than 5°C. difference between mean summer and

winter temperatures at 50 cm. depth, or at a lithic contact if this contact is shallower than 50 cm., and have a mean annual temperature of more than 8.3°C. ;

2. have (a) an umbric epipedon, or (b) have a mollie epipedon if

(1) there is 35 per cent or more montmorillonite, and the epipedon rests on materials with less than 40 per cent CaC03 equivalent, or

(2) the moist and dry values are no darker than those of underlying horizons, or

(3) the chromas, moist or dry, are 4, or 3. have an ochric epipedon and a cambic horizon.

Humitropepts (3.53)

Tropepts with mean annual temperatures of 71.6°F. (22°C.), or less, and with base saturation of less than 35 per cent (by sum of cations) in some part of the epipedon or cambic horizon.

Eutropepts (3.52)

Other Tropepts with more than 35 per cent base saturation (by sum of cations) throughout the epipedon and any cambic horizon ; with no horizon containing soft powdery secondary carbonates, and never dry for more than 60 consecutive days in any horizon in most years (7 out of 10).

Dystropepts (3.51)

Tropepts with mean annual temperatures of more than 71.6°F. (22°C.), and with base saturation (by sum of cations) of less than 35 per cent in some part of the epipedon or cambic horizon.

46

U stropepts (3.54)

Tropepts with (1) base saturation of more than 35 per cent throughout the epipedon and any cambic horizon, and/ or (2) dry in some horizon for more than 60 consecutive days in most years (7 out of 10) or with a horizon containing soft powdery secondary lime.

Typic subgroups of Tropepts

(a) With a cambic horizon (Entic if absent). (b) With organic matter contents that decreas'e regularly with

depth (Cumulic otherwise). (c) Lacking soft plinthite within 2 meters of the surface

(Plinthic if present). (d) With cation exchange oapacity in the cambic horizon of

more than 20 meq./100 grams of clay af ter correcting for organic matter, and less than 50 meq. (Oxic if too low, Andic if too high).

(e) With no Uthic contact within 50 cm. (Lithic if present). (f) With no mottles having chromas of 2 or less within 1 meter

(Aquic if pres!ent). (g) With less than 35 per cent montmorillonite in the fine earth

fractions of the cambic horizon and the epipedon (Vertic if present).

(N ote that intergrades between great groups of Tropepts are not yet provided. If needed, specifications must be added to the individual Typic subgroup definitions.)

(1) Yerokhina, A.A. & Sokoiova, T.A. - Genesis and Geography in the New (American Soil Classification) System. Soviet Soil Science, J une, 1964, p. 579.

47

ORDER 4: ARIDISOLS

The Aridisols are soils of dry places. They have an ochric ·epipedon that is normally soft when dry or that has distinct ·structure. In addition they have one or more of the following .diagnostic horizons : argillic, natric, or cambic horizons ; calcic, gypsic, or salie horizons ; or a duripan. The shifting sands of what some call the "true desert" are not included in Aridisols. If they have no vegetation they are not considered soil. If they -have an ephemeral vegetation they are considered soil and they -are classed with Psamments. The salt flats of desert playas are not considered soil unless they have some vegetation, either ephemeral or scattered.

The action of wind is particularly important in the genesis -of Aridisols. Because the soils are so frequently dry and vegeta--ti on is scattered, much of the soil surface is normally bare and exposed to blowing. Dust in the air is common. There is astrong tendency for fresh deposits to be blown away by the winds until a protective veneer of gravel has accumulated on the sur-

-face. Wind polish on the gravels is normal. The dusts are transient, for loess does not accumulate. The

·dust may fall during a caIm, onIy to be moved on by the next winds. If it should rain in the meantime, any of the salts in the dust and a part of any carbonates are carried into the soil -bel ow the desert pavement. When the dust moves on, it may leave behind some carbonates and any more soluble salts. As a resuIt, carbonates seem almost always present in Aridisols, even in soils formed in parent materials that are extremely po or in calcium. The carbonates tend to accumulate in the horizons that

-mark the average depth of penetration of moisture. The climate of most present deserts has evidently not always

oeen so dry. Shorelines of lakes now dry, thick salt deposits, . fossil bones, and solution cavities in massive thick calcic hori­. zons all suggest periods of higher rainfall during the Pleistocene.

-48

Apparently the glacial periods have been more humid, and interglacial periods dry. Radiocarbon dates of organic matter in the old lake deposits in the U.S. are conformabie with the glacial advances. Many of present land surfaces appear to have been stabie through one or more interglacial stages. Soils on these surfaces may have strongly developed argillic horizons and thick cemented ca1cic horizons, of ten containing solution cavities. On fans, there may be a series of buried soils with argillic horizons suggesting, but not proving, that the argillic horizons formed in a more humid climate than that of today. Even though the climate is arid, eros ion and deposition can be rapid because there is too little vegetation to protect the soi!. Torrents may come from heavy rains in adjacent mountains and leave their deposits high on the desert fans or aprons. Thus, the youngest soils on the landscape may be high on the land­scape, or low, or anywhere. The soil pattern can be far more complicated than is generally realized, and the variety of soils great.

Orthids (4.1)

The Orthids are the Aridisols without an argillic horizon. In the U.S., at least, they have mostly developed since late Pleistocene time, perhaps in the last 25,000 years, mostly under an arid climate. The Orthids must have a cambic, calcic, gypsic, or salic horizon, or a duripan.

Calciorthids (4.13)

These are the Orthids that have a ca1cic or gypsic horizon. They mayor may not have a cambic horizon. We require that the upper boundary of the calcic horizon be within 1 meter of the surface on the assumption that any such horizon that is deeper is almost certain to be a buried horizon. These are perhaps the most common of the Orthids in the U.S. The lime may have come from the parent material or from dust. The lime in the ca1cic horizon may be soft and powdery or hang as pendants from gravel, and this is thought to be the earliest stage of formation of the Calciorthids. With continuing accumu­lation of lime, the pores are filled and the ca1cic horizon becomes plugged with lime and cemented. In still later stages the percolating water is stopped by the ca1cic horizon and the lime is deposited in the low pI aces as laminae until the surface is smoothed. This is the petrocalcic horizon, recognized as the basis for the Petroca1cic Calciorthids.

49

N onnally all horizons above the calcic horizon remain cal­careous below the upper 5 to 10 cm., but Galciorlhids develop­ed in sland may be free of carbonates to a somewhat greater depth.

In a few Calciorlhids the accumulation is primarily of calcium sulfate (gypsum) rather than calcium carbonate. These are distinguished as sulfatic families or, if sulfate contents are low, as series.

Camborthids (4.11)

The Camborthids are the Aridisols that have only a cambic horizon. These generally are the youngest of the Aridisols, though they may be somewhat older if they had few or no carbonates in the parent material and receive little calcareous dust. The cambic horizon is generally prismatic, with the tops of the prisms only 2 to 5 cm. below the surface. The prisms are weak and easily overlooked. U sually there is a ca horizon where lime has started to accumulate.

Durorthids (4.12)

The Durorthids are the Orthids that have a duripan. They are extensive in the U.S., perhaps because we have had a great deal of volcanic activity in the region or the parent materials are washed from ignimbrites, which are also extensive in our desert region and rich in glass.

In any event, the Durorthids are rich in glass wherever they occur. Most duripans contain some lime, and the duripans grade almost imperceptible into petrocalcic horizons.

Salorthids (4.14)

The Salorthids are the Orthids that have salic horizons. There is nonnally ground water at some season in these soils, and capillary rise and evaporation concentrate the salts, usually near the surface.

Argids (4.2)

We think that the Argids are older than the Orthids, and that the argillic horizon fonned during one or more of the glacial periods when rainfall was higher. In more recent times, petrocalcic horizons and duripans may have fonned in the argillic horizons, engulfing a part of them.

There is no good evidence yet that the argillic horizons are due to differential clay formation, though this was once postulat-

50

ed, and the idea gained rather wide acceptance. While much or most of the clay formed in place in the argillic horizons, the minerals that can form clay do not seem any more depleted in the argillic horizon than in the horizons above.

Durargids

These are the Argids with duripans. Like the Durorthids, they are rich in glass.

Haplargids

These are the Argids with an argillic horizon but not with a natric horizon. They mayalso have calcic or petrocalcic horizons. Accumulations of carbonates below or in the argillic horizon seem normal, and the presence or absence of a calcic horizon may depend only on the sources of carbonates. The calcic horizon therefore does not seem to have any particular significance by itself. The petrocalcic horizon, however, stops both water and roots and indicates that percolating water is collecting on top of the horizon. This is a somewhat different process, and is recognized by the petrocalcic subgroup.

The Haplargids must either have less than 35 per cent c1ay in all subhorizons of the argillic horizon or must not have an abrupt textural change between the A and B. There are Argids with gradual transitions from A to Band others with abrupt AB boundaries and an abrupt textural change. We wanted to dis­tinguish between these if the texture of some part of the argillic horizon is fine. The Haplargids probably represent soil genesis during and since Wisconsin time. High on the desert fans we may find a series of argillic horizons, one buried by another. Toward the base of the fans the deposits are thin, and two or more of the argillic horizons are fused. Here we think that the present soil represents genesis during two or more glacial periods and the intervening interglacial periods. Where we can trace the several argillic horizons to show that they have fused into a single massive horizon we of ten have a fine textured argillic horizon, an abrupt textural change between the A and Band rather slow permeability in the argillic horizon. This kind of soil we want to exclude from the Haplargids. Be­cause the A horizons are thin, plowing of ten mixes the A with part of the Band destroys the abrupt textural change. To keep from having to change the classification af ter plowing, we re­quired that the cultivated Haplargids have less than 10 per cent difference in clay between the plow layer and the argillic hori­zon. We were not so concerned about the loamy argillic horizons, so we permitted an abrupt textural change above them.

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Nadurargids (4.24)

These are the Argids that have both a natric horizon and a duripan. The reasons for including soils with natric horizons (8010netz) with the Aridisols follow under Natrargids.

N atrargids (4.2'3)

The N atrargids also have a natric horizon. In this respect they are like the other natric great groups, Natriborolls, Natrustolls, Natraquolls, Natralbolls, Natriboralfs, Natrustalfs and Natra­qualfs. These have traditionally been grouped as 8010netz and 80-lodized 8010netz on the properties of the natric horizon - col um­nar structure and high sodium. But the natric great groups have a host of other properties that vary widely. 80me are wet, gray and mottled. 80me, like the AlfisoIs, have acid ochric epipedons. Others have mollic epipedons, and resembIe the Mollisols ; some are like the Aquolls, some like the Borolls and some like the Argialbolls or the U stolIs. Others are like U stalfs. In general, the natric great groups are very much like the soils with which they are associated in most of their properties.

The N atrargids in most properties are more like the other Argids than they are like the Natraqualfs, for one example. The Natrargids like the other Argids are usually dry, have ochric epipedons with Iittle organic matter, usually have more organic matter in the B than in the A, have the same bare surface and desert pavement, have the s'ame soft vesicular crusts, have accumulations of Urne and mo~e solublle salts at depths, and so on. Not one of these properties is common to all nat;ric great groups.

80 we kept the Natrargids together with the other Argids at the order and suborder levels. You should note that, if for any purpose, one wants to group all soils with natric horizons, the system provides a means of doing so and provides a name for the group - natric great groups. There are many threads to soil genesis that, like the fabric of a cloth, cross each other. We may look from one direction, within an order, and see emphasis on one set of properties or threads. Or, we may look from another direction, as at the natric great groups, and see emphasis on a different thread. Or, from still another viewpoint, we can look at the aquic great groups and see some of the natric great groups, Natraqualfs and Natraquolls, but not the Natrar­gids. Depending on one's purpose, one may wish to emphasize different properties at different times, and the systematic

52

nomenc1ature provides names for important kinds of group­ings. (*)

Normargids (4.25)

These, as explained under Haplargids, are thought to re­present soil development through several glacial and inter­glacial periods. They are comparable in horizonation to the Albaqualfs, Argialbolls, and Normustalfs - soils formerly called Planosols and having sIowly permeable argillic horizons. All of these consist of soils with advanced stages of horizon develop­ment.

(*) These include Cryie, Agrie, N atrie, Fragie, Duric, Plaeie, Rhodie, Glossie, and Vermie great groups. They also inelude a number of subgroups sueh as arenie, eumulie, petroealcie, plinthie, and a larger number of families.

53

ORDER 5: MOLLISOLS

The Mollisols are soils in which we think there has been decomposition and accumulation of relatively large amounts of organic matter in the presence of calcium, producing calcium saturated or calcium rich forms of humus. This requires de­composition in the soH, not on the soil. Probably the basic difference between the Eutrochrepts and Mollisols is the differ­ence between decomposition of organic matter on and in the soil.

The Mollisols therefore must have high base saturation with abundant calcium. On the whoIe, this requirement tends to restrict the Mollisols to subhumid and semi-arid regions where the leaching of bases is slow or impossible, but where moisture is adequate for relatively large annual additions of organic matter. In arid regions too little organic matter is produced. Much of the surface is bare and the surface horizons become too hot for roots to survive. Grass is important to Mollisols because of its fibrous root system, but grass is not essential. In humid regions, under forest, calcium is rather quickly lost from the soil as .a general rule. Mollisols can form if the soH is rich in bases, and the soil fauna carries the leaf litter into the soil to decompose. Mostly, this requires calcium carbonate in some or all of the soil horizons. Calcareous soils in humid regions are generally transient. Either they are on young geo­morphic surfaces, or the parent materials are extremely high in carbonates.

Traditionally, soil science has tended to distinguish between the soils formed under grass and those formed under forest. The soils do differ as a general rule, but the difference between trees and grass is not a soil difference, nor does it represent a basic difference in plant composition. The leaves of trees that drop in the fall and the dry leaves of the dormant grasses are not strikingly different in composition. If both decompose in

54

the presence of lime, the residues need not differ at all. The organic residues are microbial, and the original plant materials have vanished.

80, we have emphasized in Mollisols the nature of the soil, not the vegetation that we think was once th ere. Pollen studies suggest that many Mollisols were forested during the glacial periods and we have no way to evaluate the effects these forests had on the soils, or even where they stood. The more recent forests have been cut in many of the are as of transition from forest to grass, and we have no way to be sure what the vegetation was on a particular spot. The forests generally were encroaching on the grasslands in the last few . thousand years, and if the forest is still present, we do not know the extent to which the soil developed under grass. We can be sure of the nature of the soil by studying it.

Perhaps I should emphasize again that many soils with mollie epipedons are not Mollisols. We permit mollic epipedons in Vertisols because we think that the properties of the mollic epipedon are less important than the physical properties of the clayey cracking V ertisols. We permit mollie epipedons in most suborders of Inceptisols. In the Andepts, we feel the glass and allophane are more important. In the other suborders, the base status of the cambic horizon is low, and the mollic epipedons are found only in cultivated and limed soils. Here we do not want to change the classification simply because the soil has been limed.

Mollic epipedons can also occur in Ultisols and Oxisols. Ulti­sols with mollic epipedons have been limed. Oxisols may have mollic epipedons in virgin soils, but we attach greater signific­ance to the oxic horizon.

Albolls (5.2)

These are Mollisols of flat places and high c10sed depressions. They have perched water tables and they may have fluctuating ground waters at depth, but the B horizons are commonly so fine that the concept of a water table is meaningless. Below the mollic epipedon there is an albic horizon, and below this an argillic or natric horizon. The albic and lower horizons have mottIes of high chroma where iron has segregated. The peds of the argillic and natric horizons of ten have mottled coatings with chromas of about 1 to 3, and mottled interiors. The B horizons are slowly or very slowly permeable. In Typic sub­groups there is an abrupt textural change between the A and B. The Albolls include some soils, of ten intergrades to Aquolls,

55

that are as strongly gleyed as some of the Aquolls. We have kept such soils apart from the Aquolls and grouped them with Albolls because all Albolls show some evidences of wetness, and the albic horizon is typical of the Albolls but not of the Aquolls. The albic horizon seems to develop af ter the argillic horizon has become slowly permeable enough to cause water to perch above it. This is not typical of Aquolls, even those with argillic horizons.

Argialbolls (5.21)

These are the Albolls with argillic horizons but without natric horizons.

Natralbolls (5.22)

These are the Albolls with natric horizons. It appears that in a few of those the sodium comes from the weathering of feld­spars, and accumulates because the permeability is so slow that there is less leaching than liberation of sodium. The typic Argialboll may be the precursor, or early stage, of the Natral­boJl.

Aquolls (5.3)

These are the Mollisols of wet places. They may be found associated with Mollisols, with some Alfisols and, at the margins of their distribution, with Spodosols and Aridisols. It was point­ed out in the discussion of Aquepts why the hydromorphic soils were not grouped in a single order. Af ter drainage, the Aquolls behave like other Mollisols. In the U.S. only a few have not been drained. These few are mostly Cryaquolls or approach Cryaquolls in their temperature regimes.

Argiaquolls (5.32)

These are the Aquolls that have argillic but not natric hori­zons. The water tables fluctuate.

Calciaquolls (5.33)

These are the Aquolls that have a calcic horizon with its upper boundary within 40 cm. of the surface. If the calcic horizon is within this depth, we think it is primarily the result of deposition of carbonates from rising capillary ground water. Usually the upper boundary is much shallower, and this leads to an unsolved problem in our definition of the class. If the surface has too much disseminated carbonate, the dry color becomes too light for a mollie epipedon. Soils that have com-

56

I I

. 1

parabIe textures and humus eontents, and that belong together in the classifieation, are separated if one of them has too mueh accumulation of earbonate in the surface horizon. Caleiaquolls are associated primarily with Udolls, Borolls, Ustolls and Aridi­sols.

Cryaquolls (5.36)

These are the eold Aquolls. Cryoturbation is aetive in these soils, and the mollie epipedon normally shows evidenees of mixing with the cambic horizon in the form of eurved tongues a few cm. wide and spaeed a few cm. apart in loamy soils, and larger deep tongues a few tens of cm. wide spaeed a meter or so apart in elay soils.

Duraquolls (5.34)

These are the Aquolls with duripans.

Haplaquolls (5.31)

These are the Aquolls with only a mollie epipedon and a eambie horizon. The thiekness of the mollie epipedon is generally related to the depth of rooting of the native vegetation. On broad flats, where deposition of recent sediments is negligible and wh ere there has been little or no mixing by animaIs, the mollie epipedon reaehes a maximum thickness of about 60 cm. Adjacent to slopes where slow accumulation of materials is not only possible but probable, the thiekness of the mollie epipedon may reaeh a meter or mueh more. This overthiekened epipedon is used as the basis for defining subgroups of Cumulic Mollisols. The mollie epipedons of Cumulie Mollisols should have a gradual or diffuse lower boundary without the evidenees of mixing by animals that we find in Vermie great groups.

Natraquolls (5.35)

These are the Aquolls with natrie horizons. N ote that these should not have an albic horizon above the natric horizon.

Borolls (5.4)

The Borolls are the cool or eold Mollisols that laek evidenees of strong gleying and that have low ehromas in at least the upper part of the mollie epipedon. They were reeognized as a suborder rather than as Cryie great groups in oh ter suborders partly because the soil morphology does not fit the eryie limits ; it extends into somewhat warmer climates. In the region of the Borolls, all of the Mollisols tend to have a blaek upper epipedon, not just the Aquolls. The blaekness is due in part to humus,

57

and in part to the absence of coatings of free iron that normally impart brown hues to soils. In this respect, the Borolls and Boralfs are similar, but the mechanism of iron removal is not known. As the climate becomes either warmer or drier the epipedons take on brownish hues if the soils are not aquic. Removal of the organic matter by H20 2 leaves gray materials with Borolls and brown colored materials with Ustolls and Udolls.

The Borolls, then, are cool or cold, and the climate may be humid or subhumid. Ustolls replace Borolls as the climate becomes drier. Eventually, an additional great group will prob­ably be needed for the Borolls of humid climates. These occur but are very rare in the U.S.

Argiborolls (5.43)

These are the Borolls with argillic horizons. We think they are on older land surfaces than the Haploborolls or were forested.

H aploborolls (5.42)

These are the Borolls without argillic or natric horizons. The mollie epipedon is sometimes thickened by slow deposition of wash, but not by animaIs.

N atriborolls (5.45)

These are Borolls with natric horizons. In these, the salts were probably in the parent materiaIs. The salts may have been concentrated in some by capillary rise from ground water. Certainly they did not come from weathering of feldspars.

Vermiborolls (5.41)

These are the Borolls with epipedons overthickened by ani­mals, chiefly earthworms, assisted by burrowing mammals. They are not known in the U.S.

Rendolls (5.1)

These are Mollisols, chiefly of humid regions, formed in very highly calcareous materiaIs. Many Mollisols in subhumid climates have hard limestone at shallow depths. These mostly would be Mollisols even in the absence of limestone, so we have said Rendolls should not have a lithic contact at the base of the mollie epipedon. Likewise, we have said they cannot have the calcic horizons typical of calcareous soils of subhumid and arid climates.

We have changed the definition of Rendolls slightly, so that

58

they may have mollic epipedons as much as 50 cm. thick in the absence of gravel. The former limits in thickness, from 30 to 50 cm., depending on gravel, have been transferred to the Typic subgroup.

Udolls (5.5)

These are Mollisols of humid temperate climates with summer rains, fOrnled mainly on recent or late Pleistocene surfaces from calcareous or base rich materials in the absence of shallow ground water. There are occasional years, in some, when the soil retains a dry horizon throughout the winter. But there are frequent years when there is an excess of precipitation over evapotranspiration, and some water moves through the soil to the ground water. Soluble salts are more or less completely removed every year or so, and conductivities are very low, less than 1 millimho, in all horizons.

The present definition is not satisfactory. It is very com­plicated and difficult to understand to say the least. The troubles are due to a few Ustolls that have a shallow lithic contact. In these there may be no accumulation of salt or carbonate, but in all other respects they are like the deeper Ustolls with which they are associated. Some are very dry, almost Aridisols, and certainly should not be grouped with the Udolls. A simple defi­nition would be possible were it not for these problem soils.

The Udolls of regions of summer rain were separated from soils with similar appearance in regions of winter rains - the Xerolls. There were several reasons for this split of soils that were grouped in the 7th Approximation.

The agriculture, of course, is very different. Irrigation is needed for summer crops in Xerolls because they become almost completely dry nearly every summer. One cannot say that soils are the same if moisture regimes vary so widely. The presence of moisture during the growing season seems to lead to rather rapid and complete decomposition of plant residues. The CjN ratios of Udolls under cultivation are normally about 10 to 12 in the plow layer, and decrease with depth. In Ustolls, without irrigation, the soil is too dry when it is warm and too cold when it is moist for rapid biologic decomposition. The CjN ratios range from about 14 to 17 or higher in the plow layers of these soils and decrease with depth less rapidly than in Udolls. It was the difference in the biologic cycle that convinced us of the need for the distinction between Udolls and Xerolls. The present . distinction works weIl in the U.S., but it will doubtless give troubles somewhere in the world where rainfall is erratic.

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L

Argiudolls (5.53)

These are the Udolls with argillic horizons. They occupy late Pleistocene or older surfaces ; many had, at one time, a conifer vegetation according to pollen analyses, and may have been under grass for only 5,000 to 6,000 years.

H apludolls (5.52)

These are the Udolls without an argillic horizon, and without an epipedon overthickened by animaIs. Cumulic subgroups are recognized if the epipedon is thicker than 50 cm. The normal epipedon of the Hapludoll is thinner than that of Aquolls where we used a 60 cm. limit. The Hapludolls are mostlyon late Pleis­tocene or post-Pleistocene surfaces.

Vermudolls (5.51)

These are the Udolls with epipedons overthickened by animal mixing. They are not known in the U.S.

UstoHs (5.6)

These are the Mollisols of subhumid and semi-arid climates. Leaching is normally inadequate to remove carbonates. More soluble salts of ten accumuIate beI ow the carbonates. The very shallow soils on rock discussed under Udolls are the principal exception, though Ustolls formed in non-calcareous materials may have no zone of lime accumulation.

We have yet found no satisfactory way to separate the Ustolls that intergrade to AridisoIs from the Typic, or even from the Ustolls that intergrade to Udolls. The present defini­tion of the Aridic subgroups works weIl with the cooler soils, but does not seem to work with m'any others.

A rgiustolls (5.63)

These are the Ustolls with argillic horizons but without natric horizons. Being warmer, the Iandscapes of the U stolIs were less commonly disturbed during Wisconsin (Würm) time, and the older stabIe surfaces where we most of ten find argillic horizons are more common. Argiustolls are relatively extensive compared to Argiborolls.

This great group appears at the moment to be inconsistent with the classifications of Argids and UstaIfs. In those sub­orders we have normic great groups that are intended for soils of very considerabIe age and with strongly contrasting textures in the A and B. In Argiustolls this distinction is treated as a

60

subgroup difference, and there is no rational reason for not having comparable classifications. The difference in treatment reflects the views of different groups of our staff, one working with Ustolls, and the other with Argids and Ustalfs. It seems likely that this great group will be split, and the Albic Argi­ustolls put in a different great group, comparabie to Norm­argids.

Calciustolls (5.65)

These are the Ustolls that have a calcic horizon, but no argillic or natric horizon, and no duripan. They are comparabie in genesis to Caiciorthids except for having a mollie epipedon. Because the calcic horizon represents precipitation from soH moisture that moved down through the soH rather then up from ground water, the calcic horizon is of ten deeper in Calci­ustolls than in Calciaquolls. The limit is 1 meter in the former as against 40 cm. in the latter.

Durustolls (5.64)

The Durustolls are the Ustolls with a duripan, but without a natric horizon. The soils of the Typic subgroup have an argillic horizon above the duripan. Like other duric great groups, glass is common in these soils.

Haplustolls (5.62)

These are the Ustolls that have a cambic horizon instead of an argillic or natric horizon, and that have no calcic horizon unless carbonates have been leached from more than the upper 17.5 cm. Thils oepth limit is set to prev'ent a change in classifioa­tion of the soH by plowing.

Natrustolls (5.66)

These are the U stolls with a natric horizon. The genesis of the natric horizon is comparabie to that of the Natriborolls.

Vermustolls (5.61)

These are U stolls deeply mixed by animaIs. They are unknown in the U.S. but common in Europe.

Xerolls (5.7)

The reasons for establishing this suborder were given in the discussion of Udolls, wh ere the 7th Approximation placed these soils. They occur in regions of winter rain. The great groups are nearly comparable to those of Udolls.

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Argixerolls (5.72)

These are Xerolls, with argillic horizons.

Durixerolls (5.73)

These are Xerolls with a duripan.

Haploxerolls (5.71)

These are Xerolls without a duripan or an argillic horizon.

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ORDER 6: SPODOSOLS

This order originates in the western European (not Russian) concept of Podzols. This concept of Podzols was that of a soiI with a mor, an albic horizon and a spodic horizon. Mattson and associates pointed out that the clays of the B horizons of Pod­zols were largely amorphous j ust at the time that the general crystalline nature of soil clays was being discovered. The import­ance of his studies was not fully understood, at least by many in the U.S. The Soviets relate podzolization to the formation of the albic horizon. Traditionally they have not been concerned with the nature of the illuvial materiaIs. We think that the amorphous nature of the illuvial materials indicates a difference in kind of soiI genesis, one important enough for the definition of an order.

The illuvial materials are at least partly organic, but to me the key element is most apt to be aluminum. Iron seems to be largely accidental, and probably does not contribute to the cation exchange phenomena of the spodic horizon although it may be an important cementing agent in some ortsteins. Certainly the organic fraction of the spodic horizon that con­tributes to the high cec is not related to the conventional "fulvic" acids. If we heat or extract with dithionite a mollic or umbric epipedon that contains a mixture of "humic" and "fulvic" acids, the cec is not affected. The concepts of these acids, that are at least partly laboratory artifacts, have not been useful to us in soil classification.

We have not emphasized the albic horizon for two reasons. First it may be thin, and is of ten incorporated with the spodic horizon by plowing, pasturing, or by earthworms. Second, the albic horizon is related to the absence of coatings of free iron. This can be either the result of soil genesis or the accident of the parent material. We have many white quartz sand dunes forming today along the coast of Florida. These may be 50 cm.

63

to a meter or two thick, and if they rest on older brown sands they could be considered thick albic horizons, and strongly Podzolized soils in the Soviet sense. To us they are Quartzi­psamments, not Spodosols.

Accumulations of free iron oxides are found in many soils and are not restricted to Spo do sols (or Podzols). They are normal in argillic horizons in non-aquic great groups. The free iron accumulations in these horizons are of ten proportional to the accumulation of c1ay. Iron may be accumulated even more strongly in the laminar textural horizons of sandy soils. It can accumulate in an erratic manner in aquic soils, and in massive sheets at contacts between deposits of different texture. Cer­tainly accumulations of iron can be found in many kinds of soi!.

So, the definition of Spo do sols breaks with tradition on two counts - it reduces emphasis on albic horizons and on the importance of the movement of iron. In their place it substitutes the concept of the spodic horizon with its amorphous illuvial materials whose exchange capacities are largely destroyed by heating and by dithionite extraction.

Aquods (6.1)

The Aquods are wet soils but the hydromorphic features are less evident than in other aquic suborders. Commonly, free iron is present in such small amounts that mottIes or the blue hues of ferrous iron are absent or inconspicuous. The brown and reddish brown colors of the humus in the spodic horizon tend to mask other colors and to mimic the colors of free iron. The pres en ce of a transition horizon between the albic horizon and the spodic horizon is generally the most distinctive hydromorphic feature. The other evidences used have been the presence of an indurated albic horizon or a thin iron pan.

The Aquods range from the arctic regions to the equator. They are confined to humid regions, although th ere may be pronounced dry seasons.

Vegetation is quite variabie, ranging from palms and grass to heath and coniferous forests.

Cryaquods (6.11) These are the cold Aquods without a thin iron pan. They have

been studied very little. Cryoturbation is active, and Ruptic subgroups of various kinds are probable. Iron of ten seems to be present in small amounts in the spodic horizons.

Duraquods (6.16) These are the Aquods that have a strongly cemented albic

64

horizon. The cement seems to be chalcedony instead of opal, and it is quite possible that the cemented horizon should not have been called a duripan. Duraquods are not known in the U.S. but are known in New Zealand and Australia.

Fragiaquods (6.17)

These are the Aquods that thave a fragipan below the spodic horizon, but no thin ironpan. They are rare in the U.S.

Normaquods (6.12)

These are Aquods that have a spodic horizon enriched pri­marily with humus and aluminum. Free iron is virlually lacking in the spodic horizon (formerly Humaquods).

Placaquods (6.14)

The Placaquods have a thin involute iron pan that we will probably add to the diagnostic horizons. It seems to occur in a varietyof soils, always in perhumid climates with no dry season. We find the iron pan in Oxisols or Inceptisols ; we still lack data to be sure which, though both seem probable. It does not seem reasonable to include the tropical bauxitic clays of Hawaii with the cold skeletal and light loamy siliceous soils of Southern Alaska and Wales. So, we think it probably best to treat the thin iron pan like the fragipan as a diagnostic horizon, aplacic horizon, and use its presence to define a number of great groups. lts genesis is obscure.

In the Placaquods, the iron pan may be in the albic horizon, in the spodic horizon, or lie on top of a fragipan.

Sideraquods (6.13)

These are the Aquods, warmer than Cryaquods, that have appreciabIe amounts of free iron in the spodic horizon. They are rare in the U.S. and possibly in the world.

Tropaquods (6.15) (formerly Thermaquods)

These are the Aquods of the tropics. The 'albic horizons may be extremely thick, at least up to 5 meters, over the spodic horizons. Free iron is normally absent in the spodic horizon, but an argillic horizon with plinthite of ten underlies the spodic horizon if it is a meter or less in thickness. Some arbitrary limit in the thickness of the albic horizon is needed if the classi­fication is to be used for soil surveys. Otherwise, it is impossible to distinguish between a Quartzipsamment or Aquipsamment and a Tropaquod without special drilling equipment. We have proposed 2 meters as a practical limit.

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Ferrods (6.4)

This suborder has been provided, but it is not known in the U.S. Humus accumulations are not important; accumulations of iron are considerable ; amounts of accumulafion of aluminum are unknown. Base exchange capacities are extremely high for the measured clay contents reported in the literature. We feel confident that these exchange capacities, as in other spodic horizons, can be destroyed by gentIe heating or dithionite extraction, but we do not know that as a facto

Humods (6.2)

This suborder also is unknown in the U.S. Humus has accumulated, iron is virtually absent, and aluminum has not of ten been studied.

Orthods (6.3)

These are the Spodosols in which iron, humus, and aluminum have accumulated. Because there are striking contrasts between the albic horizon and the spodic horizon, these soils have receiv­ed a great deal of attention. We have proposed four great groups.

Cryorthods (6.31)

The cold Orlhods.

Fragiorthods (6.34)

The Orthods with fragipans.

N ormorthods (6.33)'

The Orthods without fragipans, usually sandy or light loamy, and too warm for Cryorthods.

Placorthods (6.32)

The Orthods with aplacic (thin iron pan) horizon.

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ORDER 7: ALFISOLS

The concept of Alfisols is centered on a group of soils that are usually moist, have ochric epipedons, and have argillic horizons with medium or high base status. While there are soils included with Alfisols that may lack any one of these charac­teristics, the concept of the group is easiest to grasp if we consider the exceptions later.

The Alfisols generally have a period when evapotranspiration exceeds precipitation and one or more horizons drop weIl below field capacity or reach wilting point. This is the normal mois­ture regime of soUs with argillic horizons. In addition, movement and accumulation of clay have gone on more rapidly than truncation byerosion, so the geomorphic surfaces have had some stability. At least eluviation and illuviation of clay have been more rapid than erosion. Water movement through the solum has been adequate to remove free carbonates from the fine earth in the epipedon and from most of the argillic horizon, but inadequate to remove a substantial part of the exchangeable bases held by the soiI. The rate of liberation of bases by weather­ing or additions of bases by wind or water approach the capacity· of leaching to remove bases. If this were not so, the base satur­ation would be sm all , particularly in the lower part of the argillic horizon where removals both by percolating water and by plant roots are active processes.

Similar conditions with respect to loss of bases are typical of Mollisols with argillic horizons, which we may consider to be parallel to Alfisols and to differ primarily in having a mollic epipedon. . •

The Alfisols and Argids differ primarily in moisture regimes and the salt and lime accumulations in the Argids that are the result of deficient rainfall or natural irrigation. In areas where the Alfisols have a common boundary with Aridisols, the mois­ture regime, which is difficult to determine precisely, can not be

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used objectively. For this transition we have used the nature of the epipedon as one differentia that can be applied in the field. Since the soils are rarely cultivated, the surface horizons have rarely been removed byerosion. The eluvial epipedons of many Alfisols and Ultisols tend to become massive and hard when dry, while the epipedons of Argids tend to be soft when dry. By such devices we have tried to develop a system that can be used in the field, even at the expense of simple defin­itions.

The limit between Alfisols and Ultisols has also been trouble­some to define, and is discussed later.

Transitions to Spodosols are bisequal so far as we know now. A thin Spo dos ol forms in the epipedon of the Alfisol. Over time, the Spodosol becomes thicker, and the argillic horizon of the Alfisol is moved deeper in the soil and probably is finally destroyed. So long as the solum of the Spodosol is so thin that it is destroyed by the first plowing, we classify the soils as AlfisoIs. Otherwise the classification of the soil would be changed by plowing and I have already discussed our reasons for keeping virgin and cultivated soils together in the classifi­cation. If the spodie horizon extends below normal plow depth we classify the soils with the Spodosols. Some evidence is accumulating that the amorphous materials diagnostic of spodie horizons remain for a long time in the plow layers, and it may be possible to recognize the cultivated Alfisols that formerly had very thin spo die horizons.

The traditional Russian emphasis on the importance of gray colors of the epipedon has been reduced in this classification. The colder Alfisols do have gray or light gray epipedons, and in the Russian classification some would have to be considered strongly podzolized. Total analyses show accumulations of ses­quioxides in the B horizons, but this is the re sult of accumu­lations of layer lattice clays. Exchange capacities are unaffected by heat or increase instead of decreasing. Free iron is admittedly low in the albic horizons relative to the clay fraction, but it seems to be equally low in the argillic horizons. These soils appear to lose free iron throughout the solurn, just as do some of the Borolls. The reasons are obscure ; chelation may weIl be the means. The process is closely related to temperature and to snow cover, but the precise role of temperature is not established.

Aqualfs (7.1)

These are the wet AlfisoIs. The water table fluctuates in these soils. It is very shallow during some seasons, and deep in others.

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The argillic horizon would be difficult to understand if the water table were always shallow. The gray mottled colors could be the effect of a perched water table, and this is sometimes present in the Albaqualfs and Fragiaqualfs. However, our studies even in these soils show that at some season the ground water is shallow. We have been unable to find a distinction in the water regimes between the so called Pseudogley and the Gley except that in a few gleyed soils, chiefly Hydraquepts, the ground water level remains high in all seasons. If there is appreciable organic matter in the reduced horizons, and the ground water is more or less permanent, the soils have blue or green hues. In soils with fluctuating ground water, reduction of iron decreases with depth in the soil as the organic matter decreases. The reduced iron is either removed from the horizons or concen­trated in mottIes and concretions. The reduction process is the same - biologic reduction of the iron oxides to obtain oxygen.

If the "gley soils" are drained, the color changes, and any blue or green hues and some grays vanish, except at depths generally below the soil. We want to avoid as much as we can changes in the classification as a re sult of drainage. It is the same problem that we have with plowing or liming. Changes in the classification as a result of drainage would make it diffi­cult to use the experience from drained field to predict the results of new drainage works. The pedologist himself should not be greatly bothered, but others who use soil maps will think that the soils are different if the names differ. 80, we use soil drainage works as a basis for phases.

Albaqualfs (7.11)

These are the soils with fine textured sIowly or very sIowly permeable argillic horizons. The upper boundary of the argillic horizon is abrupt, and smooth or wavy. These soils are most common in subhumid climates in which the ground water fluctuates strongIy. As in the Argialbolls, permeability is of ten so slow that we think sodium, liberated by weathering of feld­spars, accumulates in the lower horizons.

Fragiaqualfs (7.15)

These are the Aqualfs with fragipans. N ormally the fragipan is below the argillic horizon. In a few soils, the fragipan may be in the argillic horizon. In some Fragiaqualfs there is no horizon that meets all of the requirements of an argillic horizon. The soil individuals generally are very smalI, only a few pedons, associated with soils with distinct argillic horizons. The soils are similar except for the rather rapid or diffuse increases in

69

clay with depth that meet or fail to meet requirements for argillic horizons. To keep them together, we have grouped in the Fragiaqualfs the soils that have clay skins thicker than 1 mmo in the fragipan. This is one exception to the rule that Alfisols have argillic horizons.

Glossaqualfs (7.12)

These are the Aqualfs in which the albic horizon tongues into the argillic horizon in a manner that commonly suggests that the argillic horizon is being destroyed. These are generally in quite humid climates, without pronounced dry seasons.

Natraqwilfs (7.16)

These are the Aqualfs with natric horizons. Geographically, they are associated with Albaqualfs.

Ochraqualfs (7.13)

These are the Aqualfs with an ochric epipedon, and with a transitional horizon between the albic horizon · and the argillic horizon. Typically the argillic horizons are more permeable than those of Albaqualfs, and there is no seasonal perched water tabIe.

Tropaqualfs (7.17)

This great grOUp has been added tentatively for the tropical Aqualfs. The only ones we knoware comparable to the Ochr­aqualfs in physical features.

Umbraqualfs (7.14)

These are wet soils with an umbric epipedon and an argillic horizon with high or medium base status. They are the main exception to the general rule that Alfisols have ochric epipedons. They are rare in the U.S. We put them with Alfisols because they resembIe the Aqualfs in all features except the nature of the epipedon.

Boralfs (7.2)

The Boralfs are the AlfisoIs, exclusive of Aqualfs, that have an albic horizon unless the plow layer rests directly on the argillic horizon, and that show distinct evidences of destruction of the argillic horizon in the form of a tongued and broken upper boundary. All cryic Alfisols other than Aqualfs were included in this suborder. The few we know have tongued argillic horizons. We would not want to separate cold soils at

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the suborder level if some should be found without a tongued argillic horizon. Actually, we do not know that this is a real problem, but the definition needs explanation. The Boralfs are primarily in cool or cold climates, humid to subhumid. In the northern hemisphere they are associated with Borolls in sub­humid regions. They are occasionally associated with Spodosols, where the Boralfs are found in fine textured parent materials and Spodosols in medium and coarse textured materiaIs.

Generally they are colder than Udalfs. The transition zone between Udalfs and Boralfs is quite broad - a few hundred kilometers. This very gradual transition makes it difficult to apply the definitions ; we would surely like to find some better differentiae.

Cryoboralfs (7.21)

Our experience with these is very limited. We think they are extensive in the Soviet Union. · The few we knoware at high elevations in the western mountains of the U.S.

Eutroboralfs (7.22)

These are the Boralfs of subhumid regions, bordering Molli­sols in the U.S. There is a short period during the growing season when some horizons are dry, and base saturation is high. There may be accumulations of secondary lime below the argillic horizon.

Fragiboralfs (7.24)

These are the Boralfs with fragipans, mostly found in cool humid regions. They are rare in the U.S. and we have not attempted to write definitions of subgroups.

Glossoboralfs (7.25)

These are the Boralfs of cool humid regions. Typically the lower part of the albic horizon has low or very low base satur­ation ; so does the upper part of the argillic horizon. We did not call these Dystroboralfs because we doubt that they will all have low base status in some horizons, and the base status generally is much better than that of the Dystrochrepts and other Dystric great groups. Glossoboralfs are rare in the U.S.

Natriboralfs (7.23)

This great group is provided for Boralfs with a natric horizon. We have not found them in the U.S. but the Canadian pedolo­gists report them. We do not propose to try to develop the subgroup definitions of this great group.

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Udalfs (7.3)

, These are the Alfisols of humid climates with short dry seasons or periods when evapotranspiration exceeds precipita­tion. The distinction between Udalfs and Dstalfs is one that would be very difficult to apply in the field in a transitional area. In the U.S. the Udalfs and Ustalfs are separated geo­graphically for the most part and the distinction does not give us trouble. An additional differentia that can be applied in the field to transitional soils is needed, but _ we ar~ not in good position to- suggest what it should beo The problems of limits with Boralfs have already been mentioned.

Problems of the boundary with Ultisols have been exceedingly difficult and are not entirely solved. The definition of the 7th Approximation was in terms of base saturation which is difficult to estimate precisely in the field. There is a limit to the amount of laboratory work that one can afford and the data are of ten not available when they are wanted.

The typical Udults have chromas of 6 to 8 in the argillic horizons, and the typical U dalfs have chromas of 4. The chroma would make the desired distinction except that the aquic Udults (moderately weIl and somewhat poorly drained soils) have l,ower chromas, chromas that match those of the Udalfs. So, we decided to use the chroma where we cotild. This is why the definition of Alfisols excludes soils with chromas of 6 or more if the soil is never dry in any horizon, or is dry for only short periods. (The Ustalfs, dry for long periods, of ten have chromas of 6). This solved the major part of the problem, because Ultisols with chromas of 3 or 4 in the argillic horizon are not extensive. For these we retained base 'saturation as a differen­ti'a but modified the differenti'a of the 7th Approximation. We kept the Emit of 35 per cent s'aturation, but set i.t on the horizon at a depth of 125 cm. below the top of the argillic horizon. We used this limit because it kept together virgin soils and their cultivated and limed counterparls separated by a fence. Some of our farmers have traditionally used burned Iime (CaO) in small amounts, and over periods of more than 100 years. This lime has moved down, particularly into the sandy soils, to considerable depths.

Normally the most consistent difference in base saturation between Alfisols and Ultisols is rather deep. Leaching is only a partial cause. In UltisoIs, the bulk of the bases have been cycled through the plants and the roots may go deep into the soil for them. So, in the deeper horizons, leaching and removal

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of bases by the forests accentuate the differences between Alfi-· sols and Ultisols.

The revised definition has had the effect of classifying the intergrades between Alfisols and Ultisols as Ultisols. In the 7th. Approximation they were placed in AlfisoIs. At this stage, we· simply don't know enough to define intergrades in both orders ..

Agrudalfs (7.'30)

Thes'e are Udalfs with an agric horizon and possibly an anthropic epipedon. They are not known in the U.S. and we do~ not intend to try to develop their classification.

Ferrudalfs (7.37)

The Ferrudalfs probably occur in the U.S. but have not been: recognized. These have tongued or brok en argillic horizons in which there has been distinct segregation of iron along the tongues or the edges of the argillic horizon. They are kept out: of Boralfs by the absence of an albic horizon.

Fragiudalfs (7.33)

These are the Udalfs with a fragipan in or below the argillic' horizon. The position of the fragipan relative to the argillic · horizon is used to define subgroups. The Typic subgroup has an. argillic horizon above the fragipan.

Natrudalfs (7.36)

These are the Udalfs with a natric horizon. These are rare· in the U.S. They occur mostly associated with Albaqualfs and" N atraqualfs· but lack the evidences of wetness required oÎ Aqualfs.

Normudalfs (7.32)

These are the Udalfs that have no fragipan, but have an' argillic horizon with a smooth or wavy upper boundary. Mostly­these soils are found on late Pleistocene surfaces.

Ustalfs (7.4)

These are the Alfisols of climates that have pronounced warm dry seasons. Rainy seasons may be warm, or cool with occasionaf frost, but the soils rarely freeze. They are dominantly but not· exclusively subtropical and tropical. They are extensive in the· southwestern U.S., the Mediterranean area, Africa, southern and western Australia, central Chile, and northeastern Brazil.

In the warmer regions Pleistocene events caused cyclical

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changes in rainfall. Associ·ated with these were cycles of erosion and deposition, just as in today's desem. Some sur­faces, however, appear to have been stable for long periods, and the soils in these have probably had one or more periods of higher rainfall. As in Argids, one ean allso find a series of buried soids on the long f.ans and aprons, merging at the base into a single profile that has a very strongly developed argillic horizon. Typically, the soils have reddish brown or even red hues. Chromas of the argillic horizons are intermediate as a rule between those of Udalfs and Udults, mostly ranging from 4 to 6. For this reason we could not use chroma to distinguish between Ustalfs and Ultisols, so we have been forced to continue to use base saturation. The distinction between Ustalfs and Ar­gids, as mentioned earlier, is largelyon the nature of the epiped­on. In Ustalfs, transitional to Argids, the epipedon is normally hard and massive when dry.

Durustalfs (7.41)

These are the Ustalfs with duripans. Like other soils with duripans, glass is commonly abundant in the soil above the pan. While we have no evidence that all Durustalfs contain glass, they are all located in regions of present volcanic activity, or are developed from alluvium coming from deposits of tuff and ignimbrites.

Haplustalfs (7.46)

The Haplustalfs are thought to represent the Ustalfs formed on late Pleistocene surfaces. They generally have 2:1 lattice clays as the dominant clay fraction in the argillic horizon. They have no abrupt textural change between the A and the B or lack fine-textured argillic horizons. Our intent here was parallel to our intent in defining the Haplargids. The argillic horizons lack dark red colors. In excluding the dark red colors we were influenced in part by the traditional use of col or, and in part by the lack of hard massive epipedons in the soils with dark red argillic horizons.

Natrustalfs (7.42)

These are the Ustalfs with natric ·horizons. Their general similarity to the other U stalfs may be appreciated if I note that up to 1959 in Australia, the Red Brown Earths, which are included in Ustalfs, were required to have a natric horizon in some States, and were not permitted to have one in other States. The soils are similar enough that the difference in concepts had not been noticed.

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Normustalfs (7.45)

This great group, comparable to the N ormargids in many respects, is probably largely restricted to soils that have been forming through several glacial and interglacial periods. The explanation of the definition was given for N ormargids and does not need repetition.

Rhodustalfs (7.43)

These are the Ustalfs with dark red argillic horizons and with a dominance of 2: 1 lattice clays. They are common on the basic igneous rocks and on hard limestones that are low in :Mg. These are rare in the U.S. The epipedons are gene rally either absent (as in the Mediterranean region) or granular (as in Australia) .

Vetustalfs (7.44)

These are probably the oldest of the Ustalfs. The clays mostIy have al: 1 lattice, suggesting that the soils have once had much higher rainfall and lower base saturation than they have today. These also are rare in the U.S., but seem more common in the tropics.

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ORDER 8: UL TISOLS

The Ultisols are the soils that have argillic horizons, and that have low base status in or below the argillic horizon. They are primarily soils of regions with high rainfall relative to evapo­transpiration in one or more seasons, but with some season of at least limited moisture deficiency. There is excess water virtually every year for leaching. The Ultisols occur from the cool temperate to subtropical and tropical regions. The balance between liberation of bases by weathering and removal by leaching is normally such that a permanent agriculture is im­possible without fertilizers. Either shifting cultivation or fer­tilizers are used. A few Ultisols transitional to AlfisoIs, partic­ularly on basic rocks, have a moderately low base status. Others, of course, have been limed or fertilized and are highly produc­tive.

The definition in the 7th Approximation stressed that the Ultisols either had base saturation of less than 35 per cent in the argillic horizon or base saturation that decreased with depth below the argillic horizon. This was modified to a limit of 35 per cent saturation at a depth of 125 cm. below the top of the argillic horizon or 75 cm. below the top of any fragipan. The purpose was to keep some limed and cultivated soils together with their virgin counterparts.

The concept of Ultisols is one of soils in which vegetation plays a major role in the maintenance of the bases against leaching. The roots of trees go several meters deep in many soils, and the bases they extract at these depths are eventually returned to the surface of the soil. Before the bases can be moved very deeply into the soil, they are again taken up by roots. Thus, the bases are held against leaching primarily by the plants. The supply is partly a function of the species of plants. Some collect large amounts of bases, and the soil below one of these may be weIl supplied during the mature life of the

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plant. But the maintenance of bases in the surface horizons is at the expense of the supply in the deeper horizons.

When the soils are cultivated and limed, the base status of the argillic horizon is eventually raised but changes in the deepest horizons are very slow.

The age of the geomorphic surfaces of the Ultisols varies enormously. Some are post-Pleistocene and rich in weatherable minerals ; others probably are pre-Pleistocene with few weather­able mineraIs. The classification as it stands does not reflect this difference except at the family and series levels. It does not reflect the differences in age satisfactorily. Consequently, I expect a number of changes in the classification of UltisoIs, in the years to come, several probably at the great group level to reflect the morphologic differences associated with age.

We of ten find evidences of destruction of the argillic horizon in the form of a broken or irregular upper boundary. Our soil­geomorphology studies suggest that with time the A horizon becomes thicker at the expense of the argillic horizon. We have noted in soils with thick A horizons that clay skins are almost completely absent in the upper part of the argillic horizon. They may be thick and abundant in these soils at depths of 2 to 3 meters. On the oldest stabIe surfaces we know in the U.S., probably pre-Pleistocene, the A horizons vary in thickness with the texture of the argillic horizon, but range from 75 cm. to over a meter. Small amounts of plinthite are found in these soils in the lower A and through the B.

In contrast, on young (late Pleistocene?) surfaces cut into these old surfaces, the A horizons are from about 25 to 40 cm. thick in fine loamy materiaIs. Clay skins are abundant in the upper part of the argillic horizon, but are scarce below 75 cm. Plinthite is present only if inherited from the old parent materi­aIs, and then it is present in all horizons. If the young surface has cut into fresh materials that are below the old soil, plinthite is absent. Weatherable minerals vary both with the age of the soil and the parent materiaIs. They are few in soils of the old surfaces, and · in soils formed from highly siliceous parent materials - sandstones and quartzites. They are abundant in soils on young surfaces if the parent materials supplied them.

In the tropics UItisoIs seem to be replaced by Oxisols on the oldest stabIe surfaces.

It should be noted that Ultisols normally have rather large amounts of KCI extractable aluminum. They have a rather wide variety of clays. In the U.S. most have appreciable amounts of vermiculite or chlorite in addition to kaolin. If the parent

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materials contained montmorillonite or were unconsolidated basic materiaIs, montmorillonite is common.

Cation exchange capacities of Ultisols in the U.S. have a mode at about 40 meq.j100 grams of clay. A very few are below 20, and a few are over 70. Kaolin seems most common in the tropics. The intergrades to Oxisols have only small amounts of 2:1 lattice clays, and these mostly in surface horizons and deep horizons, and have cation exchange capacities of less than about 20 meq.j100 grams of clay in the argillic horizon.

Aquults (8.1.)

These are the wet UltisoIs. As in Aqualfs, the water table fluctuates. In the U.S. they are largely restricted to the lower coastal plain and to river terraces. Base status generally is extremely low.

Fragiaquults (8.14)

These are the Aquults with fragipans. On the coastal plain in the U.S. they seem to be restricted to the middle terraces, on surfaces that have been stabIe since perhaps mid-Pleistocene time. The fragipans underlie the argillic horizon. In late fall, af ter plants are dormant, water is perched on the fragipan. By spring the ground water has risen to or very near to the surface of the soil if it has not been drained. The fragipans in this great group are deeper in places than is normal for the horizon. We have found a few with upper boundaries more than a meter below the surface of the soi!. Until we have studied these soils more the subgroup definitions will have to be con­sidered tentative.

Ochraquults (8.12) The Aquults without fragipans and with an ochric epipedon

are placed here.

Plinthaquults (8.11)

These are Aquults with plinthite in appreciable amounts within shallow depths. They were defined in the 7th Approxima­tion and listed in the table of names in the June '64 supplement, but not defined. The soils are being reexamined to determine their best placement.

It has been proposed that the plinthite is more important than the argillic horizon, and that the soils we called Plinth­aquults should be grouped with the Plinthaquox. In the U.S. we have so few that we have no basis for a decision. The present status of the great group is uncertain.

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Tropaquults (8.15)

This is a new great group, set up for the Aquults of the tropics that have little or no plinthite at shallow depths. The epipedon may be either umbrie or ochric. Definitions of the subgroups have not been written.

Umbraquults (8.13)

These are the Aquults of the temperate regions that have an umbric or mollie epipedon. The mollie epipedon is unknown except in cultivatedand limed soils. Typically, the Umbraquults have had a higher water table for longer periods than the Ochraquults. The Umbraquults seem to grade into Normaquods as the textures become sandy.

Humults (8.5)

This suborder includes soils of high rainfall regions. vVe find them from the southern border of Canada to the tropies. Relative to other UltisoIs, the Humults tend to be cool. Many of the tropical members have ochric epipedons, and most of the temperate members have umbric epipedons; but there are exceptions. So, we dropped the concepts of Ochrihumults and Umbrihumults in favor of a distinction between the Humults of tropical regions and those of temperate regions, defined by the seasonal temperature differences. The changes in concept suggested changes in the names of the great groups.

Some of these soils have remarkable contents of humus inso­far as the soils of the U.S. are concerned. They may have twice the humus of Mollisols with the highest contents - excluding Cumulic subgroups.

Normihumults (8.51) (formerly Umbrihumults)

These are the Humults of temperate regions. The requirement for an umbric epipedon has been removed from the definition of the great group and added to the definition of the Typic subgroup. In the U.S. these occur in the Pacifie northwest, where precipitation ranges from about 2,000 to 5,000 mmo The argillie horizons in these soils are of ten strongly degraded, and lack clay skins to depths of 150 cm. or so.

Tropohumults (8.52) (formerly Ochrihumults)

These are the Humults of tropical regions. The former requirement for an ochric epipedon has been dropped entirely except at the series level where it may be used if it seems relevant. We have added to the definition of the Typic sub-

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-groups a requirement that the cec be more than 20 meq./100 :grams of clay (af ter correcting for organic matter). The Tropo­~humults with lower cec are placed in Oxic subgroups. The "Tropohumults that we knoware on rather steep and probably "young surfaces. The argillic horizons have the most clay skins . in the upper part.

'Udults (8.2)

The Udults are the Ultisols of humid regions with short or no ~marked dry seasons, with little orgailic matter, and without the gray mottled colors of Aquults. They may be found on "both young and on very old surfaces. The Udults are very extensive in the, southeastern U.S. We are just beginning to look carefully into the genesis of these soils and have been 'surprised at what we have found. We have had a tradition, I suppose, that by a depth of a meter we should be through "the solum. Mottled horizons, or horizons with lower chromas 'that began at about a meter were considered to be the C horizon. If we did not find an argiHic horizon (B horizon) within 75 cm. 'we assumed that the soil was a Regosol (Entisol) .

Now it appears that 'on the old ' surfaces, thick epipedons are 'normal. The mottled clays called C generally have the finest 'textures and most or all of the clay skins. An unknown number nave bisequums in which what we formerly called C horizon is an A'2. Below it, we find remnants of fragipans, and probably . I 'argillic horizons. In all these we can detect no evidence of burial 'of a soil. We considered some rather drastic revisions of the great groups of this suborder but decided that we don't know 'enough at this moment to write reasonable definitions. But we "expeet some reorganization of -the great groups -in the years j ust to come.

,Fragiudults (8.24)

These are the Udults with a fragipan in or below the argillic -horizon. The Typic subgroup has an argillic horizon above the 'fragipan, and the pan is not strongly degraded - at least 90 'per cent of the cross section of some horizon in the pan has a 'brittie consistenee when it is moist or wet. '

_N ormudults (8.23)

These are the Udults without fragipans, with only minor -amounts of plinthite, and without dark red argillic horizons.

Unfortunately, this group includes soils on young 'surfaces 'with thin sola, less than a meter, and soils on very old surfaces

:go

with sola of 2 to 3 meters, depending on one's viewpoint of what constitutes the solum. Distinct differences in morphology are associated with the differences in age and solum thiekness, and it seems certain that this great group wiIl be split.

We are tending to reduce emphasis in color in classification, and the Rhodie Normudult subgroup has been dropped and is now included in the Typic subgroup.

Plinthudults (8.21)

These are the Udults with non indurated plinthite that forms more than half of the mass or that forms a continuous phase in some subhorizon within 165 cm. These are only on old sur­faces in the U.S. Most of these are probably in the tropics. We think it might be best to center the Typic subgroup concept on tropie al soils, and consider those in the U.S. as intergrades.

Rhodudults (8.22)

These are the Udults with dark red argillic horizons. They could weIl have been considered as families of N ormudults except for tradition.

Tropudults (8.25) These are the Udults of the tropics. They are extensive in

Puerto Rico and Hawaii on basic parent materiaIs, and on relatively young surfaces, generally strongly rolling to steep. We are proposing that the Tropudults with low cec in the argillic horizon, less than 20 meq./ 100 grams of clay af ter cor­rection for organic matter, be considered as intergrades to Oxisols.

Ustults (8.4)

These are the Ultisols with prolonged dry seasons and little organic matter. In the U.S. we find them in Mediterranean climates, with virtually no rain during the growing season. In the tropics only wet and dry seasons have meaning, and we would not distinguish between summer and winter rains.

This is a new suborder, not recognized in the 7th Approxima­tion. The differences from Udults are largely the result of climate, but this has been one of the main bases for recognition of suborders in our new classification. I cannot point to distinct differences between Udults and Ustults that are preserved in samples, but my personal prejudices are strongly in favor of distinguishing between such different moisture regimes at a high categorie level. The classification is probably very in­complete because we have relatively few of these soils in the U.S.

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N o rmustults (8.41)

These are the Ustults of temperate zones without fragipans. Except for the moisture regime, these soils are much like the Normudults.

Fragiustults (8.42)

These are the Ustults with fragipans. They occur in the U.S. but are extremely rare.

Tropustults ( 8.43 )

This great group is being added tentatively for the Ustults of the tropics. They are rare in the U.S. but they should be extensive on young surfaces in the wet-dry tropics. We have had little chance to study soils of this character.

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ORDER 9: OXISOLS

Oxisols inc1ude soils that have an oxic horizon. It has been proposed that we add others with plinthite at shallow depths if the colors of the soils are gray throughout. 80 far as we know Oxisols are restricted to old or ancient surfaces in the tropics and subtropics. Oxisol is not a synonym for Latosol, although the concept of Latosol has dominated our thinking in our efforts to define the oxic horizon.

We pointed out in the 7th Approximation that the develop­ment of concepts of Oxisols had lagged behind the development of concepts of the orders discussed previously. In the 6th Approximation, the oxic horizon and the argillic horizon were mutually exc1usive. In the 7th Approximation we tested the possibility that a single horizon might be both an oxic horizon and an argillic horizon. This was not weIl received, so we have abandoned the idea. I should point out that in our proposed classification we have broken with traditional use of color to define kinds of Oxisols. We must use color to some extent to define the Aquox, the wet Oxisols. Aside from this suborder we have not distinguished between the yellow, brown and red soils. We have been unable to relate col or to behaviour. If our maps are to be useful for predictions of soil behaviour we must define the classification on more basic properties than color. The hue and chroma of the oxic horizon are such obvious proper­ties that one might want to use them to define series primarily to prove that he is not color blind. And, it is easy to understand why color has been used by those who had no ready access to laboratory data. Instead of color we have chosen the moisture und temperature regimes, the exchange capacity and base saturation, segregations of gibbsite, plinthite and organic matter levels as the primary differentiae for suborders and great groups.

The definition of the oxic horizon that follows is the first

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draft of a new approximation, actually about the fourth, and win probably be changed. It represents our present thinking based on the suggestions of others and on our experience with the few Oxisols of Hawaii and Puerlo Rico.

Oxic horizon

The oxic horizon is an altered subsurface horizon consisting of a mixture of hydrated oxides of iron or aluminum together with variabie amounts of 1 :llattice clays and accessory diluents, such as quarlz, that are highly insoluble. It has, in the fine earth fraction, no more than traces of 2: 1 lattice clayS, or primary minerals that can weather to liberate bases, iron or aluminum. The mineral part of the less than 2 micron fraction has a cation exchange capacity of 13 meq. or less per hundred grams (by NH40Ac). The oxic horizon differs from the cambic horizon primarily in its lower exchange capacity or the absence of alumino-silicates other than 1 : 1 lattice clays or both. It dif­fers from the argillic horizon in the absence or near absence of clay skins, the gradual or diffuse boundaries and usually in the lower activity of the clay fraction.

The genesis

Little is known about the genesis of the oxic horizon but there has been much speculation. It was at one time postulated that under warm humid climates, silica was selectively removed from the soil by leaching, leaving a residual concentration of iron and aluminum. The evidence for this was the low ratio of silica to aluminum and iron. This theory seems largely untenable in the light of recent studies of rock weathering. These indicate almost complete loss of combined silica in the first stage of rock alteration. The silica of the oxic horizon is either in quartz or kaolin, both of which seem to be relatively stabie in the humid or dry tropics.

Geomorphologic evidence shows that the oxic horizons are generally found in soils of very old stabie land surfaces. In any landscape, the oxic horizon is in the soil of the oldest surfaces. It is not in soils of recent surfaces with shallow regolith.

If quartz gravels are present, stone lines in or under the oxic horizon are almost normal. Their occurrence is so common that one must consider the possibility that oxic horizons are found mainly in transported sediments of ancient age. Quartz veins may rise to near the surface through cam bic or argillic horizons, but the authors have not yet seen them in oxic horizons. It should be noted, however, that quartz veins are

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very rare in Hawaii and Puerto Rico wh ere we have studied the oxic horizon most.

One must suspect that most sediments from which oxic horizons have fonned were chemically weathered before de­position, because it is so commoil to find argillic or cambic horizons in alluvium that contains fresh weathered minerals at depths of a meter or two.

Whatever the mineraIs, the great age of the oxic horizon has allowed time for animals and plant roots to mix the materials so that almost no vestige of the original rock structure is retained. Only one exception is found to this - if iron oxides or gibbsite coat and cement fragments of weathered rock, the original rock structure may be retained in the interior of the aggregates but not in the fine earth.

The age of the oxic horizons is such th at minerals weather­able in wann humid climates are absent, or present only in trace3. There is, therefore, no reserve of bases beyond those in the exchangeable fraction, and in plant tissue.

Significanee of oxic horizon to soil classification

The oxic horizon is considered to have great importance to soil classification. All soils with oxic horizons are grouped into a single order, Oxisols. While much remains to be learned about the genetic significance of the oxic horizon, it is very important that weathering has been so extreme that virtually the only remaining minerals are quartz, free oxides and 1:1 lattice clay mineraIs. Yet there is no underlying illuvial horizon. This suggests a high order of stability or immobility of the clay fraction. The stability is further suggested by the absence of water-dispersible clay in all oxic horizons that have a net negative charge, as weIl as by the great resistance to erosion.

The absence of micas, glass, hornblende and feldspars leaves no weatherable reserve of bases in the soil, and the low activity of the clay limits bases that may be held at exchange sites. The bases for plant growth in the humid tropics are largely in plant tissue, and can be lost if the plant cover is cut, burned, and not allowed to regenerate. Thus, there are rather unique soil management problems with Oxisols that stem from the high penneability, low erosivity, and low reserve of bases.

In general, if other kinds of soils are available, soils with oxic horizons are not cultivated or are in plantations. Shifting cultivation and grazing are the most common types of agri­culture on soils with oxic horizons. If the interval under natural vegetation is' too short under shifting cultivation in humid climates, the bases are lost from the soil and only a few grasses

85

and shrubs will grow. Where the climate is subhumid, leaching of bases is reduced and hazards of exhaustion of bases are reduced, but lack of water limits the agriculture.

Oxic horizons are rarely found outside of tropical or sub­tropical climates, and there they are mainly at elevations below about 1,500 to 2,000 meters. However, within the zone they appear largely independent of rainfall, suggesting that many have formed under much higher rainfall than they receive today.

Laboratory identification of oxic horizons

Positive identification of oxic horizons normally requires both field examination and laboratory analyses, but field identi­fications can be highly accurate if analyses of a few soils are available.

The identification in the laboratory normally requires : (1) petrographic studies to demonstrate the absence of weather­

able minerais, (2) determination of the presence or absence of water-dispers­

ible clay, (3) measurement of the cation exchange capacity of the

mineral fraction less than 2 microns in diameter, and (4) thin sections for identification of clay skins.

Feldspar, mica, glass and ferro-magnesian minerals should be less than 3 per cent of the fraction between 20 and 200 microns.

End over end shaking in water for 16 hours does not disperse a significant (more than 3 per cent) part of the clay fraction unless the horizon has a net positive charge. The positively charged horizons may be readily identified by comparisons of the pH in water and in KCI. If the charge is positive, the pH in KCI is higher than the pH in water.

It should be noted that surface horizons rich in organic matter usually have water-dispersible clay, particularly if there is more than 1 per cent organic matter. The upper limit of the oxic horizon and the lower limit of the epipedon come at the least depth at which water-dispersible clay is absent. A horizon that is just below the epipedon and that lacks water­dispersible clay is present even in oxic h<;>rizons with net positive charges.

Determination of the exchange capacity of the mineral fraction less than 2 microns in diameter may be either direct or indirect. Direct determination requires treatment to destroy organic matter, dispersion with sodium carbonate or hydroxide,

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decantation or siphoning of the clay, and direct measurement of the exchange capacity at pH 7. Note that phosphates must not be used for this dispersion. The capacity should be 13 meq. or less for 100 grams of clay.

Reliable estimates may be made for many soils by relating the cation exchange capacity of the fine earth to the measured clay percentage and correcting for organic matter. This method is apt to give unrealistic values if the clay is cemented by oxides or fails to disperse for other reasons. An independent estimation of the percentage of clay is essential if the capacity is estimated. Evidences of failure of dispersion may show in the lack of correlation of clay and 15 bar water contents. Or, petrographic examinations of the silt and fine sand may show the presence of aggregates.

No constant value for the exchange capacity of organic matter may be assumed. For each soil one should plot the cation exchange capacity per hundred grams of clay against organic carbon for á number of horizons to calculate the activity of the organic fraction. Calculated values should be checked by occasional direct measurements. We are currently attempting to develop a method that may be used ; no method that we know can be used for this measurement.

Thin sections of oxic horizons show either no orientation of the clay, or the pressure orientation that is discussed under the argillic horizon. Occasional clay skins may be seen in the middle or lower part of some oxic horizons. Pseudomorphs of feldspars and micas should be absent or nearly absent, as these are usually remnants of the original rock structure.

Hydrated oxides of iron and aluminum generally constitute more than 12 per cent of the fraction finer than 2 microns but this is not a universal feature, nor is the high oxide content restricted to oxic horizons. N evertheless, the high metallic oxide content is certainly an important if not the prlmary factor responsible for the high aggregate stability of oxic horizons. Most 1:1 lattice clays seem capable of adsorbing about 12 per cent of oxides. Above this percentage there seem to be surplus oxides that can cement the clays or form concretions. The failure of the clay to disperse in water is also probably largely due in part to the content of metallic oxides, and in part to the very low electrostatic charge that is responsible for the low cation exchange capacity.

Field identification of oxic horizons

Oxic horizons resembIe cam bic horizons in many respects, particularly the cambic horizons that approach the oxic horizon

87

in degree of alteration of primary mineraIs. There are never­theless a number of properties that can be observed in the field.

Perhaps first is the structure. Most oxic horizons have a weak or very weak coarse prismatic or coarse to medium blocky structure, so weak as to he visible only in old road cuts. A frag­ment of a few cc. of the oxic horizon, whether moist or dry, crushes easily between the fingers to very fine granules. Mostly these granules are extremely fine, less than 1 mmo in diameter. The moist fragment crushes before there is any visible de­formation, indicating the very low plasticity and high friability.

Subhorizon boundaries within the oxic horizon are normally diffuse except for boundaries with stone lines or plinthite. Changes in color and texture may be considerable, but they are so gradual that the differences are difficult to detect, and subhorizon boundaries are apt to be more or less arbitrary.

Pores visible to the eye or under a hand lens are common to abundant in oxic horizons. The stability of these pores is pre­sumably a reflection of the aggregate stability of the horizon. Linings of the pores by clay skins are rarely found, even though water can move rapidly through them.

Weathered rock fragments that retain the original rock structure (saprolite) are absent unless they have been coated and cemented by iron or gibbsite. The rock structure, or sapro­lite, referred to here may be readily recognized by the discrete varicolored patterns of white, red, and strong brown in a material virtually free of pores. The spots of individual colors are most of ten of 20 to 200 microns in long dimension, without regular pattern. In thin sections the colors are seen to reflect pseudomorphs of crystals of primary minerals such as feldspars and micas. These may underlie an oxic horizon, but not more than about 3 per cent by volume should be identifiable in the oxic horizon. Similarly, sand-sized kaolin crystals that smear between the fingers into mica-like flakes of sand size should be rare or absent in the oxic horizon as these too are remnants of the original rock structure.

The colors of oxic horizons are not diagnostic. They may have varying shades of gray, brown, and red, or there may be mixtures of these in medium or coarse patterns of mottles.

Texture of oxic horizons

The percentage of clay in an oxic horizon may increase, decrease, or remain constant with depth. But, in the absence of a stone line to separate horizons of different texture, in­creases of the percentage of clay with depth are gradual. An increase of 20 per cent in the clay percentage of the fine earth

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(as from 30 to 36 per cent) does not take place within a verlical distance as short as 12 inches (30 cm.) if there are c1ay skins in pores and on peds somewhere within the soil. This is the limit with the argillic horizon.

Horizons with properties of oxic horizons grade from those with virtually no sand or silt to those with virtually no c1ay. lt is characteristic of oxic horizons that the fine silt fraction (2-20 microns) is very sm all , less than one-tenth of the c1ay fraction. This refers to discrete crystals of silt size and not to aggregates cemented by metallic oxides. N everlheless a few exceptions may exist, and the determination is not easy to make.

lt would be unreasonable to consider a horizon as oxic if it had less than one per cent silt and c1ay, and over 99 per cent quartz sand. Somewhere, there must be a more or Ie ss arbitrary upper limit on the percentage of sand. This is set at the limit between loamy sands and sandy loams. Because silt percentages are low, the oxic horizons normally will have more than 15 per cent clay. These limits apply only to the fine earth fraction, less than 2 mmo Oxic horizons may be very gravelly. Hardened plinthite is quite commonly the gravel.

In summary the oxic horizon is a subsurface horizon, exclu­sive of the argillic, natric, cambic or spodic horizons, that :

1. has a cation exchange capacity of the mineral fraction Ie ss than 2 microns of less than 13 meq. per 100 grams of clay ;

2. lacks more than traces of 2: 1 lattice clays or primary alumino­silicates such as feldspars, micas, glass and ferro-magnesian minerals ;

3. lacks more than traces of water-dispersible clays in some subhorizons ;

4. has texture of sandy loam or finer in the fine earlh fraction ; 5. has mostly gradual or diffuse boundaries between its sub­

horizons.

Aquox (9.1)

The concept of Aquox has not been developed much beyond that proposed in the 7th Approximation. A place is needed for wet soils that have the other characteristics of Oxisols. None have been identified in the U.S. to date, but they seem to be present on at least two continents. There are also wet soils of seepy areas below slopes in the tropics wh ere plinthite has formed in the surface horizons. These are more or less unique soils and seem to fit best with the Aquox even though they may have some weatherable minerals at depth or even in the plinthite.

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If the native forest is cut, the surficial plinthite hardens and the soil is quickly destroyed. There is still another group of wet soils that are gray to the surface; but that have non-indurated plinthite at shallow depths. In these soils the water table fluc­tuates. The plinthite may constitute well over half of the mass of some subhorizon between 40 cm. and a meter or more, or it may fonn a continuous phase, or both. In such soils there may be horizons that meet all requirements of an argillic horizon, a cam bic horizon or an oxic horizon. Cambic horizons in such soils may approach oxic horizons in weathering, but this does not seem essential to the fonnation of plinthite. Should the presence of the plinthite in such soils be considered more important than the argillic or cambic horizons ? Is the ease of field identification of the plinthite in contrast to the difficulty of distinguishing between cambic and oxic horizons an important factor ? Is an argillic horizon more important to the genesis and behaviour of the soil than the plinthite ? We have no way to answer these questions in the U.S., but the shape of the clas­sification depends on the answers.

We can foresee the possible need for at least three great groups of Aquox : (1) those with plinthite at the surface or very near the surface

with variabIe underlying horizons ; (2) the gray soils with plinthite at depth but within the soil ; (3) the gray bauxitic soils with nodules or concretions of

gibbsite.

Humox (9.2)

These are the Oxisols, other than Aquox, of cool perhumid tropical and subtropical regions. The mean annual temperatures are less than 22°C., the soils are always moist in all horizons in most years, and base saturation of the oxic horizon is low -less than 35 per cent by NH40Ac. These soils are very rich in humus, but our data are inadequate to let us use the humus content as a differentia.

Acrohumox (9.21)

These are the Humox with very low exchange capacity in the mineral clay fraction - less than 6.5 meq.j100 grams of clay, in some parts of the oxic horizon.

They have no gravel size nodules or concretions cemented by gibbsite. With the low exchange capacity and low saturation, the Acrohumox are extremely poor in bases. With the present

90

technology in most tropical countries the agricultural potentials are slight.

Gibbsihumox (9.23)

These are like Acrohumox except that gibbsite is present as horizontal sheets or it cements aggregates of gravel size in some subhorizon within a depth of 120 cm. lndustrial use as a source of bauxite is possible for these soils but agricultural potentialities are extremely smalI. Failures of the Hawaiian plantations on these soils have been explained to me in many ways but never in terms of the most likely cause, the soil.

Haplohumox (9.22)

These are the Humox with cec of the inorganic clay of more than 6.5 meq./100 grams of clay in all subhorizons of the oxic horizon. Epipedons may be ochric or umbric, but if umbric they should not be thicker than 75 cm. if they show evidences of intensive mixing with the oxic horizon by animals - chiefly worms and termites.

The Typic subgroups of Haplohumox may be used to illustrate proposed definitions of subgroups for Oxisols.

The Typic Haplohumox have: a. less than 4 meq. KCI extractable Al/100 grams of clay, b. an ochric epipedon, or an umbric epipedon less than 75 cm.

thick, c. no plinthite that is not indurated, and no mottles with

chromas of 2 or less within 125 cm. ; d. no clay skins in the oxic horizon ; e. an oxic horizon that extends more than 120 cm. below the

surface ; f. fine-Ioamy or fine texture in the oxic horizon. 9.22-8 Ultic H aplohumox : like the Typic except for d. 9.22-3 9.22-C

9.22-1.2 9.22-AI

Inceptic Haplohumox : like the Typic except for e. Cumulic Haplohumox : like the Typic except for band with a diffuse lower boundary of the umbric epipedon. Psammentic Haplohumox : like tlhe Typic except for f. Allic (*) H aplohumox : like the Typic except for a.

Vermihumox (9.24)

These are Humox with an umbric epipedon that is thicker

(*) The Allic extragrade provides for Oxisols with appreciabIe KCI extractabIe aluminum.

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than 75 cm. and that shows a transition to the oxic horizon with evidences of mixing by animals (as in other vermic great groups). This definition is very tentative. If Vermihumox can be recognized with an ochric epipedon we should want them included with the group, but we have only seen or recognized those with umbric epipedons.

Ustox (9.3)

The Ustox are Oxisols with dry horizons at some season but that are usually moist in some horizon. They have base satur­ation of more than 35 per cent (50 per cent ?) in some part of the oxic horizon and have amineral clay fraction with a cec of more than 6.5 meq.j100 grams of clay.

The Ustox, despite the possibilities of drouth, offer the highest potentialities of all Oxisols for a simple agriculture. The high cec combined with relatively high base saturation provide some nutrient reserve.

Mollustox (9.31)

These are Ustox with a mollie epipedon. The epipedon must be at least one unit of value darker than the oxic horizon. In the Typic subgroup, the chromas of the oxic horizon are less than 5 (moist).

Haplustox (9.32)

These are Ustox without a mollie epipedon, or with one that is less than 1 unit of value darker than the oxic horizon.

Idox (9.4)

These are the Oxisols of very dry climates, with prolonged dry seasons with little or no rain. The soils are usually dry -that is, dry for more than 6 months in most years. They have ochric epipedons with moist color values of 3 or more in all subhorizons. Base saturation is either more than 50 per cent (by NH40Ac) in all horizons of the soil, or increases to more than 50 per cent in the lower part of the oxic horizon. Only one great soil group has been recognized in this suborder.

Orthox (9.5)

The Orthox are the remaining Oxisols. They have base satur­ation that is less than 50 per cent in all parts of the oxic horizon or that decreases to less . than 50 per cent in the lower

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part. They may be usually moist or always moist with mean annual soil temperatures of more than 22 °C. ; or they may have temperatures of less than 22 °C. if they have some horizon that is dry at some season in most years. The Orthox, like the Humox, mostly have limited potential for a simple agriculture.

Umbr iorthox (9.51)

These are the Orthox that either have an umbric epipedon that is at least 1 unit of value darker (moist) than the oxic horizon, or that have more than 1 per cent organic carbon to a depth of at least 75 cm. The Typic Umbriorthox should have cec of less than 6.5 meq.jlOO grams of mineral clay.

Udorthox (9.52)

These are Orthox with short dry seasons, with no horizon dry for more than 60 days in most years, and with either less organic carbon than the Umbriorthox or with no umbric epi­pedon that is darker than the oxic horizon.

Haplorthox (9.53)

These are comparable to the Udorthox except that the dry season is longer, and some horizon is dry for more than 60 days in most years.

Anthor thox (9.54)

This includes the Orthox that have an anthropic epipedon. 80 far as we know these are aboriginal kitchen middens and are not extensive.

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ORDER 10: HISTOSOLS

The definition of Histosols is not changed from the 7th Approximation. The classification has been developed further but it is still very tentative, and there seems to be no question but that additional changes will be made.

The Histosols, organic soils, have been traditionally classified with primary emphasis on the base status, as in eutrophic vs. dystrophic ; in the water regime, as in high moor vs. low moor ; in the presumed botanic origin of the plant remains ; and in the degree of their decomposition. We have dropped botanic origin of the materials to the low categories - family and series for the most part - and there we have used them only if the mat­erials were weIl enough preserved for identification. The other traditional properties have been retained with some modification except for the direct use of the concepts of high moor and low moor. We propose to reduce the 10 stages of decomposition to three that we think can be rather consistently identified.

In the U.S., students of organic soils have been interested either in the history of the bog itself, or in the bog as a source of peat for agricultural or industrial uses. Except for two States, Florida and Minnesota, the Histosols are so minor in extent relative to other soils that they have had virtually no attention. It was possible to adopt European concepts, but to be used they had to be defined. The concept of eutrophic and dystrophic peats is simple unless one has to define the terms so that they can be applied uniformly.

The Histosols are last in the list of orders, last in importance and last in the attention they have been given. (They perhaps should have been called UltisoIs).

We are proposing to add several horizons to the list of diag­nostic horizons for use in the classification of organic soils. These proposals are as foIlows :

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Diagnostic master horizons of organic soils

The following definitions of organic soil horizons are meant to separate three master horizons based on degree of decom­position of organic remains. Limits of morphological characteris­tics selected are set to describe organic soils in terms of features that can be distinguished in the field insofar as possible.

Sedimentary peats are not treated as master horizons. Their morphological characteristics normally fall outside the limits of criteria of the three diagnostic master horizons unless they have been exposed to oxidation and decomposition. Many depos­its called sedimentary peats are mineral rather than organic. The limnic horizons described later include all unoxidized and undecomposed sedimentary peat horizons and form a basis for defining subgroups.

The diagnostic master horizons of Histosols must be at least 12 inches (30 cm.) thick if drained and 18 inches (45 cm.) thick if undrained. They do not include the upper 6 inches (15 cm.) of the organic soil if drained, or the upper 9 inches (22.5 cm.) if not drained. If th ere are two diagnostic master horizons in a given soil, the uppermost has precedence for classifying the profile.

The three kinds of diagnostic master horizons and the limnic horizons are as follows :

Fibric horizon (fibrous structure; formerly peat). L. fibra, fiber.

This consists of relatively undecomposed organic materiais, wood, sedge, moss, and other plants. The morphological features are similar to those of the original plant tissues from which they are derived. The morphological properties include :

(a) fibers comprise over 2/3 of total mass ;

(b) more than 50 per cent of fibers exceed 1.0 mmo in at least one direction (note that wood is included as fiber) ;

(c) there is an increase in color value (Munsell standard) of 2 or more units when wet peat is pressed firmly in hand ; the hue mayalso change, for example, from 10 YR to 7.5 YR;

(d) no color change is apparent when rubbed gently while wet ;

(e) when wet peat is squeezed in the hand, the liquid removed is clear or only slightly turbid ; no finely-divided dark­colored organic matter oozes between fingers (low N value); plant structure remains intact and may be easily identified ;

95

(f) if rubbed gently when dry, fibers are flexible and generally stay intact with very little dustiness ;

(g) yields a saturated sodium pyrophosphate extract, when a white filter paper is inserted into a paste, that is higher in value or lower in chroma than 10 YR 7/3.

Lenic (*) horizon (slemi-fibroUis structure; rormerly peaty muck and mucky peat). L. lenis, soft.

This consists of partly decomposed organic materials with morphological features indicating physical disintegration and/ or partial biochemical decomposition. Only the gross morphological features are considered and no differentiation is made according to the botanical origin of these partly altered materiaIs. The morphological properties include :

(a) fibers, including wood, comprise 1/3 to 2/3 of total mass with over 50 per cent of fibers exceeding 1 mmo in size ; or, if the fiber content comprises over 2/3 of moss, more than 50 per cent of the fibers are Ie ss than 1 mmo in size ;

(b) there is less than 2 units increas€ in value (Munsell) when wet peat is firmly pressed ; or, either the wet organic ma­terial decreases in chroma (MunseIl) when gently rubbed ; or, the dry organic material increases at least one unit in chroma over that when wet ;

(c) when wet organic material is squeezed in the hand, liquid removed is turbid ; less than 2/3 of organic material oozes between fingers and the residue is mushy and friable with only a few of the most weIl preserved fibers that can be identified readily ; or, if there was no col or change when rubbed gently while wet, the fibers break down (disinte­grate) to finer size ;

(d) when rubbed gently while dry, fibers are brittIe to very brittIe ; mass breaks to finer sized fibers and is quite dusty.

Sapric horizon (formerly muck). Gk. sapros, rotten. This consists of highly decomposed· organic materials with

very little original plant fiber remeaning. Properties indicate an advanced stage of decomposition, and a high degree of disintegration. The morphologic properties include :

(a) fiber content is less than 1/3 of total organic mass ; the fibers, if present, have variabIe sizes, but usually are very small « 1.0 mm.) and are soft skeletal remains that are

(*) Lenic was subsequently changed to hemic.

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readily brok en down when rubbed gently, either moist or dry;

(b) no color change is apparent when wet organic material is pressed firm!ly, or if rubbed while wet until fibers disappear in the ~ass, and little or no color change is apparent on drying;

(c) if wet organic material is squeezed in the hand, material removed is very turbid ; over 2/3 of mass passes between fingers if undrained ; fibers disappear ;

(d) if present, dry fibers áre very brittIe and turn to dust with pressure ; dust persist on hand af ter handling ;

(e) yields a saturated sodium pyrophosphate extract, when a white filter paper is inserted into a paste, that is lower in value and higher in chroma than 10 YR 7/3.

Limnic layers These are not diagnostic of taxa in the suborder category,

but are used to separate classes in the subgroup category. They include both organic and inorganic materials either (a) deposited in water by precipitation, action of aquatic organisms such as algae or diatoms, or (b) derived from underwater and floating aquatic plants subsequently modified by aquatic animaIs. They include marl, diatomaceous earth, sedimentary peat, bog iron, and possibly others. Except for some of the sedimentary peats containing over 30 per cent organic matter, most of these limnic materials are inorganic in composition. Diatomite is highly siliceous ; marl is mainly calcium carbonate ; bog iron is high in iron oxide content deposited by iron bacteria in bogs. Limnic layers should be at least 10 cm. thick to be used to define sub­groups. Thinner layers should be ignored unless there are multiple thin layers of the same type. If these exist they should be used if the combined thickness of the layers reaches 10 cm. or more.

The th ree master organic horizons - fibric, lenic and sapric - which were formed from plant residues varying in stage of decomposition are given consideration at a high level (suborder) in the classification. It is possible to make a few distinctions between plant materials in different stages of decomposition ; but if such highly diverse materials as the lim­nic sediments are considered at a high categoric level in the system, many classes would be needed. Sedimentary peat, marl, diatomaceous earth and other similar materials generally occur in the lower part of an organic soil and were formed during an open water stage of bog development.

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Coprogenous earth

A coprogenous earth (sedimentary peat) layer can be defined as a limnic layer of a Histosol that : (a) contains 50 per cent or more of fecal pellets, a few hun­

dredths to a few tenths of a mmo in diameter ; (b) has a dry color value less than 5 ; (c) forms slightly viscous water suspensions, is slightly plastic,

but not sticky, or shrinks upon drying to form clods that are difficult to re-wet and that of ten tend to crack along horizontal planes ;

(d) is normally nearly devoid of fragments of plants that can be recognized with the eye ;

(e) yields saturated sodium pyrophosphate extracts, obtained by filtration of a paste, that are higher in value and lower in chroma than 10 YR 7/3, or the cation exchange capacity is less than 240 meq./100 grams of organic matter (measured by loss on igni tion) .

These layers have the range in particle size and the C/N ratios, 12 to 20, consistent with extensive decomposition. Yet they have both a low and a narrow range in cation exchange capacity values, 80 to 160 meq./100 grams of organic matter (loss on ignition) which indicates very little decomposition influenced by ' exposure to air. Occasionally these layers show a platy structure. The individual plates are a little more than 1/2 mmo in thickness and may be annual deposition increments. This structure certainly is not pedogenic. Olive to olive brown colors of organic layers of Histosols are limited to these layers.

Diatomaceous earth

A diatomaceous earth layer of a Histosol can be defined as . one that : (a) has a matrix color value of 4 -+- 1, if not previously dried,

that changes the matrix on drying from organic matter to diatoms which can be identified by microscopic (440 X) examination of dry samples ; drying produces a permanent change in color ;

(b) yields a saturation sodium pyrophosphate extract to a white filter paper from a paste that is higher in value and lower in chroma than 10 YR 7/3, or the cation exchange capacity is less than 240 meq./100 grams of organic matter (loss on ignition) .

Diatomaceous earth layers frequently are more nearly mineral than organic in composition.

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Marl

A marl layer of a Histosol can be defined as one that : (a) has a moist color value of 6 -+- 1 ;

(b) reacts with dilute HCI to evolve C02 and leave disintegrated plant remains.

Marl usually does not change matrix upon 'drying ; con se­quently, there is no permanent color change. The reason is that these layers contain too little organic matter to coat the carbon­ate even though it has not been shrunk by drying. Most marl samples from the U.S. studied to date had organic matter contents between 4.0 and 19.9 per cent inclusive, and 92 per cent had contents less than 20.0 per cent.

Bog iron Bog iron consists of authigenic deposits of hydrated iron

oxides, mixed with varying kinds of amounts of organic ma­terials. The iron may be present in large cemented aggregates, or may be dispersed and soft. Colors are normally in shades of dark reddish brown, of ten mixed with black, and show little or no change on drying. Iron oxide contents may range upward to over 20 per cent.

Horizon thickness It is characteristic of the Cryic great groups, and of other

great groups in places, that the surface is very irregular with small or large mounds. Surface elevation may vary a foot or more within a horizontal distance of a few inches. If the mounds are organic and are so closely spaced that the pedons are less than 5 square meters, the soil should be classified as though the mounds had been leveled by plowing.

If the mounds are mineral or widely enough spaced that the pedons are larger than 5 meters, that is, mounds spaced so widely that they do not occur in each pedon of 5 square meters, the soils should be classified as they now exist. It is assumed that the more widely spaced mounds are due to frost and win be formed again if leveled, but that the small mounds -may not only be due in part to t ran1pling of live stock but also will be destroyed by the first cultivation.

It is also characteristic of the cryic soils, both organic and mineral, that polygonal patterns are common in the soil; a common pattem is one of thick organic materials bordering the depressed frost cracks with thin or very thin organic materials in the raised centers of the polygons. In these patterned soils, the mounds may be either mineral or organic. If mineral, the thickest organic materials may be between the mounds, and

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leveling would cover the organic materials with mineral ma­terials. It is believed that the Ruptic intergrades provide the best means of classifying such soils, but descriptions are inade­quate to permit even tentative definitions at this time.

The control section

If the organic horizons are thinner than 40 inches (100 cm.) if drained, or 60 inches (150 cm.) if undrained, and there is no lithic contact, the control section extends into the underlying mineral soil, but the limits should be adjusted to fit a 40 inch (100 cm.) control section af ter drainage.

To determine the thickness of the control section in undrained soils, the thickness of undrained organic horizons is reduced by one third (i.e., 60 is reduced to 40, and 30 is reduced to 20). The thickness of the mineral or limnic horizons is assumed to be unchanged by drainage if the N factor is 0.5 or less (that is, if the mineral material will not pass between the fin gers on squeezing). If the N factor in the mineral or limnic horizons exceeds 0.5, the assumption is made that they too will subside on drainage, and to determine the base of the control section the mineral horizons are treated as organic horizons.

A lithic contact within the thickness limits of the control section is always used as the base of the con trol section.

Some organic soils have been drained to depths shallower than one meter. In these soils the control section should be intermediate in thickness between those of the drained and undrained organic soils. Perhaps the best general rule is to use the equation,

S Dwt + 1.5 (40 - Dwt) when S = thickness of control section, and

Dwt = depth to water tabIe. Thus, a soil drained to a depth of 30 inches (75 cm.) would

have a control section of 30 + 1.5 (10) or 45 inches (112.5 cm.). One and one half inches of organic soil material below the water table is considered equivalent to one inch above. If the soil has been drained but the water table is not controlled and fluctuates with the season, an arbitrary 40 inch (100 cm.) control section should be used.

The limits of thickness that vary with drainage compensate for the initial very rapid subsidence that follows drainage, and help prevent changes in classification that would otherwise result from drainage alone. If the drainage system is known to be less than three years old, the initial subsidence will be in­complete. On these soils, some additional adjustment of limits may be desired. It is suggested that in making these adjust-

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ments it be assumed that 50 per cent of the subsidence comes the first year af ter drainage, and about 33 per cent the second year.

Family differentiae

Family differentiae for Histosols should include at least the following : 1. In Limnic subgroups, soils with limnic horizons within the control section, the nature of the limnic horizon should be used with the following classes : marl : calcareous family ; diatomaceous earth : diatomaceous family ; bog iron: ferruginous family ; sedimentary peat : coprogenous family. 2. In subgroups with mineral soils or horizons that are within the control section and that underly the organic horizons, texture and mineralogy of the mine ral soil should be used. Texture should include only the three broad texture classes -sandy, loamy and clayey.

Mineralogy classes should be those used for mineral soils. In Cumulic subgroups, where there are alternating organic

and mineral horizons, texture and mineralogy of the mineral layers should be used only as series differentiae. 3. Distinctions in Fibrists and Lenists according to the botanic composition of the plant remains should be made at the series level for the most part. However, distlnctions between woody materials and fine fibrous materials such as sedges, tule, mosses, etc. may be useful at the family level. This would be particularly true in Fibrists where the woody materials behave somewhat as do coarse fragments in mineral soils. For those Fibrists that are more than half wood, families designated as woody may be useful. If woody families are used, those that are not woody should be designated herbaceous. 4. Sulfureous families will be needed for Histosols with enough polysulfides that drainage wiIl produce a pH of 3.5 or less (in 1 N KCl).

Saprists (10.1)

The Saprists include the Histosols that have organic materials to depths of more than 24 inches (60 cm.) if drained, and 36 inches (90 cm.) if undrained, and that have immediately below the surface 6 inches (15 cm.) if drained, or 9 inches (22.5 cm.) if undrained, or above any other diagnostic horizon a horizon that is 12 inches (30 cm.) or more thick meeting the

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requirements of a sapric horizon. Normally, the upper 6 inches or 9 inches also meet all of the requirements of a sapric horizon except position. Soils of this character have been called muck in the Soil Survey Manual. There is, however, no restriction against the presence in Saprists of other thin horizons that don't meet the requirements of a diagnostic horizon over the sapric horizon, or of other diagnostic horizons below the sapric horizon. Saprists either have been artificially drained or have had a water table that fluctuated.

The Saprists are divided into the following great groups :

Cryosaprists (10.11)

Saprists with mean annual temperatures of 0 °C or less, or having ice or snow in or on the soil more than 10 months most years.

Dyssaprists (10.12)

Saprists with pH of 5 or less in 1 N KCI in the major part of the sapric horizon.

Eusaprists (10.13)

Saprists with pH of more than 5 in 1 N KCI in the major part of the sapric horizon.

Lusaprists (10.14)

Saprists with a horizon of accumulation of illuvial humus.

Among the great groups, we would propose that the Typic subgroups have a sapric horizon throughout the con trol section below 15 cm. If limnic horizons are in the control section we would recognize a Limnic subgroup. If there are mineral horizons or buried mineral soils within the con trol section we would provide subgroups for intergrades to the appropriate suborders or great groups. If the mineral content of the sapric horizon exceeds 50 per cent in the major part of the sapric horizon, we would propose a Clastic (extragrade) subgroup. And, finally, if the control section includes other diagnostic master horizons, we would propose intergrades to other suborders of Histosols. These are assumed to lie below the sapric horizon.

lenists (10.2) (subsequently changed to Hemists)

The Lenists inc1ude the Histosols that have organic materials > 60 to 90 cm. thick according to drainage and that have a lenic horizon lying over any other diagnostic master horizon. If cultivated, the plow layer is apt to decompose rather quickly to

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muck that has all the properties of the sapric horizon except for thickness and/ or position. This surface horizon is not considered in the natural classification, but is considered in mapping, as is the former category of type, as a phase that may be very important to use and management. The surface decomposition may extend below the plow layer, but the disintegrated and decomposed organic materials must extend to 18 inches (45 cm.) if drained to that depth, before the soil is excluded from the Lenists.

Since the fibers are soft, drainage commonly results in the formation of prismatic structure immediately below the plow layer. The prisms may become coated with decomposed organic material from the plow layer. These are typical features of drained and cultivated Lenists, but are not essential to the clas­sification. If they were considered essential, the drained and undrained Lenists would be kept apart in the classification. Predictions about the behaviour of Lenists af ter drainage would be made more difficuIt because the drained and undrained soils would be in different series.

We have proposed the following great groups of Lenists :

Cryolenists (10.21)

The Lenists with mean annual temperatures of O°C. or less, or with ice or snow in or on the soil for more than 10 months most years.

Dyslenists (10.22)

The Lenists with pH of 5 or less in 1 N KCI in the major part of the lenic horizon.

Eulenists (10.23)

The Lenists with pH of more than 5 in 1 N KCI in the major part of the lenic horizon.

Subgroups of Lenists are parallel to those of Saprists.

Fibrists (10.3)

The Fibrists include Histosols with organic materials > 60 to 90 cm. thick according to drainage, and that have a fibric horizon overlying any other diagnostic master horizon. They are the least altered and decomposed organic materials formerly called peat, or raw peat. The materials may be of diverse plant material. They include moss, sedge, grass, tule, papyrus, and woods of various sorts. The common denominators are the lack of advanced decomposition and disintegration, and the thickness

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of the plant residues. The materials must be thicker than 24 inches (60 cm.) if drained, or 36 inches (90 cm.) if undrained, and there· must be at least one subsurface fibric horizon, 12 inches (30 cm.) or more thick af ter drainage.

We are proposing the following great groups of Fibrists :

Cryofibrists (10.31)

Like other cryic Histosols.

Sphagnofibrists (10.32)

Fibrists with more than half of the organic part of the fibric horizon composed of fibrous remnants of Sphagnum mosses.

Hypnofibrists (10.33)

Fibrists with more than half of the organic part of the fibric horizon composed of fibrous remnants of Hypnum mosses.

Dysfibrists (10.34)

Other Fibrists with pH of 5 or less in 1 N KCI in the major part of the fibric horizon.

Eufibrists (10.35)

Other Fibrists with pH of more than 5 in 1 N KCL

Subgroups of Fibrists are parallel to those of Saprists.

Leptists (10.4)

, The Leptists (Gk. leptos, thin) include the Histosols that are too thin for other suborders, or that lack diagnostic master horizons. For the most part, the master horizons are lacking because the organic materials are thin.

Soine Leptists, the Cumulic subgroups, consist of stratified mineral and organic materiaIs. These occur primarily'in deltas of rivers and areas receiving pumice or ash. No single horizon within control section is thick enough to be considered a master horizon though the bulk of con trol section is organic. And, a few others in Stratic subgroups (L. stratum, a covering) consist of thin layers of such varying degrees of decomposition and disintegration that there are no master diagnostic horizons, even though the organic materials are thick.

The central concept of the great groups, the Typic subgroups, are mostly set on the pedons with the fewest possible horizons and the most decomposition. These are soils with thin decom­posed organic horizons, comparable to the sapric horizon, over mineral materials that are reduced or strongly gleyed, or that

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have no diagnostic horizons. A thin darkened Al horizon is permitted in the mineral portion of the soil, but horizons that could be considered buried mollic or umbric epipedons are not permitted in Typic subgroups, except in the coldest Leptists, the Cryoleptists.

We have proposed the following great groups of Leptists :

Cryoleptists (10.41)

Like other cryic groups of Histosols.

Dysleptists (10.42)

These are the acid ones, like Dysfibrists.

Euleptists (10.4'3)

These are like the other Euic Histosols.

Luleptists (10.'44)

These are similar to the Lusaprists.

We have two kinds of subgroups proposed for Leptists but not for other suborders. These are Cumulic for sooils with stratified organic and inorganic deposits., and Stratic for those that have thin organic llayers differing in degree of decomposition.

I should note that a fifth suborder is possible, one consisting of disturbed organic soils, without diagnostic horizons, but with recognizable fragments of them without discernible order. Such a suborder, parallel to Arents, would be named Arists.

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CLASSIFICATION OF TROPICAL SOILS

Genera I principles

1. There is a fundamental difference between the soils of humid equatorial regions and those of higher latitudes. In the equatorial regions there is no winter or summer. N ear the two Tropics there are generally wet and dry seasons, but still no winter or summer. In contrast, in the middle and high latitudes there are seasonal air and soil temperature variations that limit or stop biological activity. The differences in biologie activity in the soils are most clearly reflected by the soil organic matter. Other things being equal, the organic matter undergoes more complete decomposition and has lower C/N ratios in humid equatorial regions than in temperate or boreal regions.

In arid and semi-arid regions the biologie activity is inter­rupted by seasonal lack of soil moisture. This is not unique to the equatorial and near equatorial regions for it is also charac­teristic of temperate climates.

80, we are proposing to distinguish the soils of humid tropical regions at high categorie levels - suborders and great groups, but to distinguish the soils of the dry tropical regions only at a low categorie level, the family. For a few soils we are considering differentiation by subgroups if the central concept is of a soil in the tropics and closely related soils occur in subtropical but not in temperate regions.

We are defining the tropical soils by the soil temperature regime which shows little seasonal varia ti on between the two tropics. This is a differentia that is easily applied to most soils. Climatic data are generally available for the estimation of the soil temperature, and if they are lacking the measurements of the temperature regimes are not particularly time consuming.

2. We are also proposing to use a more or less arbitrary definition of solum in humid tropical regions. Plant rooting is

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of ten quite deep, deeper than it is practical to examine the soil. One can study deep road cuts to depths of several meters, but deep cuts are not common in many parts of the tropics. We can conveniently examine the soil in the fields or forests to a lithic contact or to about 2 meters, whichever is shallower. It is very expensive and of dubious value to go much deeper because the vast bulk of the plant rooting is in the upper two meters. It is more important to spend limited funds to extend our knowledge of soils to unknown areas than to spend it in study of the nature of layers that, for most purposes, lie below the soil. In special projects, for irrigation as an example, we may need to study the layers below the soil, because they affect the drainage and salt problems under irrigation. But, we feel that deep borings should be made only if there is a reasonably firm prospect that the information is needed.

3. The classification of tropical soils should be consistent with that of other soils to provide a single system that shows the relationships, which are many. The same processes operate in most soils, through not always with the same relative inten­sities, but there is nothing unique to the tropics except the lack of winter and summer seasons. One should expect, then, to find many similarities between tropical and non-tropical soils. These should be reflected in the classification.

New suborders and great groups

We wiU need to add a considerable number of great groups in several orders if we accept the principles I have outlined. We can discuss these, one order at a time.

Entisols

Arents

No special classes are needed at a high categoric level. Subgroups are used to indicate the nature of the disturbed soH, and if these occur in humid tropical regions that fact will be shown by the subgroups.

Orthents

It would seem that a great group of Troporthents is needed for the Orthents of the humid tropical regions. We have already proposed a group of Cryorthents, and a parallel group of Troporthents would seem to be logical and useful.

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Psamments

The sands of the humid tropics are largely included now in Quartzipsamments, because the sand is primarily or entirely quartz. The need for a great group of Tropopsamments would seem more questionable than the need for Troporthents. Very little actually goes on in Quartzipsamments. Intergrades to other tropie great groups would provide about all the infor­mation that could be used.

Vertisols

We are not proposing great groups of tropical Vertisols for two main reasons. One, the soils are normally dry at some season, and two, most of the Vertisols are either tropical or subtropical. The family temperature differentiae seem adequate.

Inceptisols

In this order we have had four suborders - Aquepts, And­epts, Ochrepts and Umbrepts. The relation between humus content and col or that makes a useful distinction between Ochrepts and Umbrepts in the temperate climates does not work weIl in the humid tropics. There seems to be little or no relation between humus and color in the humid tropics, and tropical great groups of Ochrepts and Umbrepts would group soils on a property that seems meaningless and that has few or no co-varying properties. Such a split seems to cut across the natural groups that exist in the tropics. It seems necessary, therefore, to set up a suborder in the Inceptisols, a suborder of Tropepts for the tropical Inceptisols that are excluded from Aquepts and Andepts. Before taking up the proposals for this suborder, we should look at the Aquepts and Andepts.

Aquepts

A distinction seems justified between some of the Aquepts of the tropics and those of other regions. We already have a great group of Cryaquepts. A similar group of Tropaquepts, however, cuts across a number of important similarities of the great groups.

H ydraquepts

These occur both in tropical and temperate regions. Their sim­ilarities seem much greater than the differences due to lack of seasons. We should, in my opinion, keep these soils together at the great group level, and use the families to distinguish the tropiealones.

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Fragiaquepts

These are not known at present in the tropics, and can remain as a group of soils in temperate regions.

Halaquepts

These, we think, are restricted to dry regions wh ere we are not proposing recognition of tropicall soitls rexcept at the f.amiiy level.

This leaves, then, the possibility of a great group of Trop­aquepts that is exclusive of Hydraquepts, Fragiaquepts and Halaquepts. It would eliminate for the tropics any distinction between the Aquepts with umbric and ochric epipedons. This would parallel what we are proposing for the suborder of Tropepts and should be useful.

Andaquepts

A great group of Tropaquepts mayor may not properly include the tropical Andaquepts, depending on how the defini­tions are written. Such soils exist in Hawaii, but our experience with them has been very small. Because we have not established tropical great groups for Andepts, we would question the need for splitting this group.

Andepts

We have reorganized the classification of Andepts. Definitions and names that have been changed are given in the lecture on Inceptisols. While Andepts occur from low to high latitudes we have proposed recognition of the tropical Andepts as families rather than as great groups. We can look at the problem one great group at a time.

Cryandepts

This group is proposed for the Andepts of high latitudes or high altitudes. The recognition of the tropical Cryandepts would be at the family level.

Durandepts

This group occurs in both tropical and temperate climates. The duripan seems to us to be more important than the soil temperature regime, and we would distinguish the tropical soils as families.

Hydrandepts

These are known only in the tropics.

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Eutrandepts

The Typic Eutrandepts are defined without reference to soil temperature regimes of tropical or temperate regions. They appear to be restricted to regions with pronounced dry seasons. One subgroup intergrades to Tropepts and another to Vitran­depts. In these, the soil temperature regimes are part of the subgroup definitions. Otherwise, soil temperature is used as a family differentia.

Normandepts

As in Eutrandepts, tropical intergrade subgroups are recog­nized, and tropical Thapto subgroups may be recognized. The Typic subgroup, however, carries no specification as to the temperature regime.

V itrandepts

No distinctions have been made on soil temperature regimes except on the family level.

Tropepts

The suggestions for great groups and subgroups of Tropepts are given in the lecture on Inceptisols.

Aridisols

Because these soils are usually dry th ere seems to be no need to distinguish between tropical and temperate Aridisols except at the family level.

Mollisols

AZbolls

These are not known to us in tropical regions.

Aquolls

We do not know that these occur in tropical regions, but one would expect them. Particularly one might expect soils compa­rable to the Haplaquolls, and a great group of Tropaquolls might be j ustified if found.

Borolls

This suborder was intended for boreal climates, and not the cold parts of the tropics. If cold Mollisols should be found at high altitudes in the tropics, a great group of Tropoborolls would seem j ustified.

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Rendolls

These are largely in humid climates, and it would seem that a great group of Troporendolls would be justified for the Rendolls of the tropics.

Udolls, Ustolls and Xerolls

The Udolls and Xerolls are not known to us in the tropics, though they could occur. Because the Ustolls and Xerolls are so frequently dry we would doubt the need for a great group of tropical U stolIs or Xerolls.

Spodosols

We proposed great groups for tropical Spodosols in the 7th Approximation. We plan to rename these as Tropaquods and Tropohumods. If we should find Orthods or Ferrods we would propose separate great groups for them.

Alfisols

Aqualfs

A separate great group for tropical Aqualfs seems j ustified -the Tropaqualfs, for Aqualfs that have no natric horizon.

Boralfs

Like the Borolls, should this suborder be found in the cold parts of the tropics we would want a separate great group.

Udalfs

We do not know that these occur in tropical regions but we would want to distinguish such soils as Tropudalfs.

Ustalfs

Because these soils have pronounced dry seasons and because they are mostly restricted to tropical and subtropical regions, we would only propose to distinguish the tropical U stalfs as families.

Ultisols

Aquults

We are proposing a new great group of Tropaquults for the Aquults of the tropics that lack enough plinthite for Plinth­aquults. The latter group we would not propose to subdivide, since virtually all are either tropical or subtropical.

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Humults

For this suborder we are proposing a great group of tropical Humults, the Tropohumults, to replace the earlier Ochrihumults. The requirement for an ochric epipedon has been dropped. We have added a requirement to the definition of the Typic subgroup that the cation exchange capacity exceed 20 meq./100 grams of clay af ter correcting for organic matter. The Tropohumults with lower cation exchange capacities are proposed as Oxic subgroups, intergrading to Oxisols.

Udults

We are proposing two changes in this suborder. First, the Typic subgroup of Plinthudults should probably be restricted to the tropics, and those outside the tropics should be considered as intergrades to Normudults. The second change has been to add a great group of Tropudults, comparable to Normudults, but restricted to tropical regions.

Ustults

Because these are largely in regions of high rainfall but with distinct dry seasons we should not consider a great group of Tropustults. Perhaps because we are human, we have proposed the group, although we know very little about them.

Oxisols

The proposals for classification of Oxisols are discussed in the lecture on that order.

Histosols

N ew great groups would be proposed for each suborder of Histosols for tropical soils, defined by the seasonal temperature fluctuations. These would be :

Tropo fibrists

Tropohemists

Troposaprists

Tropoleptists.

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SPECIAL PROBLEMS IN TROPICAL AND SUBTROPICAL REGIONS

Tropical and subtropical regions offer great potentialities for increasing production of food. Growing seasons are either long or continuo us so that several crops may be produced on a given field. Carbohydrate production can be extremely high in com­parison with the most productive soils of temperate climates. For example, good sugar plantations in Hawaii can produce one ton of sugar per acre per month or roughly two metric tons per hectare per month. A high yield of maize in Iowa would only be from one quarter to one third of this. The absence or scarcity of frost permits selection from a wide variety of crops .

. These are very important advantages. But it is not an easy matter to capitalize on or to get the full benefits from these advantages.

Subtropical regions, for our discussion, may be considered as those with mean annual air temperatures of 21°e. or higher. Tropical regions are those that lie between the two Tropics, Capicorn and Cancer, but I am restricting the discussion to the areas with mean annual air temperatures in excess of 14 °C. Frost may occur in subtropical regions during the winter or in high tropical regions during the dry season, but they are in­frequent, and the soil does not freeze.

These regions have some unique soil problems that can be traced in part to a rather basic difference from temperate regions in geomorphology. During the Pleistocene the repeated glacial periods, assisted by cryoturbation, solifluction, and loess formation, either destroyed or buried all but traces of any pre-Pleistocene stabIe surfaces of the temperate and boreal regions with their strongly developed and of ten completely weathered materiaIs. New materials with abundant weatherable minerals covered most temperate and boreal land surfaces at the close of the Pleistocene.

In the subtropical and tropical regions these events had much

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less effect. Climatic changes and changes in ocean levels ini­tiated new erosion cycles, and in the tropical and subtropical deserts wind movement of sands and silts was probably exten­sive. Yet enormous areas of the old surfaces in Africa, South America and southern Asia persisted without significant erosion or deposition. Relatively smaller areas appear to have persisted in Australia, and Oceana has few. The areas in southern Europe and the U.S. appear to have heen intermediate but more nearly like Australia than Africa.

The common presence of ancient soils in tropical and sub­tropical regions is one factor th at makes for special problems. These soils may have serious physical or chemical problems. Fine textured slowly permeable argillic horizons, extensive plinthite that has hardened or can harden, and petrocalcic horizons can all create serious physical problems because they limit the depth of rooting of plants. The intensely leached soils with very low cation exchange capacities and high oxide con­tents create difficult chemical and physical problems for our present technology.

Problems in humid climates with high rainfall or short or no dry season

Weathering in the humid climates is rapid, and has of ten been very intense. The soils in humid climates or that have been in humid climates are typically mixtures of sand and clay, with little of the very fine sand or silt (0.1 - 0.002 mm.) that seems to contribute most to the available water holding capacities of soils.

An Orthox in Puerto Rico with clayey texture holds about 3 to 4 per cent available water below the plow layer. This com­pares with 7 to 10 per cent for a nearby HumuIt with comparable clay and humus contents, and 12 to 18 per cent for a Udoll from loess in Iowa. The bulk densities further increase the spread between the soils. The U doll actually holds about 6 times more available water when expressed as cm. available water per meter of thickness.

Clayey soils have very high total water holding capacities. The Orthox held over 40 per cent water, at field capacity, but only about 10 per cent of this water was in a form available to plants. Such soils may feel moist even though there is no available water, and we are very apt to overlook the frequency with which the plants suffer from moisture deficiency. The Udoll held less water at field capacity, only about 30 per cent, but roughly half of this water was available to plants.

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High temperatures not only tend to increase weathering and decrease available water holding capacities but also in­crease demands for water by the plants. In compensation, the high temperatures insure soil temperatures that are favorable for root growth to great depths. Perennial plants can have very deep roots if the soils are otherwise suitable, so that a large volume of soil can be available for moisture storage. But annual plants are apt to suffer from very short rainless periods.

A second problem in the humid regions is the intense leaching of bases. In the Oxisols and some Ultisols the major factor in holding bases against leaching is the reserve held in the plant tissue. Inceptisols and Entisols and other Ultisois, with some weatherable minerals in the silt and sand fractions, may have small reserves ; but unless the plants capture and hold them, they are quickly lost af ter release by weathering.

In these soils base supplies normally are greatest in the surface and decrease with depth. The distribution reflects the cycling of the bases through the plants. Calcium and potassium are relatively more abundant in the surface than magnesium.

The normal agriculture in these soils is shifting cultivation. The forest is cut and burned, and a crop is planted in the clearing. The crops and their sequences vary but cassava, maize and upland rice are common. Af ter a very few years, with the time depending largelyon the pressure of the population, the field reverts to forest. The trees again collect the bases and af ter a period of years, usually longer than the period of cul­tivation, the process is repeated. If the soil is kept too long in crops, and the interval in forest is short, the supply of bases is depleted and lost so that forest cannot regenerate. Grass takes over, and the soil is useless for the primitive cultivators. The grass, which usually burns each year, does not have a reasonable supply of bases in its tissue.

The Oxisols appear in places to have lost their bases without the intervention of men. If the oxic horizon becomes depleted of calcium, we seem to have no agricultural possibilities until we learn how to restore the calcium to the deeper horizon.

We can use Profile 27 of the 7th Approximation (page 92) to illustrate this problem. The soil has 1.4 meq. of Ca in the upper 27 cm. Theh to 45 cm. there is 0.1 meq. of Ca. Below this there is presumably none. Actually, there may be something like 0.01 meq. of Ca that would not have been detected and reporled as a trace. This soil has a dark red col or, is from a basic rock high in magnesium, and has very good physical properties and ample rain ; but it lies idle in a country where the population pressure

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on the land is very intense. The trouble seems most likely to be the deficiency of calcium, which in this soil restricts most root growth to the surface 50 cm. Plant physiology teUs us that calcium is essential to cell division and growth, and that calcium taken up by roots in the surface horizons cannot be used by the plant roots for growth in deeper horizons. Most elements essential for plant growth can be moved .more or less freely in the plant, but calcium seems to be an exception. At least in the plants in which it has been studied it can move only in one direction - up the root and up the stem. It does not move down.

There are extensive are as of similar soils in Brazil that support only scattered stunted trees, shrubs and grass. These seem about devoid of calcium in the oxic horizon. I'm sure we can learn how to correct this deficiency. In the U.S. it has been done by liming and mixing the soil to a meter or more ; but this may not be the cheapest way, and lime can have harmful effects by making micronutrients unavailable or by creating an imbalance between calcium and magnesium.

In high rainfall areas, deficiencies of calcium are not restricted to Oxisols, but mayalso be found in UltisoIs.

This brings us to what is undoubtedly the greatest single special problem in humid tropical and subtropical regions - the lack of information about soil management.

Most research on soil management has been conducted in temperate regions on soils that are very unlike many of the soils of the humid tropics, particularly the Oxisols which !ie mostlyon gentIe slopes and appear to have a great potential for agriculture. Yet one sees little native agriculture except for shifting cultivation on these soils. What native agriculture one does see is mostlyon slopes cut weU below the levels of the Oxisols, where there are Tropepts and UltisoIs. The two latter kinds of soils are more nearly like soils of temperate regions in their chemistry and mineralogy than are the Oxisols.

Detailed soil surveys and classifications are made for large regions for practical purposes - to let us transfer experience with a given kind of soil from one place to another. But the experience to transfer into the tropics is commonly lacking or unsatisfactory. Or it is experience with plantation agriculture that cannot be applied to smaU holdings. Yet if we are to make soil surveys they must be useful, for if they are not useful someone will find a better way to sp end the money that they cost.

There is no easy solution to this problem. We can only get

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the experience by experimentation if the soils are not now used. The experiments, to be useful, must be made on kinds of soil that are important. Field experiments in temperate regions have been too of ten conducted on unimportant soHs simply because the land was available or close by an installation. SoH management studies are too badly needed in the tropics to permit selection of sites to be guided by any factor but the kind of soi!. Yet I have seen experimental fields laid out without any knowledge of the nature of the soi!. If the data from these stations prove useful it wiIl be an accident.

The bulk of the world's research on soH management has been in western Europe and the northeastern U.S. SoH management systems developed have normaIly included the use of lime to correct soH acidity, and applications of phosphorus and potas­sium. The fertilizers have been developed for soHs that are highly buffered compared to many soHs of the humid tropics. We learned in the U.S. what to do, but we of ten did not know why we did it, because we most of ten measured plant response without studying what we had done to the soi!.

We do know that the Oxisols rarely have a problem of alu­minum toxicity. We know continued use of heavy applications of fertilizers has made some Hawaiian soils so acid that manganese toxicity has become a problem. We know that lime applications of ten cause deficiencies in minor elements, notably zinc and boron, and sometimes magnesium if we use a lime high in calcium.

But in general, we don't know how to correct calcium defi­ciencies effectively. In Hawaii the planters have commonly used gypsum rather than lime as a source of calcium to avoid troubles with micronutrients, only to create troubles with manganese. We don't know how to get calcium deep into the soH to encourage deeper rooting.

Two things we have proven, though not everyone has yet noticed them : one is that we should study the effects of our practices on the soH as weIl as on the crops ; if we know what we did to the soH to improve the crops, the knowledge can be applied much more widely and to a variety of soils. The other is that we should select the locations of our experiment fields because the soils are important, not because the sites are convenient.

The sun

Sunlight is not always beneficial despite the opinion of sun­bathers. If the dry soil is bare and exposed to the sun in the

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tropics we know it can become too hot at the surface for plant roots. We know that some soils dry more or less irreversibly, and become too soft and granular. They lose water holding capacity rapidly and recover it slowly. This behaviour is not typical of all tropical soils, but we do not know how to predict it because we do not understand the mechanism of the changes.

Other soils set irreversibly into hard ferruginous crusts if exposed. These have the plinthite at the surface that we mentioned in the discussion of Aquox. This seems to be primarily a problem of climates with distinct dry seasons, rather than the permanently moist ones.

Problems in subhumid and arid climates

Soils with favorable physical properties are less common in the subhumid and arid climates than in the humid climates. Chemical weathering is slow, 'vegetation scarce, and erosion relatively rapid. Many soils are too sandy, too gravelly, too shallow to rock or to petrocalcic horizons, or have too fine textures in the argillic horizons. In addition th ere are chemical problems, and many soils are too salty or too calcareous. And there is nearly everywhere an even more important problem : the soils are too dry. The broad transitions of the northern continents between humid and arid regions where we have extensive areas of Mollisols either do not exist or are very small in tropical and subtropical areas.

Irrigation is normally required for any use of the soil except extensive grazing, but irrigation requires mostly large proj ects with an appreciable area of suitable soils more or less in a single body. Small areas of soils with suitable properties for irrigation can rarely be used. In the humid tropics small areas of productive soil mixed with others are used intensively for food production. The large areas of arid soils with good physical properties are sometimes poorly located with respect to the sourees of water. A successful irrigation project in a frost-free region can be enormously productive, but prospects for many such new projects in the world are poor. We can probably in­crease food production more by improvements in existing pro­j ects than by construction of new ones.

Most of the large contiguous areas suitable for irrigation are located along the rivers, low on the large fans and aprons, or in the dry valleys. Higher on the fans the soils are apt to be too gravelly, or to have petrocalcic horizons. In the valleys we have the Entisols and Camborthids mixed with Argids that can be irrigated.

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Fertility problems

Problems with soil fertility in the subhumid and arid tropics and subtropics are relatively few, and we know how to manage most of them. Nitrogen and phosphorus are most apt to be the limiting elements under irrigation, and these are easily supplied. We have never found potassium to be deficient in the dry parts of the U.S. There is one common problem, an iron chlorosis, that seem~ primarily to be serious on soils in which we have mixed in the plow layer a part of a ca horizon with soft powdery secondary lime. We most of ten do this in leveling" a field prior to irrigation. Citrus is very susceptible to this chlorosis, but many other crops are affected. Con trol is possible by spraying with iron, but it is easier to avoid the problem than to correct it. Never leave such a ca horizon exposed. Make any cuts deep enough so that it can be buried when leveling is completed. And, never mix the soft powdery ca horizons with other horizons in the pi ow layer.

There are occasional problems with toxicities. Boron in the water from wells may be toxic to plants. Forage may be toxic to animals in our arid regions. Molybdenum and selenium have been the main offenders. Molybdenum seems to be accumulated in soils that receive seepage from granitic mountains. It accu­mulates chiefly in legumes. Selenium comes from seepage from seleniferous shales. In the U.S., selenium seems to have accu­mulated in the sedimentary rocks of cretaceous age but not in others.

The principle of interaction

One must never lose sight of the principle that maximum yield increases do not come from the cumulative effects of improved practices, but from the interaction between the prac­tices when all necessary practices have been applied.

For maximum crop production we must have optimum conditions with respect to four things : fertility, water control, plant varieties and pest control. Each plant needs an ample supply of nutrients at all times, an ample water supply and suitable soil aeration, and protection from its diseases and predators. For a successful crop, we need a plant with the genetic ability to use substantial amounts of nutrients effectively to produce carbohydrates, fats, and proteins.

The principle of interaction needs some explanation because it is not everywhere understood.

If we only supply plant nutrients, responses may be very disappointing. In many parts of the tropics, plant varieties

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have been selected to withstand very low levels of plant nutrients. These varieties may show no response to added fertilizers, even though nutrient levels are very low. This proves to some that fertilizers are not needed. Plant varieties that can make use of the higher fertility level must be introduced or developed.

If we neglect water control and allow water to run off the sloping fields, or allow a fluctuating high water tabIe, the ferlilizers and improved varieties cannot be effective. And if we do not control plant diseases and animal pests, they may destroy the crop.

The introduction of improved varieties does not help unless we supply plant nutrients. Control of water without fertilizers and good varieties may have only little effect, or even reduce yields. Irrigation in the Ganges Plain has actually reduced rice yields wh en this was the only practice applied.

Each practice may have a small effect or none; but if we apply all of the necessary practices to the same field, there is an interaction between them that gives enormous responses.

I have to take an example from our humid nearly subtropical region in Georgia for the want of a better one. Maize yields on the Normudults with our old varieties and small amounts of fertilizers have averaged about 800 kilos per hectare. The use of heavy fertilizer applications increases yields of these varieties by about 600 kilos. Improved hybrid varieties alone add about 200 kilos. The con trol of water by terracing adds about 150 kilos. Pest con trol is virtually unneeded. If we add the increases to the original yield we have 1,750 kilos per hectare. But if we apply all of these practices, we get about 8,000 kilos. A number of our southern farmers have produced in con tests about 14,000 kilos of maize per hectare on small fields in favorable years. This is what I mean by interaction.

If we lose sight of this principle in our preoccupation with a single program and forget any one of the needs of a plant, and the need for good plants, we will miss our biggest opportunity as soil scientists to make this a better world.

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RECENT TRENDS IN SOIL CLASSIFICATION

The classification systems of soils have been developed largely in government institutes. This is unlike the classifications of plant and animaIs, which come largely from universities. The difference is partly a reflection of costs but it is also a reflection of government interests in their national resources. Studies of soils have traditionally required that students see the soils. Soils may not be brought to the laboratory. The student must go to the soi!. This is expensive, particularly if a general system, applicable to large areas, is being considered. Because govern­ments are concerned with the soils within their national borders, the attention of their pedologists has been focused generally on the soils of the nation. And because it is easy to see features that differ between soils but extremely difficult to see features that are common to all the soils that one knows, classifications developed for soils in a limited geographic area tend to ignore the similarities and to stress the differences between the soils in that area. Free choice of soil classification systems by indi­vidual workers is impossible in a national program. The same system must be used by all workers or the findings in different parts of the country cannot be compared. The decisions about the classification to be used must be made by the pedologists responsible for the programs in the absence of unanimity of opinion. Such decisions, of course, need not be arbitrary. In the U.S.A. a wide discussion was conducted prior to any decision about the recent change in the classification system.

The recent trends in soil classification are impossible to under­stand without an appreciation of the influence of governmental programs and without a brief review of the situation up to the time of World War 11. It was primarily because Russia was such a large country with a very wide variety of soils that the Russian scientist Dokuchaiev was able about one hundred years ago to see that there are broad relations between climate,

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vegetation, and the kind of soi!. Dokuchaiev was concerned with classification of lands for taxation when he made his initial observations.

In the United States at that moment there was no comparable national program. Prior to 1899, studies of soils in the United States ware made excIusively by the various States through their agricultural experiment stations. While viewpoints were mostly limited to the soils of a State, one American scientist, Hilgard, pointed out many facts that Dokuchaiev had observed. Hilgard, however, made no attempt to develop a cIassification of soils. He concerned himself mostly with relationships of individual soil properties to climate and vegetation. His obser­vations were facilitated by the extreme ranges in cIimate and vegetation within the single State of California where he worked.

By 1900 the Russian school, led by Dokuchaiev, had developed the concept of soils as natural bodies, and the framework of a cIassification, with a few genetic soil types such as Chernozem and Podzol grouped into three higher classes, Zonal, Intra­zonal and Azonal soils.

At almost that same time Prof. Milton Whitney of the United States Department of Agriculture, was starting the soil survey of the United States in cooperation with the land-grant uni­versities. He devised a cIassification whose basic units were the soil series and soil type - relatively narrowly defined kinds of soil intended to serve as the bases for transferring knowledge gained by research and experience with soil use from one farm or field to all other farms where it could be safely applied. Soil types were subdivisions of soil series. The soil series we re grouped into soil provinces, which were defined as regions with relatively uniform geology. The difference in the level of generalization of the Russian genetic groups and the American series and types is indicated by the recognition by the Russians of a dozen or so soil groups af ter 30 years of work, and the recognition of 550 soil series and a thousand or two soil types and phases in the United States in the first dozen years of work. The differences between the Russian and American systems reflected the differences in the nature of the govern­mental programs. In the United States relatively large-scale maps of small governmental units, counties, were being made to help advise land operators. In Russia, small-scale soil maps we re prepared primarily to locate areas suitable for agricultural development. These maps showed only a little of the genetic nature of the soils.

Central and Western European pedologists became familiar with the Russian ideas mainly through papers and discussions

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at the International Conferences of Pedology held in Budapest in 1909, and Stockholm in 1910. Ramann, working in Gennany, was perhaps the first western pedologist to adopt the Russian ideas. These did not become generally available to the U.S. pedologists until published in German by Glinka, just before W orld War 1. The concept of the genetic soil types, or great soil groups, was quickly and widely accepted once it became known.

However, between the world wars, international communica­tion about soils was difficult. Language was only one barrier. Travel was slowand difficult, particularly into the Soviet Union. Tenns for describing soil features such as texture, color, structure, consistence, were either lacking or used with different meanings in different countries. Attempts made to apply the tenns of Glinka to similar soils of other continents were only partially successful. Sometimes names were translated and sometimes not. Yet in most countries, there arose concepts of great soil groups reflecting for the most part soil differences within the country that were associated with present differences in climate, vegetation, and relief, and named according to the dominant soil colors and broad kinds of soil development as revealed by total analyses.

The period before World War 11

In the United States, the concept of great soil groups was first added by Marbut to the soil series. Some 1,500 of these were recognized in 1930. Russian and other European great soil groups were accepted insofar as they we re understood, and new great soil groups were established for kinds of soil that did not seem to be encompassed by other groups. But the great soil groups were very imperfectly related to soil series in Mar­but's Soils of the United States, published in the 1935 Atlas of American Agriculture.

The soil series had been defined with considerable emphasis on properties important to soil behavior. Ranges in properties of series were narrow in highly productive soils, and wider in soils of low productivity or unsuited to farming. The soil maps were being used successfully for solving fann problems and for farm credit and tax assessment, rural zoning, highway con­struction, pipeline location, and a host of other purposes that required detailed knowledge of the precise nature of soils of specific farms and fields.

To meet these needs, the soil series had increased to 2,000 in 1938. As a part of the Yearbook of the U.S. Department of

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Agriculture for 1938, Soils and Men, attempts were made to sharpen the definitions of the "great soil groups," without conspicious success. The definitions of these categories, and of the taxa written then, had been vague in Professor Glinka's writing. Subsequent Soviet publications still have not defined them more c1early.

In just a little more than a decade before the war, there were enormous advances in understanding the reasons for differences in soil behaviour, made mainly in western Europe and the United States. The nature of soil c1ays was largely worked out and methods for identification developed. These simplified under­standing of the behavior of phosphorous, sodium, potassium, and calcium in the soil. N ew forms of fertilizers were developed. Methods of particle size analysis were developed. Studies of soil micromorphology were started. Mineralogical analyses ' re­placed total analyses of soils. These were radical advances. Parallel advances in geomorphology showed the recent instability of climate and vegetation, and of many land surfaces.

The reasons for distinctions between soil series that had been separated mainly for practical reasons were now of ten understandable. The potassium of illite was available to plants. The potassium of feldspars was not, and so on. But the differ­ences between many of the great soil groups that had been based largelyon the present environment could not always be related to the new knowIedge. Of ten they cut across important soil properties such as the c1ay mineralogy.

Contacts between Soviet soil scientists and the rest of the world, broken in 1914, were renewed only briefly at two con­ferences (Prague in 1922 and Rome in 1924) and at three international soil congresses, but this was not enough. There were few opportunities of travel for Soviet citizens outside the USSR, or for foreigners within the USSR. The great purges in the USSR included geneticists whose writings reflect­ed earlier western scientific findings that were at variance with party dogma. Soviet texts on soil science contained more references to the writings of Lenin, Engels, and Marx than to those of western soil scientists. This situation in fact continued weIl af ter World War 11. The bibliography of Vilenskii's book Soil Science, published in 1957, contains 225 references, none of which are from western Europe or the new world except for Karl Marx. The chapter on Soil Fertility has five references ; one to Marx, two to Lenin, and two to Soviet soil scientists. This textbook was approved for use in government universities and pedological institutes of the USSR. The contacts were brief and superficial, and Soviet soil science was isolated.

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There we re few soil survey programs in western Europe during this decade. The pedologists were commonly associated with universities and concerned themselves largely with basic research in soil chemistry, soil physics, and soil genesis. Classifications were devised and improved for forest soils, but not for arabIe soils. However, much of the world's present knowledge and ideas about soil classification comes fr om the basic work of these pedologists. In 1939, the soil survey of England and Wales was started.

The postwar period

Then the Second W orld \Var came. It brought untold miseries, but it also brought remarkable

advances in control of disease, and an unprecedented population increase. The needs for food and fiber stimulated world-wide interest in studies of soil resources by governments, particularly those of developing countries.

In the United States · the new plant varieties, new machines, chemicaIs, and cheap fertilizers that followed the war revolution­ized farming. Gone were the relative advantages that existed before the war for soils with high natural fertility. Favorable physical properties became more important ; low earth moving costs and cheap fertilizers made it economic to remake soils by shaping or fertilization or both.

Formerly unproductive soils became highly productive as a result of the interaction between better water control, higher fertilization, pest control, and the new crop varieties. The soil maps made in many areas before the war were inadequate to guide the new practices that required large investments. Ranges of properties of some series were too broad, and the detail of the older soil maps was of ten inadequate. Air photographs now made it possible to plot soil boundaries very precisely.

Efforts were started immediately af ter the war to place all of the recognized soil series into the appropriate · great soil groups. It was immediately evident that some series did not fit any known group, and others might fit equally weIl into two. The great soil groups had been defined mainly on properties of a central concept of virgin soils, but the limits we re vague. Some groups were found to be defined by properties that were not soil properties.

The first efforts were to revise the definitions of individual great soil groups and to establish necessary new groups. This effort faHed because the limits of any one great soil group are also limits of several other great soil groups. Definitions were

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proposed for given great soil groups by those who knew the soils best. These individuals, however, of ten had little experience with the several other great soil groups that their definitions affected. The attempt demonstrated that focus on a single great soil group can sharpen the definition of that group, but that it leads to difficulties with other groups. The tendency is to keep pure the group being defined and to assign the variability to other groups. We learned that one must have all competing great groups in mind before any one of them can be clearly and satisfactorily defined. If this is not done, there will be overlaps, or gaps between the definitions. And there are apt to be marked inconsistencies between the groups. 80me are apt to be narrowly defined while others serve as wastebaskets. The results of this attempt were published in a special issue of Soil Science in February 1949.

80, in 1951 we began the task of contriving a new classification of United 8tates soils that would be defined in terms of soil properties and that would include all soil series grouped into successively higher categories. The soil series then numbered 5,500. This rapid expansion in numbers largely reflects coverage of new are as by the soil survey. They number 8,000 today and will probably continue to increase in numbers to meet the demands for interpretations in terms of a wide variety of soil uses and to accomodate soils in areas still to be studied.

The large numbers of series, even though defined without re gard to any single classification system, have been essential to the solution of problems of definition of the taxa in the new system. They made it possible to test proposed definitions by seeing what kinds of groupings the definitions produced. A natural classification should group soils with the greatest simi­larities, but the definitions can specify only a few properties. Only the existence of the series definitions made possible the task of selecting a few differentiae from among the multitude that might have been used.

Af ter 14 years of effort, including help from many overseas, the new classification was put into use in the United States in January 1965. The central concepts of nearly all soil series have been related to the taxa in all higher categories, and the staff is now examining the changes needed in series definitiol1s to bring the ranges permitted within each series into conformity with limits between taxa in higher categories. Most adjustments needed are minor. A few series are centered approximately on limits between taxa in higher categories and need study to be certain what they include. And, some of the old series are defined either in terms of an old classification, or they are

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defined with somewhat different concepts in different States. They may require drastic revisions. It is generally recognized that the new system has defects, but the rapidly increasing numbers of soil series cannot be managed conveniently without a classification that facilitates comparison of very closely related soils.

Acceptance of the new classification has been nearly unan­imous among the sixty or more agencies cooperating in the United States soil survey. Only one individual with technical responsibility for an agency of a State, one that does no soH mapping, appears to be opposed to its use. This does not imply anything like unanimity of opinion amongst the entire staffs of all agencies. I doubt that these staffs would be in unanimous agreement on any subject much more controversial than whether the sun will rise tomorrow.

In the Soviet Union, a program of mapping the soils of all collective and State farms has been pushed aggressively since the war by the Ministry of Agriculture. Great soil groups were found to be of little value for this program, and categories were set up corresponding roughly in number but not in content or order to those of the United States system.

To give further emphasis to the application of soil classifica­ti on toward solution of farm problems, the Dokuchaiev Institute was transferred to the Ministry of Agriculture. The Soviet and American classifications were growing more similar under the need for classifications that had prediction value for farming. So long as the Soviet classification remains unpublished, it is impossible to compare it with other systems, but from the scattered brief summaries one can draw a few tentative con­clusions. First the system seems still to be in flux, with only partial agreement on many points. Recent papers by different authors do not always seem to agree.

The Soviet system considers most of the soil characteristics used in the American system though it seems to ignore horizons impenetrable to roots. There are provisions in the Soviet sub­group parallel to the American subgroup to indicate soils with dominant properties of one group and subordinate properties of another group. The Soviet system appears to consider some features that the Americans retain as phases, outside of the natural classification, such as applications of lime and fertilizers.

Soil surveys of the Netherlands were started in 1945 and of Belgium in 1947. In both, the mapping was very detailed, and local schemes for classification of the soils we re developed. Else­where in western Europe two important classifications were developing. Prof. Kubiena, in Soils of Europe, presented a

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classification based primarily on soil features with very heavy emphasis on the micromorphology, features visible in thin sections under the microscope. This classification presents a number of ideas that have been used in developing the U.S. classification, but it also reflects the problems inherent in the development of a comprehensive classification system by a single mind. No matter how brilliant that mind, one life is too short to permit study of the soils of enough areas to give the classifica­tion a sound base of facto For one example, the majority of the soils of Belgium, Germany, and France, formed in loess had no place in Kubiena's classification. And these are important and extensive soils. Kubiena's classification has now been improved by Prof. Mückenhausen and is being used by the German government.

The other western European classification was being developed in France, largely by Prof. Aubert. It is perhaps the best in definition of concepts, and was drawn on heavily in developing our classification.

The Belgians at this same time were developing in Africa the first soil classification of tropical soils that could "be considered rational in the light of postwar knowIedge. This effort was largely ended by the events that accompanied the early days of self government there.

In addition, in western Europe in postwar years, an FAO working party, under the leadership of Prof. Tavernier of the University of Ghent, drew together the divergent national classification systems to develop a single legend consisting of phases of great soil groups for a soil map of western Europe. This legend carries names of the great soil groups taken fr om several systems, but the concepts are mostly compatible with the new American classification. This is not surprising, because the American system draws very heavily on western European research and classifications for its concepts. N ow an effort is being made by FAO and UNESCO to combine this map with one of eastern Europe prepared by the Soviets, with a single legend.

Western European and American pedologists have been able to visit the USSR again. Soviet pedologists have traveled and studied soils in western Europe between meetings of the Inter­national Soil Science Society. Together with several western European pedologists, I had the opportunity to be in the field with Prof. Tiurin, late Head of the Dokuchaiev Institute, and some of his colleagues in Yugoslavia in 1958, and here in Belgium, in Germany, and in the USSR in 1962, in addition to the 1956 International Congress in France, and the 1960 Con-

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gress in the United States. There seemed to be reason to hope for international collaboration. There we re other joint field studies in these years, and my hopes were bright for continuing progress toward common views toward soil classification.

Then, late in 1962, Academician Gerasimov· discussed the "Problems of Soviet Soil Science in the Light of the new Program and Resolution of the 22nd Congress of the Communist Party of the Soviet Union" . He told the Second All-Union Congress of soil scientists of the USSR the following : (1)

"Hence the most important tasks of Soviet soil science at the present time are : the development of the Dokuchayev theory of soil science and purging it of all erroneous pseudoscientific overlays ; the full and systematic use of the theoretical accom­plishments of Soviet soil science in practical agriculture ; and present struggle to achieve world-wide priority for Russian and Soviet soil science in the basic, critical trends of its scientific theory .......... .

". . . . we cannot avoid mentioning the relatively inadequate representation of Soviet scientists in the governing bodies of the international society of soil scientists and its commissions, and also the less important role of representative Soviet soil science in the various international programs of a scientific and scientific-practical nature as compared with representatives of the USA and the Western Europe countries. It must be said without qualification that many representatives of foreign soil science, and first and foremost of American soil science, at the present time are extremely active in the international dissemina­tion of the principles and methods of their scientific schools. This activity goes far beyond national boundaries and is partic­ularly vigorous with respect to economically underdeveloped countries (particularly the former colonies in Africa and Asia). Moreover, the official American soil conservation service now shows a great interest in extending its influence on a world­wide scale. Evidence of this is the systematic development of a completely new system of soil classification and nomenclature which this soil organization is introducing for international use.

"NaturaIly, scientific assistance to less developed countries is an important and prestigious undertaking. It is necessary, how­ever, that such aid from the great powers be disinterested, and that it be directed to the interest of the less developed land, and that it be given on an equal international basis. . . . . . . .

"All we have said clearly indicates the principal tasks of the current stage in the struggle of Soviet soil scientists to establish world priority for their science."

In view of this charge, one is hardly surprised at the recent

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(June, 1964) issue of Soviet ,Soil Science which apparently can find nothing of value in the American soil classification system. I am tempted to say that they scraped the bottom of the barrel when complaining that the taxa of the highest category of our system combine soils with unlike management requirements. Yet it can hardly add to the prestige of Soviet soil scientists to complain about features of the American classification that are common with the Soviet classification. This one is common.

The criticisms, combined with the official party responsi­bilities of Soviet soil scientists, are disquieting. The Soviet and American soil classifications both grew- out of the needs of governmental programs, and the issue of national prestige has apparently been raised. Soviet soil scientists must "struggle to achieve world-wide priority for Russian and Soviet soil scien­tists". So far as these two countries are concerned, the pos si­bilities of agreement on soH classification seem more remote today than they did a few years ago.

FAO, which was assigned the responsibility for soils work among the international agencies, has not been particularly effective in bringing together divergent classifications. The FAO soils experts working in tropical and subtropical regions on Special Fund projects have been numerous and able. They have most of ten used classifications designed to fit their home countries. Thus, at one moment on one continent, one can find FAO experts classifying soHs in as many as four different systems. This is not surprising in the absence of any generally acceptable system for classifying tropical soils.

In addition to these soils experts in FAO, there is an indepen­dent group attempting to develop a soH map of the world through the facilities of the member nations of FAO and UNESCO. With­out adequate facilities or staff, the World SoH Resources office of FAO is attempting to devise its own soil classification in terms of a single category with a few dozen great soil groups, each defined independently of the others. If the American experience is a guide, this will never be very satisfactory. There will be gaps and overlaps between the groups. And the Soviet, the American, and all other experiences indicate that the great groups are inadequate to determine soil management require­ments or to transfer experience with soil behaviour from one continent to another. A number of categories are required in a classification intended to guide soil use.

The immediate prospect is that the FAO program for develop­ing a world soil map win only add to present confusion. They have chosen to use names from several of the classification systems that, as they say, have gained some general acceptance.

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But, to fit these together, they must change the meanings of many or most. The concepts that the names have represented are parts of very different classification systems that were never designed to be fitted together. Thus, they have adopted the new American name Vertisol, but they have given it both a restricted and vague meaning. Pedologists who use the map will know that many of the terms of the world map are used with special meanings that differ from the meaning of the same terms elsewhere. But geographers and others who see Vertisols on the FAO map and on the USA map will certainly be misled into thinking that the meanings are the same when only the names are the same.

The American classification

Some of the assumptions behind our new classification are not everywhere accepted. One has led to the greatest difference between our new system and all others, the definitions of the taxa. We wanted a system that could be applied objectively because we have some 1,500 individuals classifying soils more or less independently, and these individuals have widely diver­gent backgrounds. We were influenced by Bridgman's Logic of Modern Physics, who points out that a concept is meaningless unless it can be expressed in terms of actual operations.

For example, the concept of pedogenic lime has been used for calcium carbonate precipated in a soil from calcium feldspars, but not from calcium carbonate. Since no method has ever been devised that permits a destinction between the calcium atoms in calcium carbonate that were previously combined with silicon and those that we re previously combined with a carbonate radical, the concept resulted in a meaningless definition of "soils with pedogenic lime". We chose to define our taxa in­sofar as possible in terms of soil properties subject to measure­ment by defined methods. Thus, we have said Vertisols have more than 30 per cent clay and more than 30 milliequivalents cation exchange capacity per 100 grams of soil, and we specify the methods to be used to measure these properties. We have expressed color, when diagnostic, in terms of the Munsell color system, and so on.

In contrast, the proposed definition by FAO of Vertisols says "Soils with an A C profile. The A horizon is thick, dark in color but is relatively low in organic matter content. The upper part of the A horizon is generally granular when dry and sometimes develops a crust. The lower part of the A horizon usually develops a prismatic structure." No specifications are given for

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what is thick, what is dark, or low organic matter content. This FAO definition is not unrepresentative of a great many of the present definitions of great soil groups. The Soviet genetic groups have virtuaUy no published definitions. But such a vague definition creates confusion, for th ere are many interpretations possible of the meaning of thick and dark. This depends entirely on one' s previous experience. I t can only be said to be a meaningless definition. The function of science is to predict both what and how much. The "laws" are simply statements of quantitative relations. AU definitions of science involve its quantitative nature. Pedology cannot be said to be a science if it remains purely qualitative. Yet the American approach to definitions is inconsistent with dialectic materialism as I understand it. A definition based on operations wiU not change with time. So we find Gerasimov stating "The methodological basis of the new system is eclectic, (which is true) being primarily empirical ; this is justified by references to the most widespread modern bourgeois - philosophical subj ective -idealistic trends (operationalism)." Yet this is one of the fea­tures of our system that appeals most to the brigh t students. Having a background in chemistry, physics and mathematics, they can understand the new definitions but feel frustrated by the more common vague definitions.

The new nomenclature has also appealed to the bright students. It is a systematic nomenclature, coined largely from Greek and Latin roots. The name of each taxon indicates clearly the place of the taxon in the system and suggests some of its most important properties. The names fit any modern European language without translation. I should mention that it was developed here in Ghent with the assistance of Professors Ta­vernier and Leemans of this University. It does not appeal to many of the older pedologists who, having learned one nomenclature, dislike to have to learn another. Even some Soviets have found merit in the system, though they believe it should have used Russian formative elements instead of Greek and Latin. One can understand that viewpoint.

The American system was published in 1960 for distribution at the 7th International Soil Congress in an incomplete form and för criticism. It has been the subject of extensive discussions on the excursions of the 1962 interim meeting in New Zealand and of the 8th International Soil Congress in 1964. N ow that the major defects have been corrected and the United States has begun to use the system, it is being re-examined in a number of countries. It seems likely that it wiU eventuaUy be used in two ways, if at aU. A few countries without soil classifi-

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cation systems of their own but with active soil survey programs will likely expand the definitions to provide for any of their own soils that do not occur in the United States and use it in their work. Others, perhaps in larger numbers, will retain their existing national classifications for the present, but use the American system for international communication, especially in technical papers where most pedologists have what amounts to academie freedom to choose their own classifications. As a new generation that has learned the American system as students comes to have responsibility for the national programs, several more countries mayadopt it, with necessary modifications. Certainly, a generation from now the system will be appreciably changed. At this moment, we still have very little quantitative information about many of the soils of the United States, to say nothing of the soils of the world.

I must also note that within the past very few years, a few pedologists have become interested in a numerical classification of soils. Biologists are experimenting with a system that permits computers to compare similarities between animals to determine which species have the greatest numbers of similarities. The application of this system to soils is currently hampered by lack of data that can be coded on data-processing cards, and by inability to determine the relative weights to be given different properties. One can say, as some biologists have, that each property should have the same weight. With soils, at least, this does not solve the problem, for it in fact, is assigning a weight. Is it proper to consider the hue of a soil horizon as important to classification as the amount of humus or the cementation of a horizon by opalor lime into a hard massive stone-like horizon ? The hue may be determined by the hue of the parent rock. Is the difference between 5 and 20 per cent clay the same as the difference between 60 and 75 per cent clay ? Numerically the difference is the same but it is not proportionally the same. If different weights are given different features, what relative weights should be assigned to what features? The answers will determine the degree of similarity reported by the computer. The attempts at numerical classifications of soils have not yet solved this problem. One dare not say, however, what the future of the system may beo It looks unpromising to those who have worked with other systems, but doubtless the first mammals did not look important to the dinosaurs.

In summary the present trends seem to be toward a deepening of differences in soil classification between the Soviets and the United States. The trend outside the USSR is definitely toward more quantitative definitions of taxa. This is obvious in the

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work in the United States, Canada, Australia, N ew Zealand, and several countries of western Europe.

Because soil classifications tend to be controlled by national programs it seems certain that no one system is apt to be gene rally accepted in the foreseeable future even outside of the USSR.

The international agencies, UNESCO and FAO, have not been effective to date in developing an international soil classification. Instead, they are trying to reverse the trend toward reasonably quantitative definitions and a nomenclature that can be used internationally without translation.

(1) Soviet Soil Science, Nov. 1962. Translation printed Feb. 1964.

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TABLE OF CONTENTS

page

The purposes of soil surveys 5

The logic of soil classification 13

Entisols 25

Vertisols 30

Inceptisols 33

Aridisols 48

Mollisols 54

Spodosols 63

Alfisols 67

Ultisols 76

Oxisols 83

Histosols 94

Classification of tropical soils 106

Special problems in tropical and subtropical regions . 113

Recent trends in soil c1assification . 121