Soil Survey Manual (Complete Print Friendly Version)

Introduction

The Soil Survey Manual provides in a single volume the major principles and practices needed for making and using soil surveys and for assembling and using data related to them. The Manual is intended primarily for use by soil scientists engaged in the classification and mapping of soils and in the interpretation of soil surveys. Although the Manual is oriented to the needs of those actively engaged in preparing soil surveys for publication, workers and students who have limited soils experience or are less familiar with the soil survey process also will be able to use the information.

The Manual focuses on the major concerns of the members of the National Cooperative Soil Survey, a cooperative undertaking of the United States Department of Agriculture and an agency of each of the States: commonly, the State agricultural experiment station of a State's land-grant university. Other agencies—local, State, or Federal—cooperate under special agreements. The original Federal authority for the soil survey of the United States is contained in the record of the 53rd Congress, chapter 169, Agricultural Appropriations Act of 1896. The authority was elaborated in Public Law 74-46, The Soil Conservation Act of April 27, 1936, and again in Public Law 89-560, Soil Surveys for Resource Planning and Development, September 7, 1966. The Manual is the primary reference on principles and technical detail for local, State, and Federal contributions to soil surveys authorized under these acts. The term "the Soil Survey" is used in the Manual to refer to the National Cooperative Soil Survey.

It is hoped this third edition of the Manual will be as universally useful as were the first and second editions. Many professional people engaged in other aspects of soil science and in other disciplines have used earlier editions of the Manual as a reference. Soil Scientists concerned with soil surveys in other countries have used them as well. Teachers have used the earlier editions both as texts and as references for students. This third edition retains those attributes of the Manual that have made it useful to many groups without deviating significantly from its primary purpose of serving the needs of soil scientists in the field.

Except for isolated passages, this edition of the Manual has been rewritten. Since the second edition (1951) was printed, a new soil taxonomy has been prepared and adopted. New and more intensive uses of soils have dictated changes, and advances in soil science as well as in related disciplines have provided new and more refined concepts and techniques. The increased use of soils for purposes other than farming has prompted new interpretations, more collaboration with professional people of other disciplines, and more adaptation of soil surveys to the concepts of other disciplines. These and other changes of two decades of work have made major revision of the Manual essential. This edition retains the practice of defining terms and concepts within the context of the explanatory text. A glossary is not included; to find definitions, explanations, and uses of specific terms, refer to subject listings in the index.

Although this edition reflects the results of experience mainly in the United States, it also reports the experiences of people in other countries. Studies similar to those that preceded the earlier editions were continued, or intensified, for this third edition, especially in the areas of classification of soils and the interpretations of soil surveys. In addition, many soil scientists in the United States and abroad tested the definitions, concepts, and techniques before they were printed in this current edition.

The chapters of the Manual are arranged in the approximate chronological order in which the work required for a published soil survey is done. As background for the chapters that follow, the first chapter defines the concepts of soils and and nature of soils as geographic bodies, and the second chapter describes the nature and uses of soils surveys, the kinds of soil surveys, and the map units.

The succeeding chapters describe procedures and conventions of soil surveys from the start of a survey to its publication. Chapter 3 deals with the attributes of bodies of soil that are mapped and the details of their internal properties. The fourth chapter tells how to prepare a mapping legend and the descriptive legend, which contains the technical instructions for mapping soils and related activities. Only after these facts are known can the units to be mapped be defined and identified consistently. fourth chapter also describes the supplies, equipment, and mapping bases required for conducting a soil survey. Data are accumulating during all of these activities. Chapter 5 describes the ways in which data are recorded, stored, and retrieved.

After the mapping is completed and related data about the soils are gathered, the information must be provided to those who use it in forms they can understand. Chapter 6, therefore, discusses interpretations of soil surveys. The final product is the published soil survey. Chapter 7 describes the publication of the soil survey map, the accompanying text, and other publications based on the findings of soil surveys.

All serious users of the Manual will benefit from complementary reading in the classification and genesis of soils, geology, climatology, engineering, forestry, hydrology, and urban and country planning.

Many dedicated SCS staffers have made significant contributions to the information published in this edition of Soil Survey Manual. Work on the Manual was started by Marlin G. Cline, Professor Emeritis, Cornell University, Ithaca, New York. It was completed at the National Soil Survey Center, Lincoln, Nebraska, by Richard W. Fenwick, retired soil scientist, and Robert B. Grossman, research soil scientist. Robert J. Ahrens and Robert J. Engel, soil scientists at the National Soil Survey Center, updated this edition of the Manual in 1993.

Chapter One

Soil and Soil Survey

A soil survey describes the characteristics of the soils in a given area, classifies the soils according to a standard system of classification, plots the boundaries of the soils on a map, and makes predictions about the behavior of soils. The different uses of the soils and how the response of management affects them are considered. The information collected in a soil survey helps in the development of land-use plans and evaluates and predicts the effects of land use on the environment.

Soil surveys were first authorized in the United States in 1896. Although extensive writings on husbandry by L.J.M. Columella were published in the first century A.D., practical experience was the teacher of most farmers until the advent of agricultural chemistry in the nineteenth century. By the end of the nineteenth century the knowledge about soils that had been gained from farming, agricultural chemistry, biology, and geology grew into a unified concept of the soil itself.

Early Concepts

The first scholar to study soils in the United States was Edmund Ruffin of Virginia. He worked diligently to find the secret of liming and discovered what we now call exchangeable calcium. After writing a brief essay in the American Farmer in 1822, he published the first edition of An Essay on Calcareous Manures in 1832. Much of what Ruffin had learned about soils had to be rediscovered because his writings circulated only in the South.

E.W. Hilgard was one of the first true pedologists in the United States, but he never received the credit that his accomplishments deserved during his lifetime. The early concepts of soil were based on ideas developed by a German chemist, Justus von Liebig, and modified and refined by agricultural scientists who worked on samples of soil in laboratories, greenhouses, and on small field plots. The soils were rarely examined below the depth of normal tillage. These chemists held the "balance-sheet" theory of plant nutrition. Soil was considered a more or less static storage bin for plant nutrients—the soils could be used and replaced. This concept still has value when applied within the framework of modern soil science, although a useful understanding of soils goes beyond the removal of nutrients from soil by harvested crops and their return in manure, lime, and fertilizer.

The early geologists generally accepted the balance-sheet theory of soil fertility and applied it within the framework of their own discipline. They described soil as disintegrated rock of various sorts—granite, sandstone, glacial till, and the like. They went further, however, and described how the weathering processes modified this material and how geologic processes shaped it into landforms such as glacial moraines, alluvial plains, loess plains, and marine terraces. N.S. Shaler's monograph on the origin and nature of soils summarized the late 19th century geological concept of soils (Shaler, 1891). In 1906, other details were added by G.P. Merrill.

Near the end of the nineteenth century, Professor Milton Whitney inaugurated the National Soil Survey Program (Jenny, 1961). Professor Whitney and his coworkers in the newly organized soil research unit of the U.S. Department of Agriculture became impressed by the great variations among natural soils—persistent variations that were in no way related to the effects of agricultural use. Whitney and his coworkers emphasized soil texture and the capacity of the soil to furnish plants with moisture as well as nutrients. Professor F.H. King of the University of Wisconsin was also reporting the importance of the physical properties of soils about this time (King, 1910).

Early soil surveys were made to help farmers locate soils responsive to different management practices and to help them decide what crops and management practices were most suitable for the particular kinds of soil on their farms. Many of the early workers were geologists because only geologists were skilled in the necessary field methods and in scientific correlation appropriate to the study of soils. They conceived soils as mainly the weathering products of geologic formations, defined by landform and lithologic composition. Most of the soil surveys published before 1910 were strongly influenced by these concepts. Those published from 1910 to 1920 gradually added greater refinements and recognized more soil features but retained fundamentally geological concepts.

Early field workers soon learned that many important soil properties were not necessarily related to either landform or kind of rock. They noted that soils with poor natural drainage had different properties from soils with good natural drainage and that many sloping soils were unlike level ones. Topography was clearly related to soil profile differences. As early as 1902, soil structure was described in the soil survey of Dubuque County, Iowa. The 1904 soil survey of Tama County, Iowa, reported that, on similar parent material, soils that had formed under forest contrasted markedly with soils that had formed under grass.

The balance-sheet theory of plant nutrition dominated the laboratory and the geological concept dominated field work. Both approaches were taught in many classrooms until the late 1920s. Although broader and more generally useful concepts of soil were being developed by some soil scientists, especially E.W. Hilgard (Hilgard, 1860) and G.N. Coffey (Coffey, 1912) in the United States and soil scientists in Russia, the necessary data for formulating these broader concepts came from the field work of the soil survey during the first decade of its operations in the United States. After the work of Hilgard, the longest step toward a more satisfactory concept of soil was made by G.N. Coffey, who determined the ideal classification to be a hierarchical system that was based on the unique characteristics of soil as "a natural body having a definite genesis and distinct nature of its own and occupying an independent position in the formations constituting the surface of the earth" (Cline, 1977).

Beginning in 1870, the Russian school of soil science under the leadership of V.V. Dokuchaiev and N.M. Sibertsev was developing a new concept of soil. The Russian workers conceived of soils as independent natural bodies, each with unique properties resulting from a unique combination of climate, living matter, parent material, relief, and time (Gedroiz, 1927). They hypothesized that properties of each soil reflected the combined effects of the particular set of genetic factors responsible for the soil's formation. Hans Jenny later emphasized the functionally relatedness of soil properties and soil formation. The results of this work became generally available to Americans through the publication in 1914 of K.D. Glinka's textbook in German and especially through its translation into English by C.F. Marbut in 1927 (Glinka, 1927).

The Russian concepts were revolutionary. Properties of soils no longer were based wholly on inferences from the nature of the rocks or from climate or other environmental factors, considered singly or collectively; rather, by going directly to the soil itself, the integrated expression of all these factors could be seen in the morphology of the soils. This concept required that all properties of soils be considered collectively in terms of a completely integrated natural body. In short, it made possible a science of soil.

The early enthusiasm for the new concept and for the rising new discipline of soil science led some to suggest the study of soil could proceed without regard to the older concepts derived from geology and agricultural chemistry. Certainly the reverse is true. Besides laying the foundation for a soil science with its own principles, the new concept makes the other sciences even more useful. Soil morphology provides a firm basis on which to group the results of observation, experiments, and practical experience and to develop integrated principles that predict the behavior of the soils.

Under the leadership of Marbut, the Russian concept was broadened and adapted to conditions in the United States (Marbut, 1921). As mentioned before, this concept emphasized individual soil profiles to the subordination of external soil features and surface geology. By emphasizing soil profiles, however, soil scientists at first tended to overlook the natural variability of soils which can be substantial even within a small area. Overlooking the variability of soils seriously reduced the value of the maps which showed the location of the soils. This weakness soon became evident in the United States, perhaps because of the emphasis here on making detailed soil maps for their practical, predictive value. Progress in transforming the profile concept into a more reliable predictive tool was rapid because a large body of important field data had already been accumulated. By 1925, a large amount of morphological and chemical work was being done on soils throughout the country. The data available by 1930 were summarized and interpreted in accordance with this concept, as viewed by Marbut in his work on the soils of the United States (Marbut, 1935).

Furthermore, early emphasis on genetic soil profiles was so great as to suggest that material lacking a genetic profile, such as recent alluvium, was not soil. A sharp distinction was drawn between rock weathering and soil formation. Although a distinction between these sets of processes is useful for some purposes, rock and mineral weathering and soil formation are commonly indistinguishable.

The concept of soil was gradually broadened and extended during the years following 1930, essentially through consolidation and balance. The major emphasis had been on the soil profile. After 1930, morphological studies were extended from single pits to long trenches or a series of pits in an area of a soil. The morphology of a soil came to be described by ranges of properties deviating from a central concept instead of by a single "typical" profile. The development of techniques for mineralogical studies of clays also emphasized the need for laboratory studies.

Marbut emphasized strongly that classification of soils should be based on morphology instead of on theories of soil genesis, because theories are both ephemeral and dynamic. He perhaps overemphasized this point to offset other workers who assumed that soils had certain characteristics without examining the soils. Marbut tried to make clear that examination of the soils themselves was essential in developing a system of Soil Classification and in making usable soil maps. In spite of this, Marbut's work reveals his personal understanding of the contributions of geology to soil science. His soil classification of 1935 depends heavily on the concept of a "normal soil," the product of equilibrium on a landscape where downward erosion keeps pace with soil formation.

Clarification and broadening of the concept of a soil science also grew out of the increasing emphasis on detailed soil mapping. Concepts changed with increased emphasis on predicting crop yields for each kind of soil shown on the maps. Many of the older descriptions of soils had not been quantitative enough and the units of classification had been too heterogeneous for making yield and management predictions needed for planning the management of individual farms or fields.

During the 1930s, soil formation was explained in terms of loosely conceived processes, such as "podzolization," "laterization," and "calcification." These were presumed to be unique processes responsible for the observed common properties of the soils of a region (Jenny, 1946).

In 1941 Hans Jenny's Factors of Soil Formation, a system of quantitative pedology, concisely summarized and illustrated many of the basic principles of modern soil science to that date (Jenny, 1941). Since 1940, time has assumed much greater significance among the factors of soil formation, and geomorphological studies have become important in determining the time that soil material at any place has been subjected to soil-forming processes. Meanwhile, advances in soil chemistry, soil physics, soil mineralogy, and soil biology, as well as in the basic sciences that underlie them, have added new tools and new dimensions to the study of soil formation. As a consequence, the formation of soil has come to be treated as the aggregate of many interrelated physical, chemical, and biological processes. These processes are subject to quantitative study in soil physics, soil chemistry, soil mineralogy, and soil biology. The focus of attention also has shifted from the study of gross attributes of the whole soil to the co-varying detail of individual parts, including grain-to-grain relationships.

In both the classification of Marbut and the 1938 classification developed by the U.S. Department of Agriculture, the classes were described mainly in qualitative terms. Classes were not defined in quantitative terms that would permit consistent application of the system by different scientists. Neither system definitely linked the classes of its higher categories, largely influenced by genetic concepts initiated by the Russian soil scientists, to the soil series and their subdivisions that were used in soil mapping in the United States. Both systems reflected the concepts and theories of soil genesis of the time, which were themselves predominantly qualitative in character. Modification of the 1938 system in 1949 corrected some of its deficiencies but also illustrated the need for a reappraisal of concepts and principles. More than 15 years of work under the leadership of Guy Smith culminated in a new soil classification system. This became the official classification system of the U.S. National Cooperative Soil Survey in 1965 and was published in 1975 as Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys (Soil Survey Staff, 1975).

Categories and classes of the new taxonomy are direct consequences of new and revised concepts and theories. The system of soil classification discussed in Soil Taxonomy is dynamic and will change as new knowledge is obtained. Its most significant contribution comes from defining class limits quantitatively. The theories on which the system is based are tested every time the taxonomy is applied. For soil survey, the application of quantitatively defined classes to bodies of soil produces quantitatively defined mapping units. This permits the soil maps to be interpreted with more precision than was formerly achieved. Furthermore, this soil-classification system simplifies and accelerates the process of soil correlation.

In addition to the new soil classification system, several other techniques have contributed to the increased precision of soil survey. The use of aerial photographs as mapping bases became almost universal in detailed soil mapping during the late 1930s and early 1940s. Using aerial photographs has greatly increased the precision with which soil boundaries can be delineated on maps. At the same time, the scale of published maps was increased from about 1:63,360 to 1:24,000 to 1:15,840. The smallest area that can be delineated legibly at a scale of 1:63,360 is about 15.8 ha; areas of 1 ha can be delineated legibly at a scale of 1:15,840.

Another factor has had an immense impact on soil survey, especially during the 1960s. Before 1950, the primary applications of soil surveys were farming, ranching, and forestry. Applications for highway planning were recognized in some States as early as the late 1920s, and soil interpretations were placed in field manuals for highway engineers of some States during the 1930s and 1940s. Nevertheless, the changes in soil surveys during this period were mainly responses to the needs of farming, ranching, and forestry. During the 1950s and 1960s non-farm uses of the soil increased rapidly. This created a great need for information about the effects of soils on those non-farm uses.

Beginning about 1950, cooperative research with the Bureau of Public Roads and State highway departments established a firm basis for applying soil surveys to road construction. Soil scientists, engineers, and others have worked together to develop interpretations of soils for roads and other non-farm uses. These interpretations, which have become standard parts of published soil surveys, require different information about soils. Some soil properties that are not important for growth of plants are very important in evaluating soils for building sites, sewage disposal systems, highways, pipelines, and recreation. Many of these uses of soil require very large capital investments per unit area; errors can be extremely costly. Consequently, the location of soil boundaries, the identification of the areas delineated, and the quantitative definition of map units have assumed great importance.

Modern Concept of Soil

Soil is "the collection of natural bodies in the earth's [sic] surface, in places modified or even made by man of earthy materials, containing living matter and supporting or capable of supporting plants out-of-doors. Its upper limit is air or shallow water. At its margins it grades to deep water or to barren areas of rock or ice. Its lower limit to the not-soil beneath is perhaps the most difficult to define. Soil includes the horizons near the surface that differ from the underlying rock material as a result of interactions, through time, of climate, living organisms, parent materials, and relief. In the few places where it contains thin cemented horizons that are impermeable to roots, soil is as deep as the deepest horizon. More commonly soil grades at its lower margin to hard rock or to earthy materials virtually devoid of roots, animals, or marks of other biologic activity. The lower limit of soil, therefore, is normally the lower limit of biologic activity, which generally coincides with the common rooting depth of native perennial plants" (Soil Survey Staff, 1975).

The "natural bodies" of this definition include all genetically related parts of the soil. A given part, such as a cemented layer, may not contain living matter or be capable of supporting plants. It is, however, still a part of the soil if it is genetically related to the other parts and if the body as a unit contains living matter and is capable of supporting plants.

The definition includes as soil all natural bodies that contain living matter and are capable of supporting plants even though they do not have genetically differentiated parts. A fresh deposit of alluvium or earthy constructed fill is soil if it can support plants. To be soil, a natural body must contain living matter. This excludes former soils now buried below the effects of organisms. This is not to say that buried soils may not be characterized by reference to taxonomic classes. It merely means that they are not now members of the collection of natural bodies called soil; they are buried paleosols.

Not everything "capable of supporting plants out-of-doors" is soil. Bodies of water that support floating plants, such as algae, are not soil, but the sediment below shallow water is soil if it can support bottom-rooting plants such as cattails or reeds. The above-ground parts of plants are also not soil, although they may support parasitic plants. Rock that mainly supports lichens on the surface or plants only in widely spaced cracks is also excluded.

The time transition from not-soil to soil can be illustrated by recent lava flows in warm regions under heavy and very frequent rainfall. Plants become established very quickly in such climates on the basaltic lava, even through there is very little earthy material. The plants are supported by the porous rock filled with water containing plant nutrients. Organic matter soon accumulates; but, before it does, the dominantly porous broken lava in which plant roots grow is soil.

More than 50 years ago, Marbut's definition of soil as "the outer layer" of the Earth's crust implied a concept of soil as a continuum (Marbut, 1935). The current definition refers to soil as a collection of natural bodies on the surface of the Earth, which divides Marbut's continuum into discrete, defined parts that can be treated as members of a population. The perspective of soil has changed from one in which the whole was emphasized and its parts were loosely defined to one in which the parts are sharply defined and the whole is an organized collection of these parts.

Factors that Control the Distribution of Soils

The properties of soil vary from place to place, but this variation is not random. Natural soil bodies are the result of climate and living organisms acting on parent material, with topography or local relief exerting a modifying influence and with time required for soil-forming processes to act. For the most part, soils are the same wherever all elements of the five factors are the same. Under similar environments in different places, soils are similar. This regularity permits prediction of the location of many different kinds of soil.

When soils are studied in small areas, the effects of topography or local relief, parent material, and time on soil becomes apparent. In the humid region, for example, wet soils and the properties associated with wetness are common in low-lying places; better drained soils form in most instances in higher lying areas. The correct conclusion is that topography or relief is important. In arid regions, the differences associated with relief may be salinity or sodicity, but the conclusion is the same. In a local environment, different soils are associated with contrasting parent materials, such as residuum from shale and from sandstone, and the correct conclusion is that parent material is important. Soils on a flood plain differ from soils on higher and older terraces where there is no longer deposition of parent material on the surface. The correct conclusion is that time is important. The influence of topography, parent material, and time on the formation of soil is observed repeatedly while studying the soils of an area.

With the notable exception of the contrasting patterns of vegetation in transition zones, local differences in vegetation are closely associated with differences in relief, parent material, or time. The effects of microclimate on vegetation may be reflected in the soil, but such effects are likely associated with differences in local relief.

Regional climate and vegetation influence the soil as well as topography or relief, parent material, and time. In spite of local differences, most of the soils in an area typically have some properties in common. The low-base status of many soils in humid or naturally acid rock or sediment regions stands in marked contrast to the typical, high-base status in arid or calcareous sandstone or limestone regions. To one who has studied soils only on old landscapes of humid regions, however, low base status is so commonplace that little significance is attached to it.

Regional patterns of climate, vegetation, and parent material can be used to predict the kinds of soil in large areas. The local patterns of topography or relief, parent material, and time, and their relationships to vegetation and microclimate, can be used to predict the kinds of soil in small areas. Soil surveyors learn to use local features, especially topography and associated vegetation, as marks of unique combinations of all five factors. These features are used to predict boundaries of different kinds of soil and to predict some of the properties of the soil within those boundaries.

Soil-Landscape Relationships

Geographic order suggests natural relationships. Running water, with weathering and gravitation, commonly sculptures landforms within a landscape. Over the ages, earthy material has been removed from some landforms and deposited on others. Landforms are interrelated. An entire area has unity through the interrelationships of its landforms.

Each distinguishable landform may have one kind of soil or several. Climate, including its change with time, commonly will have been about the same throughout the extent of a minor landform. The kinds of vegetation associated with climate also likely will have been fairly uniform. Relief varies within some limits that are characteristic of the landform. The time that the material has been subjected to soil formation has probably been about the same throughout the landform. The surface of the landform may extend through one kind of parent material and into another. Of course, position on the landform may have influenced soil-water relationships, microclimate, and vegetation.

Just as different kinds of soil are commonly associated in a landscape, several landscapes are commonly associated in still larger areas. These areas cover thousands or tens of thousands of square kilometers. Many can be identified on photographs taken from satellites. From this vantage point, broad geomorphic units—the East Gulf Coastal Plain, the Allegheny Plateau, the Laramie Basin, and the Great Valley of California—are apparent. These broad units usually have some unity of landscape, which is characterized by such terms as "plain," "plateau," and "mountain." These physiographic units are composed of many kinds of soil.

The main relief features of a physiographic unit are usually the joint products of deep-seated forces and a complex set of surface processes that have acted over long spans of time. Within a physiographic unit, groups of minor landforms are shaped principally by climate-controlled processes. The climate and biological factors, however, vary much less within a geomorphic unit than across a continent.

Still broader than the geomorphic units are great morphogenetic regions having distinctive climates. For example, one classification recognizes glacial, periglacial, arid, semiarid-subhumid, humid-temperate, and humid-tropical climatic regions associated with distinctive sets of geomorphic processes. Other major regions characterized by seasonal climatic variation are also recognized. These geomorphic-climatic regions are related to soil moisture and soil temperature regimes.

Thus, the great climatic regions are divided into major geomorphic units. Landforms and associated soil landscapes are small parts of these units and are commonly of relatively recent origin.

The landforms of concern in soil mapping may include constructional units, such as glacial moraines, and elements of local sequences of graded erosional and constructional land surfaces. These bear the imprint of local, base-level controls under climate-induced processes. Most surfaces that have formed within the last 10,000 years have been subject to climatic and base-level controls similar to those of the present. Older surfaces may retain the imprint of climatic conditions and related vegetation of the distant past. Most landforms of the present started to form during the Quaternary Period; some started in late Tertiary time. In many places conditions of the past differed significantly from those of the present. Understanding climatic changes locally and worldwide far into the past contributes to understanding the attributes of landforms in the present.

Geomorphic processes are important in mapping soils. Soil scientists need a working knowledge of local geomorphic relationships in areas where they map and should understand the interpretations of landforms and land surfaces made by geomorphologists. The intricate interrelationships of soil and landscape are best studied by a collaboration between soil scientists and geomorphologists.

Development of the Soil Survey

Soil surveys were authorized in the United States by the U.S. Department of Agriculture Appropriations Act for fiscal year 1896, which provided funds for an investigation "of the relation of soils to climate and organic life" and "of the texture and composition of soils in field and laboratory."

In 1899 the U.S. Department of Agriculture completed field investigations and soil mapping of portions of Utah, Colorado, New Mexico, and Connecticut. Reports of these soil surveys and similar works were published by legislative directive. At the same time, the State of Maryland, using similar procedures and State funds, completed a soil survey of Cecil County. Since then many soil surveys have been initiated, completed, and published cooperatively by the Department of Agriculture, State agencies, and other Federal agencies. The total effort is the National Cooperative Soil Survey (NCSS).

The early soil surveys investigated the use of soils for farming, ranching, and forestry. As experience was acquired in the use of soil surveys, predictions were made about other uses, such as highways, airfields, and residential and industrial developments. As the making and the use of soil surveys expanded, the knowledge about soils—about their nature, occurrence, and behavior for defined uses and management—also increased. The Highway Department of Michigan was applying soil survey experience to assist in planning highway construction in the late 1920s. At about the same time soil surveys in North Dakota were used in tax assessment.

Soil surveys published between 1920 and 1930 reveal a marked transition from earlier concepts to give emphasis to soil profiles and soils as independent bodies. The maps retained significant geologic boundaries as soil maps do today. Many of the surveys of that period provide excellent general maps for evaluating engineering properties of geologic material. In addition, maps and texts of the period show more recognition of other soil properties significant to farming and forestry than do earlier surveys and have value for broad generalizations about farming practices in large areas.

The use of aerial photographs for soil mapping, which began during the late 1920s and early 1930s, greatly increased the precision of plotting soil boundaries. To meet the needs for planning the management of individual fields and farms, greater precision of interpretation was required. The changing objectives of soil surveys initiated changes in methods and techniques that made surveys more useful and forced reconsideration of the concept of soil itself.

Beginning in the 1930s, the Soil Conservation Service (SCS) emphasized the control of soil erosion as it used soil surveys for the resource conservation planning of farms and ranches. In the 1950s, extensive use was made of soil survey information in urban land development in Fairfax County, Virginia, and in the subdivision design of suburban areas of Chicago, Illinois. Soil surveys were an important base for resource information in regional land-use planning in southeastern Wisconsin. Rural land zoning has also relied on soil surveys.

Soil surveys necessarily involve thousands of different kinds of soils—as many as there are significantly different combinations of genetic factors. The history of a soil and evidence of its potential for use are contained in the properties soil scientists are able to identify through observation and research in the field and laboratory. These properties determine the limitations, suitability, and potential for rural and urban land use of soils. Soil surveys are particularly valuable because they identify specific soil properties and help soil scientists make broad generalizations significant to farming and forestry practices.

The program of the NCSS can be divided into soil mapping, description of the mapping concepts, and the prediction of the behavior of these mapping concepts for various uses. Soil behavior prediction relies on the evaluated and named soil properties to interpret the concept of map units.

Soil Survey and the Soil Map

The different kinds of soil used to name soil map units have sets of interrelated properties that are characteristic of soil as a natural body. This definition is intended to exclude maps showing the distribution of a single soil property such as texture, slope, or depth, alone or in limited combinations; maps that show the distribution of soil qualities such as productivity or erodibility; and maps of soil-forming factors, such as climate, topography, vegetation, or geologic material. A soil map delineates areas occupied by different kinds of soil, each of which has a unique set of interrelated properties characteristic of the material from which it formed, its environment, and its history. The soils mapped by the NCSS are identified by names that serve as references to a national system of soil taxonomy.

The geographic distribution of many individual soil properties or soil qualities can be extracted from soil maps and shown on separate maps for special purposes, such as showing predicted soil behavior for a particular use. The number of such interpretative maps that can be derived from a soil map is large, and each such map would differ from the others according to its purpose. A map made for one specific interpretation can rarely serve a different purpose.

Maps to show one or more soil properties can be made directly from field observations without making a basic soil map. Such maps serve their specific purposes but have few other applications. Predictions of soil behavior can also be mapped directly; however, most such interpretations need to be changed with changes in land use and in the cultural and economic environment. A map showing the productivity of crops on soils that are wet and undrained, for example, has little value after drainage systems have been installed. If the basic soil map is made accurately, interpretative maps can be revised as needed without doing additional fieldwork. In planning soil surveys, this point needs to be emphasized. Occasionally, "short-cut" inventories are made for some narrow objective, perhaps at a cost lower than that of a soil survey. Such maps quickly become obsolete. They cannot be revised without fieldwork because vital data are missing, facts are mixed with interpretations, or boundaries between significantly different soil units have been omitted.

The basic objective of soil surveys is the same for all kinds of land, although the number of mapping units, their composition, and the detail of mapping vary with the complexity of the soil patterns and the specific needs of the users. Thus a soil survey is matched to the soils and the soil-related problems of the area. Soil surveys increase our general knowledge about soils and serve practical purposes. They satisfy a need for soils information about specific geographic areas for State, county, and community land-use plans. These plans include resource conservation plans for farms and ranches, development of reclamation projects, forest management, engineering projects, as well as other purposes.

The storage and retrieval of soil survey data are possible through the use of Automatic Data Processing (ADP). ADP helps develop important interpretations and policy decisions for both the present and the future.

Chapter Two

Soil Systematics

Introduction

The factors of soil formation are discussed in chapter 1. Climatic and biological factors generally produce broad geographic patterns. These have given rise to a zonal concept and definition of soil distribution. Parent materials contribute to soil variations within climatic and vegetative zones. Local topographic patterns add further complexity, affecting both the time of exposure to processes of soil formation and the kinds of processes. The complex interactions among these factors occur in repetitive patterns which lead to the formation of repetitive combinations. These are the basis for defining, identifying, and mapping soils.

The repetitive patterns imprinted in soils by the soil-forming factors can be observed at scales ranging from continental to microscopic. The patterns are the basis for soil identification and mapping at vastly different scales. A system of terminology, definitions, and operations can be ascribed to the various scales. Hierarchical systems of classes and subclasses can be set up to produce groupings at the different scales.

The National Cooperative Soil Survey of the United States has systems of descriptive terminology, class definitions, hierarchical soil groupings, and operations that are applicable to various scales and appropriate to a wide variety of uses. Development of such flexibility has, in turn, required a fairly complex system in which it is important to understand a number of philosophical, conceptual, and operational relationships. Foremost among these are the relationships between mapping units and taxonomic units, site data and mapping unit data, conceptual models and the real entities in the landscape.

Map units are designed to carry important information for the more common uses of soils in the survey area such as small grain, rural living, and small community development. The map units must also be easily recognizable and mappable at scales compatible with the available base maps, the time allocated to collecting the data, and the skills of the surveyors.

In the United States, soil surveys vary in scale and in intensity of observations. The components of map units are designated by taxa in Soil Taxonomy (Soil Survey Staff, 1975) and miscellaneous areas if they are present. Soil taxa are modified with phases, such as slope and stoniness to convey more specific information. The phase often is a portion of the range of properties exhibited by the taxon. For example, a certain soil series may have slopes from 3 to more than 60 percent, but map units are shown with narrower ranges—such as 3 to 8, 8 to 15, 15 to 25—in order to provide information that is useful in managing the soils in the area.

Kinds of soils are known best by the characteristics embodied in small samples. Field descriptions of soil include horizon designation; depth and thickness; color; moist and dry; features of ped surfaces and interiors; texture; structure; consistence at several water states; and special features such as roots, pores, nodules, salts, rock fragments, pH, and boundary conditions. Site characteristics of the soil and its immediate surrounding are usually described. Chemical and physical data are obtained from horizon samples analyzed in a laboratory. Special features of microbial entities and activities are not routinely determined but may be carried out for research or special purposes.

All samples and all models of soils have a purpose in the soil survey program. Small samples from peds and profiles help us describe the properties of points and how they are organized. This mainly gives us a perspective of the results of soil genesis. We only know what is present by the techniques of measurement that are used, although we may speculate about what has been removed, changed, added, or translocated. Profile features are combined into models of soil formation and the processes and events of geomorphology that have influenced and helped shape the hypothesized features. These mental processes of model building permit us to shift readily from considering an ion in solution to the arrangement of horizons in profiles and their stratigraphic relationships across landscapes.

Purposive sampling of soil map units depends on whether the answers or relationships we desire are related in a meaningful way with the features of the soil map units. The actual clues are not necessarily soil properties at all but are features of identification that we associate with the unseen soil models. Mapping in most surveys involves delineating segments of the landscape, cutting out geographic areas, and putting the boundaries on base maps. Tonal shades and patterns on aerial photographs are used to indicate possible changes of vegetation, drainage conditions, materials, and so forth. The patterns of the gray tones are used to delineate areas on maps. As we look at the existing vegetation, we see differences of tones and composition of the species makeup, and we verify or modify the boundary locations of the units accordingly. Configurations of the visible surface of the land, stones, and other features are used as evidence of changes important enough to be recognized as separate areas. Finally, the soils are examined at a few locations to verify the models being used in the mapping process.

Soil surveys are conducted so that all the clues, features, and pieces of evidence that support the delineations that are called soil map units are in fact surrogates for the models that have been established. The measure of models of landscape evolution and soil formation relative to observable landscape areas is provided by the constant testing that goes on in the soil survey. The outdoors is a laboratory in which variability is subject to some level of systematic portrayal. Thus the small items that are used to assist in locating, verifying, modifying, and developing soil models are similar to the criteria used to identify the basis of differentiation in the classification of soils.

Predictions of properties that exist in soil map units and the predictions made about the qualities and suitabilities and responses of areas of land are all based on the relationships that exist between the desired or expected result and the actuality that is represented by the models used in mapping.

Many schemes have been proposed and tested for determining the composition of map units. The same can be said for the distribution of properties that exist, or are thought to exist, in areas of the landscape that can be delineated consistently on base maps. It is fairly well accepted that certain features of soils and of landscapes are not in accord with existing models of distributions in systematic and predictable ways. The frequency of random events can readily be predicted and tested; however, the location of the occurrences associated with events is, and likely will remain, a probabilistic phenomenon. Such aberrant features are what gives rise to most of the inclusions in map units because their occurrence cannot be predicted and mapped with models even at larger scales. It is the nonsystematic features that make all models approximations of what actually takes place. The composition of map units can never be known. It can only be approximated from samples of them.

It is common to employ transects to estimate the composition of map units. The first aspect of composition is to identify the taxonomic components because they are the things that we have learned to identify and recognize. These can be translated or interpreted as responses or properties or whatever has an acceptable relationship. If results are not satisfactory or favorable, it is because the models are being used beyond their capability. As long as one is aware of the probability of accuracy or known variability, then soil survey information can have relevant use.

Transecting carries with it some important assumptions when it is used as a means of purposive sampling for map unit composition. A major assumption is that the points observed along a line will be representative of areas on the land that are shown as map units. This line varies in utility with different landscapes and models developed for soil surveys.

Transects crossing landscape units must cross all parts of the landscape, rather than line up with known patterns of variability as this would unduly bias the results. For example, transects that follow small steam valleys overweigh the alluvial-colluvial material relative to what occurs on the hillslopes and uplands. The differences between accuracy of line and point transects for grid points or sample plots is not the same for all landscapes and is better determined by some preliminary checking. Point transects have proven to be satisfactory for most surveys in the United States. Observations made at points along a transect are usually identified as belonging to a class... often a particular taxon, but they could be a combination of properties such as depth, a thickness, a color, or a structure. Because the assumption that point-to-area relationships are satisfactory, the number of observations in the various classes are handled as samples, and statistical inferences are made about the mean values and the ranges that are thought to exist.

Pedon and Polypedon

In soil surveys the individual parts that make up the soil continuum are classified. The classes are defined to include bodies of soil of significant kinds and sizes. The classes are concepts, not real soils, but they are related to their representatives in nature—the pedon and the polypedon.

Pedon.—The pedon is presented in Soil Taxonomy (Soil Survey Staff, 1975) as a unit of sampling within a soil. The limits on the area of a pedon establish rules for deciding whether to consider one or two or more kinds of soil within a small-scale pattern of local lateral variability. A pedon is regarded as the smallest body of one kind of soil large enough to represent the nature and arrangement of horizons and variability in the other properties that are preserved in samples.

A pedon extends down to the lower limit of a soil. It extends through all genetic horizons and, if the genetic horizons are thin, into the upper part of the underlying material. The pedon includes the rooting zone of most native perennial plants. For purposes of most soil surveys, a practical lower limit of the pedon is bedrock or a depth of about 2 m, whichever is shallower. A depth of 2 m provides a good sample of major soil horizons, even in thick soil. It includes much of the volume of soil penetrated by plant roots, and it permits reliable observations of soil properties.

The surface of a pedon is roughly polygonal and ranges from 1 m² to 10 m² in area, depending on the nature of the variability in the soil. Where the cycle of variations is less than 2 m and all horizons are continuous and nearly uniform in thickness, the pedon has an area of approximately 1 m². Where horizons or other properties are intermittent or cyclic over an interval of 2 to 7 m, the pedon includes one-half of the cycle (1 to 3 1/2 m). If horizons are cyclic over an interval greater than 7 m, each cycle is considered to contain more than one soil. The range in size, 1 to 10 m², permits consistent classification by different observers where important horizons are cyclic or repeatedly interrupted over short distances.

Polypedon.—The pedon is considered too small to exhibit more extensive features, such as slope and surface stoniness. The polypedon is presented in Soil Taxonomy as a unit of classification, a soil body, homogeneous at the series level, and big enough to exhibit all the soil characteristics considered in the description and classification of soils (fig. 2-1).

In practice, the concept of polypedon has been largely ignored and many soil scientists consider a pedon or some undefined body of more or less similar soil represented by a pedon large enough to classify. Polypedons seldom, if ever, serve as the real thing we want to classify because of the extreme difficulty of finding the boundary of a polypedon on the ground and because of the self-contradictory and circular nature of the concept. Soil scientists have classified pedons, regardless of their limited size, by deliberately or unconsciously transferring to the pedon any required extensive properties from the surrounding area of soil.

George Holmgren incorporated this pragmatic, flexible view of the pedon in his proposal of the point pedon which combines the fixed position of a pedon with consideration of whatever area is needed to identify and measure the properties under consideration (Holmgren, 1988). This concept, combined with criteria for the scale of lateral variability to be considered within one kind of soil, could establish the pedon as the basic unit of classification and eliminate the need for the polypedon; however, the term "polypedon" will be used in this manual.

Polypedons link the real bodies of soil in nature to the mental concepts of taxonomic classes.

Soil Series

The soil series category is the most homogeneous category in the taxonomy used in the United States. As a class, a series is a group of soils or polypedons that have horizons similar in arrangement and in differentiating characteristics. The soils of a series have a relatively narrow range in sets of properties.

Soil series are differentiated on all the differentia of the higher categories plus those additional and significant characteristics in the series control section. Some of the characteristics commonly used to differentiate series are the kind, thickness, and arrangement of horizons and their structure, color, texture, reaction, consistence, content of carbonates and other salts, content of humus, content of rock fragments, and mineralogical composition. A significant difference in any one of these can be the basis for recognizing a different series. Very rarely, however, do two soil series differ in just one of these characteristics. Most characteristics are related, and generally several change together.

New series, variants, and taxadjuncts

Some soils are outside the limits of any recognized soil series and have unique sets of properties. These are potential new series. When such a soil is first recognized, it is described and identified as a taxon of the lowest category in which it can be classified. A phase of that taxon can be used to identify a map unit. In some surveys, including virtually all detailed surveys, greater refinement of definition is needed. For these, the soil is proposed as a new series, but the new series remains tentative until its properties can be described in detail, its extent determined, and any conflicts with established series resolved. If the soil proves to be unique and significant in extent, it is established as a new series.

Before October 1988, a soil that had characteristics outside the limits of any defined series and was less than 800 hectares (2,000 acres) in extent was designated as a variant. Variants differed enough in one or more properties from the series for which they were named that major interpretations for comparable phases were different. They were named by adding the word "Variant" to the name of a closely related series, preferably one within the survey area. Variants were potential soil series, and the soil was established as a new series if a significant area of a variant was eventually recognized.

Taxadjuncts are polypedons that have properties outside the range of any recognized series and are outside higher category class limits by one or more differentiating characteristics of the series. The differences in properties are small so that major interpretations are not affected. A taxadjunct is given the name of an established series that is most similar in characteristics. It is an adjunct to, but not part of, the named series. It is treated as if it were a member of the named series, and its interpretations are similar to those for comparable phases of the series for which it is named. The difference from the established series is described. Example: A potential series is in a fine-silty family particle size class, marginal to fine-loamy; however, it differs from an established fine-loamy series in only particle size and no appropriate fine-silty series has been identified. The potential series is given the name of the established series, and a new series is not proposed.

Phases

If a property of a taxon has too wide a range for the interpretations needed or if some feature outside the soil itself is significant for use and management, these are bases for defining phases. Phases commonly include only part of the range of features exhibited by a taxon, but phases can be based on attributes such as frost hazard, character of the deeper substratum, or physiographic position that are not characteristics used to identify taxa but, nevertheless, affect use and management. If these vary from place to place within the survey area, phases can be defined to accommodate the differences.

A soil map unit that bears the name of a phase of a taxon consists dominantly of that phase of the taxon, but it also includes other soil components. The other components are included because of the limitations imposed by the scale of mapping and the number of points that can be examined. When the limits of soil taxa are superimposed on the pattern of soil in nature, areas of taxonomic classes rarely, if ever, coincide precisely with mappable areas. Some polypedons are too small to be drawn on the map and are included in delineations and named for another soil. The boundaries between polypedons are not always so obvious that they can be plotted precisely on a map, so part of one polypedon is commonly included in the delineation of an adjacent polypedon. Some polypedons are so intimately intermingled that mappable areas are necessarily identified in terms of two or more taxa. Other polypedons are not easily distinguished from similar adjacent ones and are inadvertently or deliberately included in delineations named for other soils because apparent differences in use and management are small.

Classes of soil properties are not necessarily used directly as phases. Defined class limits of properties are designed for a convenient description of soil, and they can also be used to define phases of soil where appropriate. But they are not useful for all soils. Distinctions significant for one kind of soil are not significant for every other kind. Any single property is significant only through its interactions with other properties. The usefulness of each phase must be repeatedly tested and verified during a survey. Separate phases of a taxon must differ significantly in behavior. If no useful purpose is served by separating them in mapping, similar phases of different taxa may be combined, and the combination described. The interpretations prepared during the course of a survey provide evidence of similarities and differences among map units.

The justification for most phases rests on the behavior of the soils under use. At least one statement about soil behavior must be unique to each phase of a taxon, and the differences of soil properties must exceed normal errors of observation.

Miscellaneous Areas

Some land areas have little or no soil and thus support little or no vegetation without major reclamation. Rock outcrop is an example. Such areas are called miscellaneous areas. The names of the different kinds of miscellaneous areas (discussed later) are used in the same manner as the names of soil taxa to identify map units.

Map Units

A map unit is a collection of areas defined and named the same in terms of their soil components or miscellaneous areas or both. Each map unit differs in some respect from all others in a survey area and is uniquely identified on a soil map. Each individual area on the map is a delineation.

Map units consist of one or more components. An individual component of a map unit represents the collection of polypedons or parts of polypedons that are members of the taxon or a kind of miscellaneous area. Parts of polypedons are common when phases are used to divide a taxon. Classes of miscellaneous areas are treated the same as soil taxa in soil surveys. A taxonomic unit description describes the ranges in soil properties exhibited in the polypedon for the maps in a survey area that are referenced by that taxonomic unit. The limits of these ranges are set for the taxonomic class of which a taxonomic unit is a member, but generally the full range allowed by the taxonomic class is not exhibited in a small survey area (<200,000 ha).

A delineation of a map unit generally contains the dominant components in the map unit name, but it may not always contain a representative of each kind of inclusion. A dominant component is represented in a delineation by a part of a polypedon, a complete polypedon, or several polypedons. A part of a polypedon is represented when the phase criteria, such as a slope, requires that a polypedon be divided. A complete polypedon is present when there are no phase criteria that require the subdivision of the polypedon or the features exhibited by the individual polypedon do not cross the limits of the phase. Several polypedons of a component may be represented if the map unit consists of two or more dominant components and the pattern is such that at least one component is not continuous but occurs as an isolated body or polypedon. Similarly, each inclusion in a delineation is represented by a part of a polypedon, a complete polypedon, or several polypedons. Their extent, however, is small relative to the extent of the dominant component(s). Soil boundaries can seldom be shown with complete accuracy on soil maps, hence parts and pieces of adjacent polypedons are inadvertently included or excluded from delineations.

A few delineations of some map units may not contain any of the dominant components named in the map unit description, but contain very similar soils. In most survey areas there are a few soils that occur as mappable bodies, but they have very limited total extent. They are normally included with other map units, if, for all practical purposes, interpretations are the same.

The kinds of map units used in a survey depend primarily on the purposes of the survey and the pattern of the soils and miscellaneous areas in the landscape. The pattern in nature is fixed and it is not exactly the same in each delineation of a given map unit. In soil surveys these patterns must be recognized and map units designed to meet the major objectives of the survey. It must be remembered that soil interpretations are made for areas of land and the most useful map units are those that group similarities.

Designing Map Units

While studying the soil patterns in different landscapes, the soil scientist must keep in mind how best to relate the patterns observed to appropriate map units. The kinds of map units, the level of soil taxa, and the phases needed to satisfy the survey objectives must be determined. This requires many judgements. Every map unit that is tentatively identified is evaluated by two tests: (1) Can it be mapped consistently? (2) Is it needed to meet the objectives of the survey?

Designing map units to indicate significant differences in behavior among soils is particularly important to meet the current objectives of a survey. Reflecting differences in genesis and morphology is also important, even if no immediate differences in interpretations are known. Differences in soil properties that do not affect current interpretations may be important in the future; however, having too many delineations seriously reduces the immediate usefulness of a soil map. A potential benefit must be weighed carefully against the costs incurred in making additional separations. One objective of every soil survey is to record knowledge about soils, but this does not mean that the soil map must show the location of every kind of soil in a survey area or that the publication must record all that has been learned about the soils.

Taxonomic classes provide the basic sets of soil properties with which soil map units are defined. They summarize an immense amount of research and experience related to the significance of soil properties and combinations of properties. They provide predefined sets of soil properties that have been tested for genetic relationships and for interpretative value. Taxa provide a firm base for recognizing the components of potential map units in an unfamiliar area. Using established taxa is much easier than independently sorting out sets of properties and determining significant class limits.

The objectives of a survey determine the kind of map units and the taxonomic level used to identify components of map units. For the more detailed surveys, decisions must be made about what criteria to use to recognize phases of soil series, how broadly or narrowly to define the phases, and whether similar phases of different series have such similar interpretations that they can be combined. For the less detailed surveys, decisions must be made about how the complexities of soil in large areas can be best identified for purposes of the survey, what combinations of soils characterize useful and mappable units, what taxonomic level should be used in naming map units, and which phases contribute to the usefulness of the map units.

The names of soil taxa, along with one or more modifying terms are used to identify the soils in map units. For example, the name "Tama silt loam, 2 to 5 percent slopes," indicates that soils of the Tama series (a Udoll) are dominant in that map unit. The names of taxa of higher categories are also used in map unit names, especially on small scale maps. "Udolls, rolling," for example, identifies a map unit consisting dominantly of soils of the Udoll suborder, which includes Tama and other series. The name of a taxon of the lowest category that accurately identifies the dominant soil is commonly used.

Within each survey, soil maps can be designed with component taxa of low or high categories that reflect narrowly or broadly defined ranges of soil properties. In addition, soil map units can be designed with different compositions of soil taxonomic units and mapping inclusions. This flexibility permits the design of map units that will be most useful for the purposes of a specific survey as well as for the attainment of as much uniformity in mapping as possible.

As methods of measuring soil properties are refined, as experience in the field increases, and as use and management requirements are intensified, progressively narrower ranges in soil properties can be recognized or established. Narrow ranges of properties are not established just because methods permit it. Unnecessary separations are time consuming to delineate consistently, and they make the survey difficult to use. Not separating two significantly different, mappable units, however, makes a survey less useful. The significance of each map unit in meeting the objectives of the survey must be constantly evaluated during the mapping process.

Improper use of phases to designate map units and misinterpretations of soil survey procedures can result in unreasonably detailed soil maps delineating unnecessary map units or ones in less detail than is needed to accomplish the objectives of the survey. Using pre-established classes of selected soil properties—surface texture, depth, slope, accelerated erosion, and stoniness—as phase criteria and then using all combinations of these in defining phases, creates problems. Meaningless map units cause an unnecessary expense.

Phase distinctions must be compatible with natural variability. To illustrate, a series may range in depth to bedrock from 1.5 m to more than 2 m. For some uses, a separation at 1.5 m would be significant. If within a survey area the soils of a series range in depth to bedrock from slightly less than 1.75 m to slightly more, designating two depth phases cannot be justified. The mapping is likely to be inconsistent and the difference of a few centimeters in depth is likely to be of minor significance. In this case it is far better to designate only one phase on the basis of depth to bedrock. The description of the map unit should, of course, give the depth range.

Another example: In some areas a slope of 8 percent is about the upper limit for cropping many soils without special practices for erosion control; yet, in some series a large part of the soil has slopes of less than 3 percent, most of the rest has slopes of 6 to 10 percent, and a small acreage has slopes of 3 to 6 percent. Dividing phases of such soils at 8 percent slope would produce a large number of delineations having a gradient a little below 8 percent and a large number having a gradient a little above 8 percent. The differences in interpretations of the phases thus defined would probably not be consistently significant. For these soils, slope phases could be set at 0 to 3 percent, 3 to 6 percent, and 6 to 10 percent; or, if there is little or no significance of the break at 3 percent, they could be set at 0 to 6 percent and 6 to 10 percent.

Phases must also be compatible with practical needs. In a hypothetical survey area that is relevant to farming, the polypedons of Alpha soils are similar in all properties except stoniness and slope. The areas range from nearly stone-free to very stony and from undulating to steep. The most important single distinction for farming is the distinction between areas that can be cultivated feasibly and areas that cannot. As many as three mappable classes of stoniness could be combined with four mappable classes of slope—a total of 12 potential phases. Four of these twelve phases might be used to distinguish combinations of degrees of slope and degrees of stoniness within the limits that permit cultivation. Using the remaining eight to subdivide the nonarable areas would confuse the user with unnecessary detail more than it would help. Perhaps two, and probably no more than three, phases are adequate for all significant distinctions among nonarable areas if the survey area is to be used primarily for farming. A list of potentially useful phases is as follows:

1. Arable areas:
   1. An undulating, nonstony phase
   2. An undulating, moderately stony phase
   3. A rolling, nonstony phase
   4. A rolling, moderately stony phase
2. Nonarable areas:
   1. An undulating and rolling, very stony phase
   2. A hilly phase (ranging from nonstony to very stony)
   3. A steep phase (ranging from nonstony to very stony)

If the soil map is to be used for planning operations related to forestry or other nonfarming activities, other distinctions may be needed.

The object of surveying soils is to gather relevant facts, record the facts on maps, and then interpret the facts. Field records and observations together with other relevant information must be coordinated in defining phases and map units that meet the objectives of a soil survey.

Kinds of Map Units

Soils differ in size and shape of their areas, in degree of contrast with adjacent soils, and in geographic relationships. Four kinds of map units are used in soil surveys to show the relationships: consociations, complexes, associations, and undifferentiated groups.

Consociations—In a consociation, delineated areas are dominated by a single soil taxon (or miscellaneous area) and similar soils. As a rule, at least one-half of the pedons in each delineation of a soil consociation are of the same soil components that provide the name for the map unit¹. Most of the remainder of the delineation consists of soil components so similar to the named soil that major interpretations are not affected significantly. The total amount of dissimilar inclusions of other components in a map unit generally does not exceed about 15 percent if limiting and 25 percent if nonlimiting. A single component of dissimilar limiting inclusion generally does not exceed 10 percent if very contrasting. The amount of dissimilar inclusions in an individual delineation of a map unit can be greater than this if no useful purpose would be served by defining a new map unit. The soil in a consociation may be identified at any taxonomic level.

A consociation named for a kind of miscellaneous area is dominated by the kind of area for which it is named to the extent that inclusions do not significantly affect the use of the map unit. Generally, this means that less than about 15 percent of any delineation is soil or less than about 25 percent is other kinds of miscellaneous areas. Percentages may vary, depending on the kind of miscellaneous area and the kind, size, and pattern of the inclusions.

Complexes and associations—Complexes and associations consist of two or more dissimilar components occurring in a regularly repeating pattern. Only the following arbitrary rule related to mapping scale determines whether the name complex or association should be used. The major components of a complex cannot be mapped separately at a scale of about 1:24,000 (fig. 2-2). The major components of an association can be separated at a scale of about 1:24,000 (fig. 2-3). In either case, the major components are sufficiently different in morphology or behavior that the map unit cannot be called a consociation. In each delineation of either a complex or an association, each major component is normally present, though their proportions may vary appreciably from one delineation to another. The total amount of inclusions in a map unit that are dissimilar to any of the major components does not exceed about 15 percent if limiting and 25 percent if nonlimiting, and a single kind of dissimilar limiting inclusion generally does not exceed 10 percent if very contrasting.

Undifferentiated groups—Undifferentiated groups consist of two or more taxa components that are not consistently associated geographically and, therefore, do not always occur together in the same map delineation. These taxa are included as the same named map unit because use and management are the same or very similar for common uses. Generally, they are included together because some common feature such as steepness, stoniness, or flooding determines use and management. If two or more very steep soils geographically separated are so similar in their potentials for use and management that defining two or more additional map units would serve no useful purpose, they may be placed in the same unit. Every delineation has at least one of the major components and some may have all of them. The same principles regarding proportion of inclusions apply to undifferentiated groups as to consociations.

Inclusions Within Map Units

In all soil surveys, virtually every delineation of a map unit includes areas of soil components or miscellaneous areas that are not identified in the name of the map unit. Many areas of these components are too small to be delineated separately. The location of some components cannot be identified by practical field methods. Some mapping inclusions are deliberately placed in delineations identified as another map unit to avoid excessive detail of the map or the legend.

Inclusions reduce the homogeneity of map units and may affect interpretations. The objective is to define map units that will contain as few inclusions as practical of components that behave differently from the naming components. Also, map units must be so defined that they can be recognized and delineated consistently in the field.

The number of inclusions reflects the taxonomic purity of map units. The number and degree of contrast of inclusions with the reference taxa can be used to estimate the interpretative purity of map units. The actual amount of inclusions is estimated from observations made during the survey. Adjustments in mapping are made if appropriate.

In the definition of map units, judgement must be exercised about the effects of inclusions on management and about how much effort is justified to keep the amount of inclusions small. In exercising these judgements, visualizing two kinds of differences between components is useful. If differences are small, the components are compared as similar. If differences are large, the components are contrasted as dissimilar.

Similar components are alike or much alike in most properties and share limits of those diagnostic properties in which they differ. Differences are beyond the limits of the reference taxon or phase class, but they generally are within or slightly beyond normal errors of observation. Because only a few limits are shared or the range is small, interpretations for most common uses are alike or reasonably similar and the interpretative value of a map unit is not affected.

Dissimilar components on the other hand, differ appreciably in one or more properties, and the differences generally are great enough to affect major interpretations. Some dissimilar components are limiting, and others are nonlimiting relative to the interpretations being considered.

If an inclusion does not restrict the use of entire areas or impose limitations on the feasibility of management practices, its impact on predictions for the map unit is small. Inclusions of soil components that have less severe restrictions on use than the dominant soil of a map unit do not adversely affect predictions about the unit as a whole. They may even be beneficial. Such inclusions are nonlimiting and the interpretative purity of a map unit for most interpretations is not altered.

For example, the inclusion of many small areas having slopes of 4 to 8 percent in an area having slopes mainly of 15 to 25 percent has no adverse effect on use of the area for most purposes; however, if an inclusion has significantly lower potential for use than the dominant component in the map unit or affects the feasibility of meeting management needs, a small amount in a map can affect predictions greatly. These are the most critical inclusions because they decrease the interpretative purity of map units. Even a small area having slopes of 15 to 25 percent in a map unit dominated by slopes of 4 to 8 percent can seriously affect the use of the area for many purposes. Even small inclusions of Typic Epiaqualfs in areas of Aquic Hapludalfs, for an example, may control and limit the uses of the dominant soil.

Soils that cannot be used feasibly for the same purposes as the surrounding soils are especially critical. They are separately delineated if the map scale permits it and if showing them will improve the usefulness of the map for the major anticipated uses. Areas too small to delineate may be identified and located on the map by special symbols.

Naming Map Units

All map units in a soil survey are named. Different conventions are used for each of the four kinds of map units so that the kind of unit can be determined at a glance. In general, names are as short as is practical; the name of a map unit should be only as long as is necessary to distinguish it from all others in the survey. At times an extra term, not needed to distinguish a phase from all others in the survey, is used so that comparable phases in other areas have the same name.

Although some of the conventions for naming map units are discussed in this chapter, a more complete discussion is provided in the National Soils Handbook (Soil Conservation Service). Soil names indicate the categorical level used for identification, but soil taxonomic names are never used alone.

Phases are groupings created to serve specific purposes in individual soil surveys. Phases can be defined for any class or classes of any category. The classes are helpful in describing the soil phases that are important for the survey. Differences in soil or environmental features that are significant to use, management, or behavior are the bases for designating soil phases.

Any property or combination of properties that does not duplicate class limits for a taxon can be used to differentiate phases, and any value of a property can be set to divide phases. The choice of properties and limits are determined by the purpose of the survey and by how consistently the phase criteria can be applied. Because objectives differ from one soil survey to another, limits and ranges of a property or attribute may also differ from one survey to another. In general, phase criteria are given a smaller range where soil use is intensive (as for irrigated farming or urban development) and a larger range where use is extensive (as for forestry or grazing).

The attributes most commonly used in defining phases in soil surveys are:

Texture of the surface layer.—As the surface layer has special significance for use of the soil, texture of the surface layer is commonly indicated in the names of consociations bearing the names of soil series. These phases generally identify the dominant texture of a mineral layer about equal in thickness to that commonly mixed in tillage, which is generally 12 to 25 cm. If the layer has not been mixed, the texture that would be produced by mixing is estimated. If, after mixing, the layer is organic, terms for organic material are used to name the phase. In some areas such as deserts, where the surface layer is normally thin and cultivation is unlikely, the texture of the A horizon can be used in naming phases even if it is less than 12 cm thick.

Some mineral soils have a thin layer at the surface that contrasts sharply with the next layer. Such soils may be designated as separate phases if they are unlikely to be tilled and if clearly significant to use or management. On rangeland, for example, the texture of the uppermost few centimeters of soil is important. A thin layer of sandy material at the surface may mean the difference between success and failure of seedings on some soils. For some surveys, such layers need to be recognized and their areas delineated. Moreover, a thin cover of silt or clay is an important distinction.

Conceivably, other kinds of phases based on texture could be useful in mapping, but they should be used only if the indicated property has a major and lasting effect on interpretations.

Deposits on the surface.—Some soils have received deposits of material thick enough to influence interpretations of the soil but not thick enough to change the classification. Depositional phases of the buried soils may be recognized:

Overblown: A recent deposit of wind-blown material on the surface of an older soil can be identified consistently throughout the area and is thick enough to influence use, management, or behavior.

Wind hummocky: Recent wind-laid deposits form a fine pattern of hummocks that markedly alter management requirements of the soil. The original soil is identifiable throughout most of the area, although it is covered in spots.

Overwash: Material deposited by water that contrasts with the underlying soil and is thick enough to influence management requirements significantly. Ordinarily, overwash phases are not used for very young alluvial soils that have weakly expressed genetic horizons.

Texture terms in the names of map units describe the material currently at the surface. Phases may be recognized for soils covered by a thin layer of volcanic ash. Such phases are generally used only if needed to distinguish the phase from another phase that lacks the ash cover.

Rock fragments.—Rock fragments at the surface and in the surface layer are commonly used as phase distinctions. Kinds of rock fragments are defined by shape and size in chapter 3.

Phases of the smaller rock fragments accommodate most of the detailed phase distinctions that generally can be made accurately by field methods. The term "gravelly" is used in the examples of the names that follow, but the adjectives for each of the other kinds of rock fragments, such as cobbly or channery, are substituted as appropriate. The effect of 20 percent fine gravel on the use of arable soil, for example, is quite different from the effect of 20 percent flagstones; therefore, the phase limits may differ for larger fragments.

The following definitions are applicable to arable soils. For other uses—such as forestry, range, or recreation—the sizes, shapes, amounts, and mixtures of rock fragments have different significance. Pebbles, cobbles, and stones influence forestry much less than they do cultivation, although they could affect access and reforestation.

Nongravelly. The surface layer may contain enough pebbles to affect special uses that tolerate few if any rock fragments, but the pebbles do not interfere with the tillage of such field crops as corn. The volume is less than 15 percent. A slightly gravelly phase can be recognized for soils that are used for special purposes, such as growing turf.

Gravelly. The surface layer contains enough pebbles to interfere with tillage of common field crops. Generally, however, tillage is performed in the same manner and with the same equipment as for soils free of fragments. The pebbles are a nuisance. They may cause some equipment breakage, but they cause few major delays in field operations. The effects of the pebbles on the quality of tillage are small or moderate, depending on the kind of operation. The volume of pebbles is 15 to 35 percent.

Very gravelly. The surface layer contains enough pebbles to interfere seriously with the tillage of common field operations. The quality of tillage operations is affected. The kinds of crops that can be grown is restricted, the precision of planting and of fertilizer placement is reduced, and young plants are frequently covered during tillage. The volume of pebbles is 35 to 60 percent.

Extremely gravelly. The surface layer contains so many pebbles that tillage of the common field crops is often impractical, although not necessarily impossible. Tillage implements must force their way through a mass of pebbles that have fine earth between them. The volume of pebbles is more than 60 percent.

The rock fragments in the surface layer commonly span two or more size classes. The included fragments may be of more than one shape. The name of the kind of fragment that is judged most important in limiting the management of the soil is used in the phase designation. Generally, the largest fragments that are present in significant amounts are the most restrictive on soil use.

Classes of stoniness and boulderiness (chapter 3) are also used to define phases. The following phases of the larger rock fragments represent about the maximum detail that can be mapped consistently in most soil surveys. "Bouldery" is substituted for "stony" as appropriate.

Stony. The areas have enough stones at or near the surface to be a continuing nuisance during operations that mix the surface layer, but they do not make most such operations impractical. Conventional, wheeled vehicles can move with reasonable freedom over the area. Stones may damage both the equipment that mixes the soil and the vehicles that move on the surface. Usually, these areas have class 1 stoniness. If necessary in a highly detailed survey, these areas may be designated as "slightly stony" and "moderately stony."

Very stony. The areas have so many stones at or near the surface that operations which mix the surface layer either require heavy equipment or use of implements that can operate between the larger stones. Tillage with conventionally powered farm equipment is impractical. Wheeled tractors and vehicles with high clearance can operate on carefully chosen routes over and around the stones. Usually, these areas have class 2 stoniness.

Extremely stony. The areas have so many stones at or near the surface that wheeled powered equipment, other than some special types, can operate only along selected routes. Tracked vehicles may be used in most places, although some routes have to be cleared. Usually, these areas have class 3 stoniness.

Rubbly. The areas have so many stones at or near the surface that tracked vehicles cannot be used in most places. Usually, these areas have class 4 or 5 stoniness. If necessary in a highly detailed survey, they may be designated as "rubbly" and "very rubbly." If the soil has stones, boulders, and smaller fragments, the name includes the kind of rock fragment that are most limiting in the use or management of the soil. This is not necessarily the kind that is most abundant or the kind that is used to modify texture class of horizons in the profile description.

Rockiness. Map units, consisting of about 0.1 to 10 percent Rock outcrop, can be named either as "rocky" phases or as complexes or associations of soil and Rock outcrop. Map units consisting of more than 10 percent outcrop are normally named as complexes or associations of soil and Rock outcrop. Where rockiness phases are used, both "rocky" and "very rocky" phases can be named. Commonly, map units with less than 2 percent Rock outcrop are named "rocky" and those with 2 to 10 percent "very rocky."

Slope. The slope range of some soil taxa is narrow; in others, it is wide enough to include differences that are important for soil use and management. Slope phases are used to divide soil series or other taxa as may be needed for the purpose of the survey.

Slope gradient, complexity, shape, length, and aspect are all potential bases for phase distinctions. By far the most commonly used is gradient. Complexity is also used in many surveys. Slope length can often be appraised directly from delineations on the map. In many cases, the significance to use and management of slope length depends on the kind of landscape in which the soil occurs. Shape is seldom used as a phase distinction; differences in shape are commonly related to differences in internal properties that distinguish taxa. Slope aspect is used mainly in high latitudes.

Phases defined on the basis of slope should fit the landscape. They should be so distinct that they can be identified and mapped consistently without adding useless complexity to the map. In addition, such phases should separate areas that have significant differences in suitability or management needs.

A uniform system of slope classes should not be used indiscriminately as the basis for differentiating phases. Slope phases that have narrow ranges in gradient may be needed for soils that can be used intensively. For other soils having additional limitations, such as stoniness, these narrow ranges may be useless. The limits of slope phases should be based on data or experience in order to indicate the most useful distinctions for each kind of soil. Range in the slope of a phase of one series may encompass the ranges of two or more phases of another series. A single set of slope classes that would serve as phase distinctions for all soils is impractical because of the varied relationships of slope to mappable landscapes and to the use and management of different kinds of soils.

Table 3-1 defines slope classes in terms of flexible limits for both simple and complex slopes. The flexible limits permit use of terms to identify most distinctions of slope that may be needed. Names may use either numerical gradient limits, with or without designations of complexity, or descriptive terms. Slope terms for map units for taxa above the series are generally given in descriptive terms. The word "slopes" is used if gradient is specified as a percentage, but it is omitted if descriptive terms are used.

Depth.—Soil depth phases are used where variations in depth to a contrasting layer are significant to soil use, management, or behavior. Terms for depth classes in Chapter 3 are generally used in naming phases, but modifications are needed in some areas. For instance, the class "moderately deep," ranging from 50 to 100 cm, may be too broad to satisfy the objective of a particular survey. This range can be divided, with perhaps one class ranging in depth from 50 to 75 cm and the other from 75 to 100 cm, if the more narrowly defined phases occur in a consistent pattern within the survey area and can be mapped. Generally, the phase that covers the least acreage is given the depth designation. If the deeper phase is more extensive, "moderately shallow" is used to designate the shallower phase. If the shallower phase is more extensive, "moderately deep" is used to designate the deeper phase. In some surveys, using the standard class terms may be misleading. For example, if a series that is normally more than 175 cm deep to bedrock has a phase that is 150 to 175 cm deep, calling the less extensive phase "deep" could be construed to mean that the phase is deeper than normal. In such cases, depth limits can be specified in the phase name, or substratum phase terminology can be used instead.

Substratum.—Where underlying material contrasts sharply with the material above and interpretations are affected, substratum phases are used. The kind of contrasting material is indicated in the name of the map unit. Some examples of commonly used substratum phases are: gravelly substratum, sandy substratum, silty substratum, shale substratum, and till substratum. These terms are descriptive and not mutually exclusive. Where there is a choice between using a depth phase or a substratum phase to identify a map unit, a depth phase is generally used if the contrasting layer is bedrock.

Soil water.—Phases are used to distinguish differences in soil-water state, water table level, drainage, and the like where the range of the taxon in one or more of these properties needs to be divided for purposes of the survey. Significant differences in these factors are commonly reflected in differences in soil morphology and are distinguished at the series level. In some soils, however, evidence of wetness, such as redoximorphic features, does not fully reflect the natural drainage or wetness of the soil. These soils may not be differentiated at the series level with the refinement needed for the purposes of the survey.

Examples of soil water phases commonly used are: high water table, poorly drained, slightly wet, and drained. Some soils have properties that reflect former wetness, but they have been drained artificially; "drained" phases can be used to separate drained areas from undrained. In other soils, a water table fluctuates below the depth where properties are criteria for defining series; "water table" phases can be used to identify such soils.

Salinity.—Saline phases are used to distinguish between degrees of salinity that are important for soil use or management. Electrical conductivity values and observations of plant growth are guides for recognizing phases.

Designation of salinity phases depends on the various uses likely to be made of the soils and the effect of excessive amounts of salt on the uses. In farming areas, the crops most likely to be grown must be considered. Management induced salinity that fluctuates widely with management practices generally would not be a basis for phase distinctions.

Vegetation, especially the native cover, often shows the location of saline soils and their boundaries. Using vegetation, landform, and other features as guides and correlating these field observations with laboratory or field analyses of soil samples, the surveyor can usually draw boundaries with reasonable accuracy. Plants, however, vary in their tolerance of salt by species, variety, age, and perhaps other factors. Some plants are not good indicators of salinity because they grow well in soils that may have excessive amounts.

Other problems must also be considered in designating salinity phases. Different kinds of salts and combinations of salts have varied effects on soil behavior. In many soils, salts are transitory; in others, they are permanent. Excessive sodium may or may not be associated with excess salinity. The following classes of salinity, which are a general guide to naming phases, refer to the presence of salts in the soil. Salinity classes are defined in chapter 3.

Nonsaline: Effects of salinity on plant growth are negligible. Salinity is mainly class 0 and 1. "Nonsaline" is omitted from the names of mapping units unless the soil taxon is typically saline. A very slightly saline phase may be useful in some surveys for crops extremely sensitive to salts.

Slightly saline: Growth of many plants is affected. Yields of such plants as bromes, sunflower, corn, and peas are reduced seriously. Western wheatgrass, kale, and barley are affected little. Salinity is mainly class 2.

Moderately saline: Only plants tolerant of salinity, such as western wheatgrass, beets, and barley, grow well. Yields of these are commonly reduced. Salinity is mainly class 3.

Strongly saline: Only the most tolerant halophytic plants, such as saltgrass, grow well. Salinity is mainly class 4.

Terms for saline phases follow terms for surface texture in phase names.

Sodicity.—For some soils, recognizing a "sodic" phase is useful. The term "sodic" is used as a phase designation, if needed, generally without terms for degrees of sodicity.

Physiography.—Landform or physiographic position may be used as a phase criterion to distinguish phases of a single taxon. A soil in a deposit of loess 3 meters thick on a terrace, for example, may be so much like a soil in a similar deposit on a till plain that the two are members of the same taxon. For some uses, however, the two soils need to be distinguished on the map. A physiographic phase can be used to identify the less extensive soil.

Examples of terms that have been used to designate physiographic phases are: bench, depressional, fan, karst, ridge, and terrace. The terms generally identify phases that differ in position from what is typical for the soil. The typical physiography is not given in a phase name. The physiographic phase designation follows the term for surface texture and precedes any terms for slope or erosion.

Erosion.—Differences in soil potential for use, management needs, or performance because of accelerated erosion are a basis for recognizing phases. Phases of eroded soil are identified on the basis of the properties of the soil that remains, although the amount of soil lost is estimated and noted. In some places, erosion has changed the taxonomic classification of a soil.

Properties related to natural erosion are a part of the definition of a taxon, not bases for erosion phases. Erodibility, too, is an inherent quality of a soil and not itself a criterion for erosion phases.

Eroded phases are defined so the boundaries on the soil maps separate soil areas of unlike suitabilities and soil areas of unlike management needs and responses.

Guidelines for naming phases of soil that are eroded by water are as follows. Erosion classes are defined in chapter 3.

Slightly eroded: Erosion has changed the soil enough to require only slight modification of management from that of the uneroded soil; potential use and management remain generally the same. Most slightly eroded soils have class 1 erosion. Slightly eroded areas are not distinguished from uneroded areas in most surveys.

Moderately eroded: Generally, the plow layer consists of a mixture of the original A horizon and the underlying horizons. Most mapped areas of moderately eroded soils have patches in which the plow layer consists wholly of the original A horizon and others in which it consists wholly of underlying horizons. Shallow gullies may be present in some places. Erosion has changed the soil to such an extent that required management or the response to management differs in major respects from that of the uneroded soil. In most moderately eroded soils, ordinary tillage implements reach through the remaining A horizon or well below the depth of the original plowed layer. Most moderately eroded soils have class 2 erosion.

Severely eroded: Severely eroded phases commonly have been eroded to the extent that the plow layer consists essentially of material from underlying horizons. Patches in which the plow layer is a mixture of the original A horizon and underlying horizons may be present within some delineations. Shallow gullies, or a few deep ones, are common in some places. Erosion has changed the soil so much that (1) the eroded soil is suited only to uses significantly less intensive than the uneroded soil, such as use for pasture instead of crops; (2) the eroded soil needs intensive management immediately or over a long period to be suitable for the same uses as the uneroded soil; (3) productivity is reduced significantly; or (4) limitations for some major engineering interpretations are greater than on the uneroded soil. Most severely eroded soils have class 3 erosion.

A "gullied" phase can be recognized if gullied land occupies less than about 10 percent of the map unit. Gullied phases are used for areas having gullies so deep that intensive measures, including reshaping, are required to reclaim the soil. Where the areas are more than 10 percent gullied land, the map units are named as complexes or associations of soil and gullied land.

Guidelines for designating phases of soil eroded by wind are as follows:

Eroded (blown): Wind has removed enough soil that required management differs significantly from that of the uneroded soil, but suitabilities for use remain the same. The term "moderately" is understood.

Severely eroded (severely blown): Wind has removed much of the soil or has shifted it from place to place within the area. Suitability for use is different from that of the uneroded soil, unless extensive reworking is done and/or intensive management practices are used.

Many areas identified as moderately and severely wind eroded are, in fact, mixtures of small areas of uneroded soil and soil eroded to various degrees. The amount of erosion throughout a delineation can be described only in general terms.

Thickness.—The solum and the various horizons in soil have characteristic ranges in thickness for each taxon. Thickness phases may be used to divide the range of thickness of the solum or of the upper horizons. Phases are not used to differentiate thickness of the subsoil or the substratum. Four thickness phases are used:

Thick surface: The thickness of the A horizon or of the A and E horizons combined is within the thicker half of the range for the taxon.

Thin surface: The thickness of the A horizon or of the A and E horizons combined is within the thinner half of the range for the taxon.

Thick solum: The thickness of the solum is within the thicker half of the range for the taxon.

Thin solum: The thickness of the solum is within the thinner half of the range for the taxon.

A term is used for the less extensive of two thickness phases. For example, most delineations of a given soil may have an A horizon that is dominantly between 25 and 35 cm thick. If the A horizon is dominantly 35 to 40 cm thick in other delineations of the same soil and the difference is significant for purposes of the survey, a thick-surface phase can be recognized. The phase in which the A horizon is dominantly 25 to 35 cm thick is the norm; thickness of the A horizon is described for this phase but is not identified in the name.

Climate.—In some places, especially in mountainous or hilly areas, precipitation or air temperature can differ greatly within short distances, yet these differences may not be reflected in internal properties of the soil. Air drainage can differ enough from one location to another to produce a difference in the dates of the last killing frost in the spring or the first in the fall, or one area may be frost free. Climatic phases are used for these situations.

Only two climatic conditions are recognized for a given taxon: (1) the common climate, the climate that influences the greatest extent of the taxon, from which the climate designation is omitted, and (2) a departure from the common climate, for which a climatic designation is used. The departure may be in either of two directions from the norm: warm or cool, high precipitation or low precipitation. Each of the terms is connotative only in reference to the common climate of the taxon and must be described specifically for each phase to which it is applied.

In many places, especially on plains, precipitation or temperature changes gradually over distance. A soil in a single survey area commonly includes only part of the range for the series. Climatic phases generally are not used if only part of the range is within a soil survey area. Climatic phases are local distinctions. They are used where temperature or precipitation differs markedly between parts of a survey area.

Other.—A great variety of phase distinctions can be made. In addition to those already described, others may be needed to provide suitable map units; for example: frequently flooded, occasionally flooded, burned, calcareous, leached surface, dark surface. "Burned," for example, might be used for organic soils that have lost enough of their organic material by fire to alter their potential use or their management requirements.

The phases designated by special terms are defined to fit special kinds of soils. Such phases are defined according to the common properties of the taxon of which they are members. Thus, the terms usually have different specific meanings when used for different taxa and in different survey areas.

Miscellaneous Areas

Miscellaneous areas have essentially no soil and support little or no vegetation. This can be a result of active erosion, washing by water, unfavorable soil conditions, or man's activities. Some miscellaneous areas can be made productive but only after major reclamation efforts. Map units are designed to accommodate miscellaneous areas, and most map units named for miscellaneous areas have inclusions of soil. If the amount of soil exceeds the standards for inclusions defined in this chapter, the map unit is named as a complex or association of miscellaneous area and soil.

Following are discussions of recognized kinds of miscellaneous areas.

Badland is moderately steep to very steep barren land dissected by many intermittent drainage channels. Ordinarily, the areas are not stony. Badland is most common in semiarid and arid regions where streams cut into soft geologic material. Local relief generally ranges between 10 and 200 meters. Potential runoff is very high, and erosion is active.

Beaches are sandy, gravelly, or cobbly shores washed and rewashed by waves. The areas may be partly covered with water during high tides or storms.

Blown-out land consists of areas from which all or most of the soil material has been removed by extreme wind erosion. The areas are generally shallow depressions that have flat or irregular floors. In some places the floor is a layer of material that is more resistant to wind than the removed material or is a layer of pebbles or cobbles; or, the floor may have been formed by exposure of the water table. Areas covered by water most of the year are mapped as Water. Some areas have a few hummocks or small dunes. Few areas of blown-out land are large enough to be delineated; small areas can be shown by spot symbols.

Cinder land is composed of loose cinders and other scoriaceous magmatic ejecta. Water-holding capacity is very low, and trafficability is poor.

Cirque land consists of areas of rock and rubble in characteristically cirque shape. The shape is caused by glacial erosion.

Dumps are areas of smoothed or uneven accumulations or piles of waste rock and general refuse. Dumps, mine consist of areas of waste rock from mines, quarries, and smelters. Some dumps with closely associated pits are mapped as Dumps-Pits complex.

Dune land consists of sand in ridges and intervening troughs that shift with the wind.

Glaciers are large masses of ice formed, at least in part, on land by the compaction and recrystallization of snow. They may be moving slowly downslope or outward in all directions because of the stress of their own weight; or, they may be retreating or be stagnant. A little earthy material may be on or in the ice.

Gullied land consists of areas where erosion has cut a network of V-shaped or U-shaped channels. The areas resemble miniature badlands. Generally, gullies are so deep that extensive reshaping is necessary for most uses. Small areas can be shown by spot symbols. Phases indicating the kind of material remaining may be useful in some places.

Gypsum land consists of exposures of nearly pure soft gypsum. The surface is generally very unstable and erodes easily. Trafficability is very poor. Areas of hard gypsum are mapped as Rock outcrop.

Lava flows are areas covered with lava. In most humid regions, the flows are of Holocene age, but in arid and very cold regions they may be older. Most flows have sharp, jagged surfaces, crevices, and angular blocks characteristic of lava. Others are relatively smooth and have a ropy glazed surface. A little earthy material may be in a few rocks and sheltered pockets, but the flows are virtually devoid of plants other than lichens.²

Oil-waste land consists of areas where liquid oily wastes, principally saltwater and oil, have accumulated. It includes slush pits and adjacent areas affected by the liquid wastes. The land is barren, although some of it can be reclaimed at high cost.

Pits are open excavations from which soil and commonly underlying material have been removed, exposing either rock or other material. Kinds include Pits, mine; Pits, gravel; and Pits, quarry. Commonly, pits are closely associated with Dumps.

Playas are barren flats in closed basins in arid regions. Many areas are subject to wind erosion and many are saline, sodic, or both. The water table may be near the surface at times.

Quarries (see Pits).

Riverwash is unstabilized sandy, silty, clayey, or gravelly sediment that is flooded, washed, and reworked frequently by rivers.

Rock outcrop consists of exposures of bare bedrock other than lava flows and rock-lined pits. If needed, map units can be named according to the kind of rock: Rock outcrop, chalk; Rock outcrop, limestone; Rock outcrop, gypsum. Many rock outcrops are too small to be delineated as areas on soil maps but can be shown by spot symbols. Some areas are large, broken by only small areas of soil. Most rock outcrops are hard rock, but some are soft.

Rubble land consists of areas of cobbles, stones, and boulders. Rubble land is commonly at the base of mountains but some areas are deposits of cobbles, stones, and boulders left on mountainsides by glaciation or by periglacial processes.

Salt flats are undrained flats that have surface deposits of crystalline salt overlying stratified very strongly saline sediment. These areas are closed basins in arid regions. The water table may be near the surface at times.

Scoria land consists of areas of slag like clinkers, burned shale, and fine-grained sandstone remaining after coal beds burn out.(Scoria land should not be confused with volcanic slag.)

Slickens are accumulations of fine-textured material, such as that separated in placer-mine and ore-mill operations. Slickens from ore mills consist largely of freshly ground rock that commonly has undergone chemical treatment during the milling process. Slickens are usually confined in specially constructed basins.

Slick spots are areas having a puddled or crusted, very smooth, nearly impervious surface. The underlying material is dense and massive. The material ranges from extremely acid to very strongly alkaline and from sand to clay.

Urban land is land mostly covered by streets, parking lots, buildings, and other structures of urban areas.

Water includes streams, lakes, ponds, and estuaries that in most years are covered with water at least during the period warm enough for plants to grow; many areas are covered throughout the year. Pits, blowouts, and playas that contain water most of the time are mapped as Water.

Records and Definitions of Soil Taxa

Keeping definitions and names of soil taxa up to date is essential for identification of map units, for correlation of soils nationwide, and