Soil Taxonomy
1. 1. United States Department of Agriculture Natural Resources Conservation Service Soil Taxonomy A Basic System of Soil Classification for Making and Interpreting Soil Surveys Second Edition, 1999
2. 2. Soil Taxonomy A Basic System of Soil Classification for Making and Interpreting Soil Surveys Second Edition, 1999 By Soil Survey Staff United States Department of Agriculture Natural Resources Conservation Service Agriculture Handbook Number 436
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4. 4. 5 Table of Contents Foreword .................................................................................................................................... 7 Chapter 1: The Soils That We Classify ...................................................................................9 Chapter 2: Soil Taxonomy and Soil Classification ................................................................. 15 Chapter 3: Differentiae for Mineral Soils and Organic Soils ................................................. 19 Chapter 4: Horizons and Characteristics Diagnostic for the Higher Categories ................... 21 Chapter 5: Application of Soil Taxonomy to Soil Surveys .................................................. 115 Chapter 6: The Categories of Soil Taxonomy ...................................................................... 119 Chapter 7: Nomenclature .................................................................................................... 125 Chapter 8: Identification of the Taxonomic Class of a Soil .................................................. 159 Chapter 9: Alfisols .............................................................................................................. 163 Chapter 10: Andisols .............................................................................................................. 271 Chapter 11: Aridisols ............................................................................................................ 329 Chapter 12: Entisols .............................................................................................................. 389 Chapter 13: Gelisols .............................................................................................................. 445 Chapter 14: Histosols ........................................................................................................... 473 Chapter 15: Inceptisols ......................................................................................................... 489 Chapter 16: Mollisols ........................................................................................................... 555 Chapter 17: Oxisols ............................................................................................................... 655 Chapter 18: Spodosols ......................................................................................................... 695 Chapter 19: Ultisols .............................................................................................................. 721 Chapter 20: Vertisols ............................................................................................................. 783 Chapter 21: Family and Series Differentiae and Names ........................................................ 819 Chapter 22: Soils of the United States .................................................................................. 837 Chapter 23: World Distribution of Orders and Suborders .................................................... 851 Appendix ................................................................................................................................ 857 Index ....................................................................................................................................... 863 Maps of the United States and of the World
5. 5. 7 Foreword The second edition of Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys is the result of the collective experience and contributions of thousands of pedologists from around the world. This new edition includes many improvements. Two new soil orders, Andisols and Gelisols, are added. Low-activity clays are defined, and taxa are developed. The Aridisol, Alfisol, Histosol, Inceptisol, Mollisol, Oxisol, Spodosol, and Vertisol orders are updated. Aquic conditions, episaturation, and oxyaquic subgroups are defined. Additions and improvements are made at the family level. We are indebted to our many colleagues throughout the world who contributed soil descriptions and data, comments, suggestions, and criticisms. We are especially grateful to all of those who organized and hosted workshops and training sessions. Many pedologists provided input to the International Committees (ICOM’s), and we are thankful for their participation. Although we cannot list everyone who offered assistance, we do want to acknowledge the chairpersons of the various ICOM’s. ICOM Chairperson Institute Low Activity Clays ................ Frank Moormann ............. Univ. of Utrecht Oxisols ............... Stan Buol ......................... North Carolina State Univ. Andisols ............. Frank Leamy .................... Soil Bureau, Lower Hutt Aquic Soils ........ Johan Bouma ................... Agricultural Univ., Wageningen Spodosols ........... Robert Rouke ................... Univ. of Maine Vertisols ............. Juan Comerma ................. Univ. Centro Venezuela Aridisols ............ Ahmed Osman ................. Arab Center for the Studies of Arid Zones and Dry Lands Soil Families ...... Ben Hajek ........................ Auburn Univ. Gelisols .............. James Bockheim .............. Univ. of Wisconsin Although many improvements have been made since Dr. Guy Smith headed the effort to publish the first edition of Soil Taxonomy, there are still areas that will require a concerted effort to improve. The taxonomic system will continue to evolve as the science matures. The taxonomic system does not adequately address the anthropogenic effects on soils. Soils in urban/industrial areas can be drastically altered by landfills, farming, earth movement, and heavy metal contamination. Agricultural areas have undergone erosion, ripping, and land leveling. Drastically disturbed soils are common in regions where precious metals, rock aggregate, and fossil fuels have been mined. The International Committee on Anthropogenic Soils (ICOMANTH), chaired by Dr. Ray Bryant, is currently meeting the challenge of developing appropriate taxa for these unique soils. Soil moisture regimes and intergrades of soil moisture regimes need to be better defined. Some of the temperature regimes need refinement. The International Committee on Soil Moisture and Temperature Regimes (ICOMMOTR), chaired by Dr. Ron Paetzold, is gathering data to make needed improvements. The system of soil taxonomy currently does not provide for paleosols formed under remarkably different paleoenvironments. With age, the properties of soils from paleo and contemporaneous environments become welded. Yet, when paleosols are well preserved, they are valuable proxies of the biological and physiochemical evolution of the earth. Many paleosols are deeper than the 2 m limit set by the current system of soil taxonomy. There is now and will continue to be pressure to observe and classify soils beyond the 2 m limit. Many pedologists developed proposals, made comments and suggestions, and reviewed chapters for this second edition. Because of the concerted effort of many, the author of this publication is identified as the “Soil Survey Staff.” We would like to acknowledge those who helped write chapters or provide data for figures, maps, and tables. They include Dr. Arnt Bronger, Dr. Hari Eswaran, Dr. Samuel Indorante, Dr. John Kimble, Henry Mount, Loyal Quandt, Paul Reich, Sharon Waltman, and Dr. John Witty. Dr. Stanley Anderson had the arduous task of editing the second edition. Suzann Meierdierks and Dr. Patricia West provided their able assistance in the editing and formatting process. Adrian Smith,
6. 6. 8 Christopher Roll, and Nathan Kress provided invaluable GIS expertise. Lastly, Dr. Robert Ahrens coordinated the effort. He and Robert Engel worked tirelessly during the past few years to prepare this edition. Assistance in acquiring photographs for this publication was provided by the Kentucky Association of Soil Classifiers; the Washington Society of Professional Soil Scientists; the University of Nebraska Press and Andrew A. Aandahl; the Alaska/Yukon Society of Professional Soil Scientists; the Florida Association of Professional Soil Classifiers; the Society of Soil Scientists of Southern New England—Massachusetts; the Kansas Association of Professional Soil Classifiers; the Soil Classifiers Association of Michigan; the Professional Soil Classifiers Association of Alabama; the Professional Soil Scientists Association of Texas; and members of the National Cooperative Soil Survey. Horace Smith Director, Soil Survey Division
7. 7. 9 CHAPTER 1 The Soils That We Classify T he word “soil,” like many common words, has several meanings. In its traditional meaning, soil is the natural medium for the growth of land plants, whether or not it has discernible soil horizons. This meaning is still the common understanding of the word, and the greatest interest in soil is centered on this meaning. People consider soil important because it supports plants that supply food, fibers, drugs, and other wants of humans and because it filters water and recycles wastes. Soil covers the earth’s surface as a continuum, except on bare rock, in areas of perpetual frost or deep water, or on the bare ice of glaciers. In this sense, soil has a thickness that is determined by the rooting depth of plants. About 1870, a new concept of soil was introduced by the Russian school led by Dokuchaiev (Glinka, 1927). Soils were conceived to be independent natural bodies, each with a unique morphology resulting from a unique combination of climate, living matter, earthy parent materials, relief, and age of landforms. The morphology of each soil, as expressed by a vertical section through the differing horizons, reflects the combined effects of the particular set of genetic factors responsible for its development. This was a revolutionary concept. One did not need to depend wholly on inferences from the underlying rocks, the climate, or other environmental factors, considered singly or collectively; rather, the soil scientist could go directly to the soil itself and see the integrated expression of all these in its morphology. This concept made it not only possible but also necessary to consider all soil characteristics collectively, in terms of a complete, integrated, natural body, rather than individually. Thus, the effect of any one characteristic or a difference in any one depends on the others in the combination. Experience has shown that no useful generalizations about single characteristics can be made for all soils. Characteristics are given weight according to the knowledge gained through research and experience in soil genesis and the responses of soil to management or manipulation. Both research in genesis and the responses of soils have vital roles, but they are themselves one step removed from the taxonomy of the soil, which is based on combinations of soil characteristics. In short, the new concept made pedology possible. The Russian view of soils as independent natural bodies that have genetic horizons led to a concept of soil as the part of the earth’s crust that has properties reflecting the effects of local and regional soil-forming agents. The solum in that concept is the set of genetic horizons developed by soil-building forces, but the parent material beneath is nonsoil. This concept has limitations. If a solum is 1 or 2 m thick, there is little conflict between the concept of soil as solum and the concept of soil as the natural medium for the growth of terrestrial plants. If genetic horizons are thin or absent and unconsolidated parent material lies at or only a few centimeters below the surface, there is serious conflict between the concepts. Dokuchaiev realized this conflict and, despite the lack of horizons, included young alluvium and peat in his classification of soil. Soil in this text is a natural body comprised of solids (minerals and organic matter), liquid, and gases that occurs on the land surface, occupies space, and is characterized by one or both of the following: horizons, or layers, that are distinguishable from the initial material as a result of additions, losses, transfers, and transformations of energy and matter or the ability to support rooted plants in a natural environment. This definition is expanded from the previous version of Soil Taxonomy to include soils in areas of Antarctica where pedogenesis occurs but where the climate is too harsh to support the higher plant forms. The upper limit of soil is the boundary between soil and air, shallow water, live plants, or plant materials that have not begun to decompose. Areas are not considered to have soil if the surface is permanently covered by water too deep (typically more than 2.5 m) for the growth of rooted plants. The horizontal boundaries of soil are areas where the soil grades to deep water, barren areas, rock, or ice. In some places the separation between soil and nonsoil is so gradual that clear distinctions cannot be made. The lower boundary that separates soil from the nonsoil underneath is most difficult to define. Soil consists of the horizons near the earth’s surface that, in contrast to the underlying parent material, have been altered by the interactions of climate, relief, and living organisms over time. Commonly, soil grades at its lower boundary to hard rock or to earthy materials virtually devoid of animals, roots, or other marks of biological activity. The lowest depth of biological activity, however, is difficult to discern and is often gradual. For purposes of classification, the lower boundary of soil is arbitrarily set at 200 cm. In soils where either biological activity or current pedogenic processes extend to depths greater than 200 cm, the lower limit of the soil for classification purposes is still 200 cm. In some instances the more weakly cemented bedrocks (paralithic materials, defined later) have been described and used to differentiate soil series (series
8. 8. 10 control section, defined later), even though the paralithic materials below a paralithic contact are not considered soil in the true sense. In areas where soil has thin cemented horizons that are impermeable to roots, the soil extends as deep as the deepest cemented horizon, but not below 200 cm. For certain management goals, layers deeper than the lower boundary of the soil that is classified (200 cm) must also be described if they affect the content and movement of water and air or other interpretative concerns. In the humid tropics, earthy materials may extend to a depth of many meters with no obvious changes below the upper 1 or 2 m, except for an occasional stone line. In many wet soils, gleyed soil material may begin a few centimeters below the surface and, in some areas, continue down for several meters apparently unchanged with increasing depth. The latter condition can arise through the gradual filling of a wet basin in which the A horizon is gradually added to the surface and becomes gleyed beneath. Finally, the A horizon rests on a thick mass of gleyed material that may be relatively uniform. In both of these situations, there is no alternative but to set the lower limit of soil at the arbitrary limit of 200 cm. Soil, as defined in this text, does not need to have discernible horizons, although the presence or absence of horizons and their nature are of extreme importance in soil classification. Plants can be grown under glass in pots filled with earthy materials, such as peat or sand, or even in water. Under proper conditions all these media are productive for plants, but they are nonsoil here in the sense that they cannot be classified in the same system that is used for the soils of a survey area, county, or even nation. Plants even grow on trees, but trees are regarded as nonsoil. Soil has many properties that fluctuate with the seasons. It may be alternately cold and warm or dry and moist. Biological activity is slowed or stopped if the soil becomes too cold or too dry. The soil receives flushes of organic matter when leaves fall or grasses die. Soil is not static. The pH, soluble salts, amount of organic matter and carbon-nitrogen ratio, numbers of microorganisms, soil fauna, temperature, and moisture all change with the seasons as well as with more extended periods of time. Soil must be viewed from both the short-term and long-term perspective. Buried Soils A buried soil is covered with a surface mantle of new soil material that either is 50 cm or more thick or is 30 to 50 cm thick and has a thickness that equals at least half the total thickness of the named diagnostic horizons that are preserved in the buried soil. A surface mantle of new material that does not have the required thickness for buried soils can be used to establish a phase of the mantled soil or even another soil series if the mantle affects the use of the soil. Any horizons or layers underlying a plaggen epipedon are considered to be buried. Soil Taxonomy A surface mantle of new material, as defined here, is largely unaltered, at least in the lower part. It may have a diagnostic surface horizon (epipedon) and/or a cambic horizon, but it has no other diagnostic subsurface horizons, all defined later. However, there remains a layer 7.5 cm or more thick that fails the requirements for all diagnostic horizons, as defined later, overlying a horizon sequence that can be clearly identified as the solum of a buried soil in at least half of each pedon. The recognition of a surface mantle should not be based only on studies of associated soils. The Pedon, a Unit of Sampling Few soil properties can be determined from the surface. To determine the nature of a soil, one must study its horizons, or layers. This study requires pits or some means of extracting samples of material from the surface to the base of the soil. The visible and tactile properties of samples can be studied in the field. Soil moisture and temperature regimes are studied by observations of changes over time at points selected to be representative. Other properties of a soil must be learned by studies of samples in an appropriate place, usually a laboratory. In other words, one learns about most of the properties of a soil by studying samples extracted to represent a sampling unit, not by study of the whole soil body that is classified. A concept of what to sample must be developed before soils can be classified in a manner that meets the needs of the soil survey, and different concepts might lead to different classifications. The concept presented in this text is not the only one possible, and, in fact, its logic has been scrutinized (Holmgren, 1988). A soil commonly is not uniform in all its properties. Variability may be due to accidents; events that lack definite order, such as the development of fractures in a hard rock; variations in deposits left by running water; or the placement of seeds by wind or by animals. The influence of the biotic factors tends to produce many examples of variability in a soil. Burrowing animals, taprooted plants, falling trees, and plants that collect different elements do not operate uniformly over large areas. A filled burrow or a trace left by a taproot can result in holes in horizons filled by contrasting materials. Salts collected by a desert shrub remain concentrated below the shrub until it dies. Shrink-swell and freeze-thaw processes are other factors that contribute to soil variability. The transition between two soils that differ in a particular property or set of properties may be of at least two kinds. Normally, a given horizon of one soil disappears over horizontal distance by a gradual weakening of its expression. However, in some places the horizons become intermittent either with or without a marked decrease in the strength of expression. The transitional forms having discontinuous horizons or horizons that vary greatly in thickness or other properties are not the rule, but the soils have been troublesome to classify. One must decide whether the area is one soil in which a horizon is discontinuous or variable, or two soils.
9. 9. The Soils That We Classify 11 Trouble cannot be avoided by arbitrarily saying that two soils are present if a diagnostic property or horizon is present in some spots and not present in others. Some limit of area must be set. If one sets no limit, a vertical hole made by a burrowing animal would be considered “nonsoil.” It would become a soil when filled or, if a coating were present, the coating would be considered a soil. This would be absurd. Such a soil could not support plants, could not have structure, and could not be sampled for determination of its properties. The view that a minimum areal limit of “a soil” cannot be set, if carried to the extreme, leads to other odd conclusions. For example, if columns or prisms were present, the exteriors of the prisms would be different soils from the interiors wherever there are coatings on the exteriors. In a structureless soil, a definition of the smallest area of “a soil” as equivalent to the size of the largest ped would have no meaning. No escape from a minimum limit to the area of “a soil” seems possible. The concept of the pedon (Gr. pedon, ground; rhymes with head on) offers a partial solution to this problem and provides a clear basis for soil descriptions and for the selection of soil samples. A pedon has the smallest volume for which one should describe and sample the soil to represent the nature and arrangement of its horizons and variability in the properties that are preserved in samples. A pedon is comparable in some ways to the unit cell of a crystal. It has three dimensions. Its lower limit is the somewhat vague limit between the soil and “nonsoil” below. Its lateral dimensions are large enough to represent the nature of any horizons and variability that may be present. A horizon may vary in thickness or in composition, or it may be discontinuous. The minimal horizontal area of a pedon is arbitrarily set at 1 m2, but it ranges to 10 m2, depending on the variability in the soil. In the usual situation, where all horizons are continuous and of nearly uniform thickness and composition, the pedon has a horizontal area of about 1 m2. Photo 1 shows the normal situation in which horizons are continuous and relatively Photo 1.—A soil that has continuous horizons, in an area of Wyoming.
10. 10. 12 Soil Taxonomy Photo 2.—A sandy soil near Brugge, Belgium. uniform in thickness over considerable areas. The mollic epipedon and calcic horizon extend for hundreds of meters in areas of this Wyoming landscape. Each pedon includes the range of variability that is present in a small volume. The pedon is roughly polygonal. One lateral dimension does not differ greatly from any other. The size of a pedon can be determined only by examination of a volume that is appreciably larger than the pedon. Where horizons are intermittent or cyclic and recur in linear intervals of 2 to 7 m (roughly 7 to 23 ft), the pedon includes onehalf the cycle. Thus, each pedon includes the range of variability that occurs within these small areas, but not necessarily the total variability included in other similar pedons studied over a large area. Where the cycle is less than 2 m, the horizontal area of a pedon is the minimum size, 1 m2. Depending on the concept of soil and of the pedon, there could be different classifications of the soils. With the concept of soil and of the pedon that is outlined here, the pedons of some soils may include markedly differing sequences of horizons. The following examples clarify the concept of a pedon that has intermittent horizons. Photo 2 illustrates a soil near Brugge, Belgium, in an area that is covered by eolian sand of Wisconsin (Wurm) age. The plow layer, 35 cm thick, is very dark brown fine sand or loamy fine sand. Most sand grains are free of visible coatings. The lower boundary of the plow layer is abrupt and irregular and shows many clear spade marks. The next layer is a discontinuous B horizon that consists of at least three materials. The first of these is dark brown (7.5YR 3/4, moist) fine sand with nodules. The nodules range from about 5 to 20 cm in diameter and are firm or friable in the interior but have a very firm crust about one-half cm thick. The crust has stronger chroma and redder hue than the interior, suggesting the segregation of iron. The interiors of the nodules are free of roots. The second material is very friable, massive, grayish brown (10YR 5/2, moist) fine sand that has many fine fibrous roots. It would normally be considered parent material, the C horizon,
11. 11. The Soils That We Classify 13 Photo 3.—A soil in the Yukon Territory of Canada. where it underlies the nodules of the B horizon; however, it surrounds the nodules and continues down with little change to a thin layer of buried muck that has been dated by radiocarbon as Allerod (Two Creeks), about 11,000 years B.P. The third material is very friable, massive fine sand that is present in gross, more or less tubular forms as much as 60 cm in diameter. The sand is similar to the second material in color but has many weak, fine dark gray and very dark gray lamellae or fibers that are comparable to those in or below the B horizon of the sandy soils formed under heath. The history of this soil has been studied by the staff of the Institute for Soil Survey, IRSIA, Ghent.1 While under forest, the soil was brown and had no clearly expressed eluvial or illuvial horizons. After clearing of the forest and invasion of the heather (Calluna vulgaris), a dark colored illuvial horizon that contained amorphous compounds or mixtures of organic matter, iron, and aluminum (see spodic horizon) formed. During the 17th and 18th centuries, flax became an important crop in Flanders, and the linen was woven in the farm homes 1 Personal communication from R. Tavernier. in the winter. To obtain high yields of high-quality flax, large amounts of manure and chalk were applied to the fields. The influence of the calcium and nitrogen was to destroy the B horizon of amorphous materials, first in spots and then completely. Photo 2 shows that the B horizon has been partly destroyed. Because discontinuous horizons recur at intervals of less than 1 m, the pedon has an area of 1 m2. The processes of either formation or destruction of many horizons may not operate uniformly and may first produce intermittent horizons. In places the forces operate with remarkable uniformity and produce faint but continuous horizons. Genetically, therefore, the discontinuous horizons can have significance equivalent to weakly expressed but continuous horizons. Many cold soils are subject to physical disturbance as a result of freezing and thawing. The forces generated through freezing often produce cyclic or intermittent horizons. Photo 3 shows a soil from the Yukon Territory of Canada. The small orange squares mark the boundary between permafrost and the active layer. The organic layer is about 40 cm thick in the lowlying areas and about 20 cm thick in the areas of higher
12. 12. 14 microrelief. This pattern is repeated at linear intervals of about 1 m. The pedon in photo 3 is 1 m2. Soil taxonomy has taxa at the subgroup level of Gelisols to deal with the range in thickness of the organic layers. Although every pedon can be classified, not every pedon need be classified. The pedon should represent a segment of the landscape. Sometimes, pedons that represent a segment of the landscape are referred to as polypedons. Soil scientists should try to sample, characterize, and classify representative pedons. Soil taxonomy provides a means of comparing, describing, and differentiating these various pedons. Summary Since the genesis of a soil may not be understood or may be disputed, it can be used only as a guide to our thinking in selecting criteria and forming concepts. Generally, a more or less arbitrary definition of a pedon serves the purpose of classification better at this time than a genetic one. For that reason, the following definition is used: A pedon is a threedimensional body of soil that has lateral dimensions large enough to include representative variations in the shape and relation of horizons and in the composition of the soil. Its horizontal area ranges from 1 to 10 m2, depending on the nature of the variability in the soil, and its volume varies, depending on the depth of the soil. Where the cycle of variations is less than 2 m long and where all horizons are continuous and of nearly uniform thickness, the pedon has a horizontal area of approximately 1 m2. Where horizons or other properties are intermittent or cyclic and recur at linear intervals of 2 to 7 m, the pedon includes one-half of the cycle. If horizons are cyclic but recur at intervals of more than 7 m, the pedon reverts to an area of approximately 1 m2 and more than one soil is usually represented in each cycle. Literature Cited Glinka, K.D. 1927. Dokuchaiev’s Ideas in the Development of Pedology and Cognate Sciences. 32 p. In Russian Pedology. Invest. I. Acad. Sci. USSR, Leningrad. Holmgren, G.G.S. 1988. The Point Representation of Soil. Soil Sci. Soc. Am. J. 52: 712-716.
13. 13. 15 CHAPTER 2 Soil Taxonomy and Soil Classification T he primary objective of soil taxonomy is to establish hierarchies of classes that permit us to understand, as fully as possible, the relationship among soils and between soils and the factors responsible for their character. A second objective is to provide a means of communication for the discipline of soil science. Soil taxonomy was originally developed to serve the purposes of soil survey. During the last few decades, it has evolved into a means of communication in soil science. Taxonomy is a narrower term than classification. Classification includes taxonomy, but it also includes the grouping of soils according to limitations that affect specific practical purposes, such as the soil limitations affecting the foundations of buildings. Taxonomy is the part of classification that is concerned primarily with relationships. Classifications are contrivances made by humans to suit their purposes. They are not themselves truths that can be discovered. A perfect classification would have no drawbacks when used for the purpose intended. Each distinctly different purpose, to be served best, demands a different classification. For the different purposes of the soil survey, classes are needed that can be grouped or subdivided and regrouped to permit the largest number and the most precise predictions possible about responses to management and manipulation. Consequently, not one but many classifications can be drawn from the basic taxonomy. Flexibility in the classes of the taxonomic system is achieved by the use of phases and by the nomenclature. The phases are used to subdivide taxa according to the practical needs for the purposes of a particular survey or interpretation. They are discussed later in this chapter. Flexibility in the hierarchy permits grouping taxa into successively smaller numbers as one goes from lower to higher categories. For some purposes it is useful to group taxa that have been separated at a higher level in the system. For example, one might want a group that includes all soils that are waterlogged for extended periods. For other purposes one might want a group comprised of all soils that have a B horizon affected by sodium or of all soils that have a fragipan or permafrost. Some soils might be in several of these groups, so that no matter how a single hierarchy is arranged, it is not possible to have all desired groups. Therefore, no single hierarchy can best serve all our purposes. The way we attain flexibility in the hierarchy is explained in the discussion of the nomenclature. As knowledge expands, new facts or closer approximations of truths not only make improvements in classification possible but also make some changes imperative. Thus, classifications are not static but require change as knowledge expands. Since the original edition of Soil Taxonomy was published in 1975, eight international committees have made proposals that have been approved and incorporated. These committees include the International Committee on Low Activity Clays (ICOMLAC), the International Committee on Oxisols (ICOMOX), the International Committee on Andisols (ICOMAND), the International Committee on Spodosols (ICOMOD), the International Committee on Aquic Moisture Regimes (ICOMAQ), the International Committee on Vertisols (ICOMERT), the International Committee on Aridisols (ICOMID), the International Committee on Families (ICOMFAM), and the International Committee on PermafrostAffected Soils (ICOMPAS). Taxonomy of soils is a controversial subject. In part, controversy reflects differences in the purposes for which taxonomic classifications are made and differences in concepts of soil as well as differences in opinion about the taxonomy of soils. One cannot say that one taxonomic classification is better than another without reference to the purposes for which both were made, and comparisons of the merits of taxonomies made for different purposes can be useless. The Attributes of Soil Taxonomy Soil surveys require many nontaxonomic classifications that can be related to the real bodies of soil and that facilitate comparisons of both similarities and differences among them for a great variety of purposes. These classifications are used to determine whether experience at one location is applicable to the soils of other locations. The classifications may have to be used by a pedologist to apply the experience of others for soils that are unfamiliar. Many persons with diverse backgrounds and training are expected to use the classifications accurately to transfer experience with the behavior of soils under a variety of uses. These intended uses of the classifications impose some specific requirements on the taxonomy that stands behind the classifications. The attributes of soil taxonomy are described in the following paragraphs. First, the definition of each taxon carries as nearly as possible the same meaning to each user. Definitions in soil taxonomy are operational. It is insufficient to say that the soils in one taxon are differentiated from others by high organic-
14. 14. 16 matter content because what is considered high in one place may be considered low in another. The disadvantage of definitions, of course, is that distinctions are made that may not be meaningful for every conceivable use of the soil. Only by operational definitions can competent pedologists with diverse backgrounds arrive at the same classification of the same kind of soil. Second, soil taxonomy is a multicategoric system. Many taxa are needed in the lower categories because many properties are important to the use of a soil. Specific properties can vary independently of others, and their importance depends on their combination with other properties. Taxa in the lower categories, therefore, must be defined as specifically as possible in terms of many properties. This requirement results in more taxa in the lower categories than the mind can comprehend. Consequently, the taxa must be grouped on some rational basis into progressively smaller numbers of classes of higher categories in a manner that permits the mind to grasp the concepts and relationships of all taxa. The mind readily grasps 5 to 12 items, but it cannot deal simultaneously with 100 to 1,000 items without some ordering principle. Higher categories are necessary for organizing and understanding the lower categories and, in addition, they can be useful in comparing soils of large areas. They have only limited value for transferring experience to a specific site for a specific use. Third, the taxa represent real bodies of soil that are known to occupy geographic areas. Pedologists are concerned with mapping real bodies of soil, and a classification related to these real bodies facilitates the mapping (Cline, 1963). Soil taxonomy does not try to provide for all possible combinations of properties because the classification of kinds of soil that have not been studied should not be prejudiced by a closed system that covers all contingencies. Rather, soil taxonomy provides a means to recognize new taxa when discovery leads to new combinations of properties important to our purposes. Fourth, differentiae are soil properties that can be observed in the field or that can be inferred either from other properties that are observable in the field or from the combined data of soil science and other disciplines. Some of the most important properties of the soil are chemical properties, and soil taxonomy uses criteria in some taxa based on laboratory measurements. Often data from laboratory measurements can be interpolated to other areas, or pedologists discover physical or morphological properties that reflect chemical characteristics. Soil temperature, soil moisture, and other properties that fluctuate with the seasons are difficult to use in taxonomy unless they can be inferred by reasoning from the combined data of soil science and other disciplines, such as meteorology. Soil mineralogy can usually be inferred by reasoning from the combined data of soil science and geology. If there are no data that permit inferences about important but invisible soil properties, it is probably best to defer classifying a soil until some knowledge of its important properties is Soil Taxonomy available. A classification that is based on extremely limited knowledge of an object has little utility. Fifth, soil taxonomy is capable of modification to accommodate new knowledge with a minimum of disturbance. Taxa can be added or combined in any category without disturbance of the rest of the system at the same or a higher categorical level. If the highest category includes a number of taxa defined by a variety of properties, the number can be increased or decreased by combining or subdividing taxa whenever experience convinces us that this is advisable. If one taxon in the highest category is divided, no others in that category need be affected. If two or parts of two are combined, only those two or those parts are affected. Obviously, combining taxa at a high level changes classes of lower categories if they are members of those taxa. Adding taxa may have no effect on the lower categories if the soils concerned were not previously included in the system. If the addition is a consequence of combining classes, it affects the lower categories. Sixth, the differentiae keep an undisturbed soil and its cultivated or otherwise human-modified equivalents in the same taxon insofar as possible. Changes produced by a single or repeated plowing that mixes the surface soil to a depth of 18 to 25 cm (7 to 10 in), for example, have the least possible effect on the placement of a soil in soil taxonomy. Truncation by erosion does not change the classification of a soil until horizons or diagnostic features important to the use or identification of the soil have been lost. Consequently, insofar as possible, the diagnostic horizons and features should be those below the part of the soil affected by human activities. However, significant changes in the nature of the soil by humans cannot be ignored. Seventh, soil taxonomy is capable of providing taxa for all soils on a landscape. Soils form a continuum. The continuum is broken into a reasonable number of segments that have limited and defined ranges in properties so that quantitative interpretations of soil behavior can be made. Eighth, soil taxonomy provides for all soils that are known, wherever they may be. Many kinds of soil are poorly represented or are unknown in the United States. A system that includes all known soils helps us to see the soils of the United States in better perspective, particularly if a kind of soil is poorly represented or is very extensive. It also helps us to draw on experience in other countries with kinds of soil that are poorly represented or are not extensive in the United States as a whole but that are extensive locally. Selection of Differentiae To serve the purpose of the soil survey, the pedon should be classified by its own properties and the taxa defined strictly in terms of soil properties. In soil definitions, a given property, such as particle-size distribution or pH, cannot be treated in an
15. 15. Soil Taxonomy and Soil Classification identical way for all soils. The significance of a difference in any one property depends on the others in the combination that makes a soil of a certain kind. Soil color and the soil horizons are obvious properties that have been used as differentiating characteristics at high categoric levels in most taxonomies. Color per se seems to have no accessory characteristics. For example, if one considers all the soils that have brown color, no statement can be made about them except that they are brown. There are accessory characteristics for some colors in combination with other properties, and the use of color as a differentiating characteristic should be limited to these situations. A more useful classification can be devised if properties that have more accessory properties than color are used as differentiae in the highest categories. Soil horizons are the result of the dominance of one or more sets of processes over other processes through time. The processes themselves are not now suitable for use as differentiae. The illuviation of clay, for example, cannot be observed or measured in a soil. If illuviation has been a significant process in the genesis of a soil, however, there should be marks in the soil that indicate this process. These marks need not be the same everywhere, but if the proper marks are selected, the classification can reflect the dominance of illuviation over other processes, such as those that mix horizons and those that prevent the movement of clay. The nature of the horizons is useful in defining the taxa of soils that have horizons but is useless for soils that do not have them. Of course, the absence of horizons is itself a mark of significance. Many important properties of soils, however, are not necessarily reflected by the combinations of horizons, and many important processes do not themselves produce horizons. Intensive mixing of soil by animals can destroy horizons. The leaching of bases, particularly calcium, and the cycling of bases by plants in humid climates can be reflected by changes in base status with increasing depth but can be independent of the kinds of horizons in a soil. The horizons, therefore, are not the sole differentiating characteristics in defining taxa. Some soil properties influence or control specific processes and, through them, the genesis of the soil. Silicate clays cannot form in a soil composed entirely of quartz, and apparently they do not form if a soil is too cold. The soil moisture regime influences the base status of a soil and the formation of horizons with an accumulation of illuvial clay or of carbonates. These are examples of soil properties that are causes of other properties and that require consideration when properties are selected to be used as differentiae for taxa. The differentiae should be soil properties, but the most useful properties for the higher categories may be either those that result from soil genesis or those that affect soil genesis because they have the greatest number of accessory properties. For example, the clay percentage in soils commonly increases and then decreases with increasing depth. In many soils differences in the content of clay are the result of eluviation 17 and illuviation. In other soils they may be only the result of stratification of the materials in which the soils developed. If the horizons are genetic, they have accessory properties, although the accessory properties may vary with the kind of soil. If the climate is humid, the eluvial horizons and at least part of the illuvial horizons are free of finely divided carbonates because carbonates tend to immobilize clay and because the leaching required to form an illuvial horizon is greater than the leaching required to dissolve and remove the carbonates. Time of the order of some thousands of years without significant erosion is required. During this time there is opportunity for nutrients used by plants to be systematically concentrated in various horizons. In soils that formed under grass in humid temperate regions, phosphorus seems to be concentrated in the surface horizons, a considerable part of it in organic compounds. If the clay distribution in a soil is due solely to stratification of parent materials, few other statements can be made about that soil. The soil may be calcareous or acid. This example illustrates why properties that are the result of soil genesis or that affect soil genesis are important. They have accessory properties. Some of the accessory properties are known, but it is likely that many are still unknown. In soil surveys the pedologist is commonly concerned with finding the boundaries between map units. The boundaries are in places where there has been or is a difference in one or more of the factors that control soil genesis. The mapper learns to look for these places and uses a knowledge of soil genesis to improve the accuracy and efficiency of mapping. Genesis is fundamental to soil taxonomy and to the soil survey. Genesis itself, however, is unsuitable for direct use in soil taxonomy. Because the genesis of a soil cannot be observed or measured, pedologists may have widely differing opinions about it, and the classification of a given pedon is affected by the background of the pedologist. Forming and Defining Taxa When forming and defining the taxa, one must consider all the known properties, although only a few can be differentiating. The differentiating properties should be the ones that are the most important for our purposes or that have the most important accessory characteristics. Research and experience indicate that some properties are important to plant growth. Soil taxonomy attempts to make the most important statements possible about the taxa. Those properties that are important to plant growth and that result from or influence soil genesis are considered in the higher categories. Those that are important to plant growth but are unrelated to genesis should be considered only for the lowest categories. For example, in soils that are only slightly weathered, the nature and amount of clay may be the result of geologic accidents. If the differences are not extreme, the course of soil genesis is not necessarily affected. Although the
16. 16. 18 difference between illite and smectite is important to plant growth, it is used as a differentiating characteristic only at a low category in the system, the family. Determining the similarities among soils is not always a simple matter. There may be similarity in particle-size distribution to the members of one taxon and in base status to the members of another. One must decide which property is the more important, and this decision must rest on the nature of the statements that one can make about the classes if the kind of soil is grouped one way or the other. The best grouping determines the definition; the definition does not determine the grouping. If the grouping has imperfections, so does the definition. The statements are about the nature of the soils and the interpretations that can be made for the various phases of a taxon. Interpretations are predictions of the consequences of specific uses of soils, commonly in terms of plant growth under specified systems of management but also in terms of engineering soil behavior after a given manipulation. Interpretations of the soils indicate the reasonable alternatives for their use and management and the expected results. The best grouping is one that helps us to make the most precise and most important interpretations. Soil taxonomy must continue to be tested by the nature of the interpretations that can be made. The taxonomic classification used in soil surveys requires flexibility in the classes. It is commonly necessary to subdivide taxa and regroup those subdivisions into new classes of another classification for the greatest number and most precise interpretations possible. Soil taxonomy was designed to facilitate interpretations, but the interpretations themselves require at least one additional step of reasoning (Cline, 1963). The interpretations may also require information that is not available from the taxonomy. Slope and stoniness are soil characteristics that must be known or assumed for one to predict consequences of farming with heavy machinery. Invasions of locusts, hurricanes, or frequent hailstorms are not soil characteristics, but their probability must be known or assumed when crop yields are predicted. These and other important characteristics may be used as bases for defining phases of taxa that are necessary for interpretations for specific fields or farms. The phases are not a part of the taxonomy. Their nature is determined by the foreseeable uses of the soils in a particular survey area. Quite different phases might be differentiated for the same soils in an area of general farming in contrast to a national forest or an area being developed for housing and in an irrigated area in contrast to the desert grazing land that is above the irrigation canal. The phases represent a number of classifications superimposed on the taxonomic classification to give part of the flexibility that is needed for the wide variety of uses made of soil. Inevitably, the conclusions of a large group of scientists include some compromises of divergent points of view. Members of a group representing unlike interests and experience are likely to see soils differently. Different points of view about soil produce different ideas about its classification. Consequently, compromises between the conflicting desires of a number of individuals not only are necessary but also are likely to produce a system that has more general utility than a system that represents a single point of view. Compromise may not be the exact word. The truth has many facets; each person has a somewhat different view of the truth, and no person can see the whole truth clearly. Soil taxonomy allows changes in the system as new information about soils becomes available. Since its inception, soil taxonomy has been amended many times. Probably, no one person will approve of all the details of these changes; few will be able to agree on all the changes. Literature Cited Cline, M.G. 1963. Logic of the New System of Soil Classification. Soil Sci. 96: 17-22.
17. 17. 19 CHAPTER 3 Differentiae for Mineral Soils1 and Organic Soils S oil taxonomy differentiates between mineral soils and organic soils. To do this, first, it is necessary to distinguish mineral soil material from organic soil material. Second, it is necessary to define the minimum part of a soil that should be mineral if a soil is to be classified as a mineral soil and the minimum part that should be organic if the soil is to be classified as an organic soil. Nearly all soils contain more than traces of both mineral and organic components in some horizons, but most soils are dominantly one or the other. The horizons that are less than about 20 to 35 percent organic matter, by weight, have properties that are more nearly those of mineral than of organic soils. Even with this separation, the volume of organic matter at the upper limit exceeds that of the mineral material in the fine-earth fraction. Mineral Soil Material Mineral soil material (less than 2.0 mm in diameter) either: 1. Is saturated with water for less than 30 days (cumulative) per year in normal years and contains less than 20 percent (by weight) organic carbon; or 2. Is saturated with water for 30 days or more cumulative in normal years (or is artificially drained) and, excluding live roots, has an organic carbon content (by weight) of: a. Less than 18 percent if the mineral fraction contains 60 percent or more clay; or b. Less than 12 percent if the mineral fraction contains no clay; or c. Less than 12 + (clay percentage multiplied by 0.1) percent if the mineral fraction contains less than 60 percent clay. Organic Soil Material Soil material that contains more than the amounts of organic carbon described above for mineral soil material is considered organic soil material. In the definition of mineral soil material above, material that has more organic carbon than in item 1 is intended to 1 Mineral soils include all soils except the suborder Histels and the order Histosols. include what has been called litter or an O horizon. Material that has more organic carbon than in item 2 has been called peat or muck. Not all organic soil material accumulates in or under water. Leaf litter may rest on a lithic contact and support forest vegetation. The soil in this situation is organic only in the sense that the mineral fraction is appreciably less than half the weight and is only a small percentage of the volume of the soil. Distinction Between Mineral Soils and Organic Soils Most soils are dominantly mineral material, but many mineral soils have horizons of organic material. For simplicity in writing definitions of taxa, a distinction between what is meant by a mineral soil and an organic soil is useful. To apply the definitions of many taxa, one must first decide whether the soil is mineral or organic. An exception is the Andisols (defined later). These generally are considered to consist of mineral soils, but some may be organic if they meet other criteria for Andisols. Those that exceed the organic carbon limit defined for mineral soils have a colloidal fraction dominated by short-range-order minerals or aluminum-humus complexes. The mineral fraction in these soils is believed to give more control to the soil properties than the organic fraction. Therefore, the soils are included with the Andisols rather than the organic soils defined later as Histosols. If a soil has both organic and mineral horizons, the relative thickness of the organic and mineral soil materials must be considered. At some point one must decide that the mineral horizons are more important. This point is arbitrary and depends in part on the nature of the materials. A thick layer of sphagnum has a very low bulk density and contains less organic matter than a thinner layer of well-decomposed muck. It is much easier to measure the thickness of layers in the field than it is to determine tons of organic matter per hectare. The definition of a mineral soil, therefore, is based on the thickness of the horizons, or layers, but the limits of thickness must vary with the kinds of materials. The definition that follows is intended to classify as mineral soils those that have both thick mineral soil layers and no more organic material than the amount permitted in the histic epipedon, which is defined in chapter 4. In the determination of whether a soil is organic or mineral, the thickness of horizons is measured from the surface of the
18. 18. 20 soil whether that is the surface of a mineral or an organic horizon, unless the soil is buried as defined in chapter 1. Thus, any O horizon at the surface is considered an organic horizon if it meets the requirements of organic soil material as defined later, and its thickness is added to that of any other organic horizons to determine the total thickness of organic soil materials. Definition of Mineral Soils Mineral soils are soils that have either of the following: 1. Mineral soil materials that meet one or more of the following: a. Overlie cindery, fragmental, or pumiceous materials and/or have voids2 that are filled with 10 percent or less organic materials and directly below these materials have either a densic, lithic, or paralithic contact; or b. When added with underlying cindery, fragmental, or pumiceous materials, total more than 10 cm between the soil surface and a depth of 50 cm; or c. Constitute more than one-third of the total thickness of the soil to a densic, lithic, or paralithic contact or have a total thickness of more than 10 cm; or d. If they are saturated with water for 30 days or more per year in normal years (or are artificially drained) and have organic materials with an upper boundary within 40 cm of the soil surface, have a total thickness of either: (1) Less than 60 cm if three-fourths or more of their volume consists of moss fibers or if their bulk density, moist, is less than 0.1 g/cm3; or (2) Less than 40 cm if they consist either of sapric or hemic materials, or of fibric materials with less than three-fourths (by volume) moss fibers and a bulk density, moist, of 0.1 g/cm3 or more; or 2. More than 20 percent, by volume, mineral soil materials from the soil surface to a depth of 50 cm or to a glacic layer or a densic, lithic, or paralithic contact, whichever is shallowest; and 2 Materials that meet the definition of cindery, fragmental, or pumiceous but have more than 10 percent, by volume, voids that are filled with organic soil materials are considered to be organic soil materials. a. Permafrost within 100 cm of the soil surface; or b. Gelic materials within 100 cm of the soil surface and permafrost within 200 cm of the soil surface. Definition of Organic Soils Organic soils have organic soil materials that: 1. Do not have andic soil properties in 60 percent or more of the thickness between the soil surface and either a depth of 60 cm or a densic, lithic, or paralithic contact or duripan if shallower; and 2. Meet one or more of the following: a. Overlie cindery, fragmental, or pumiceous materials and/or fill their interstices2 and directly below these materials have a densic, lithic, or paralithic contact; or b. When added with the underlying cindery, fragmental, or pumiceous materials, total 40 cm or more between the soil surface and a depth of 50 cm; or c. Constitute two-thirds or more of the total thickness of the soil to a densic, lithic, or paralithic contact and have no mineral horizons or have mineral horizons with a total thickness of 10 cm or less; or d. Are saturated with water for 30 days or more per year in normal years (or are artificially drained), have an upper boundary within 40 cm of the soil surface, and have a total thickness of either: (1) 60 cm or more if three-fourths or more of their volume consists of moss fibers or if their bulk density, moist, is less than 0.1 g/cm3; or (2) 40 cm or more if they consist either of sapric or hemic materials, or of fibric materials with less than three-fourths (by volume) moss fibers and a bulk density, moist, of 0.1 g/cm3 or more; or e. Are 80 percent or more, by volume, from the soil surface to a depth of 50 cm or to a glacic layer or a densic, lithic, or paralithic contact, whichever is shallowest. It is a general rule that a soil is classified as an organic soil (Histosol) if more than half of the upper 80 cm (32 in) of the soil is organic or if organic soil material of any thickness rests on rock or on fragmental material having interstices filled with organic materials.
19. 19. 21 CHAPTER 4 Horizons and Characteristics Diagnostic for the Higher Categories T his chapter defines the horizons and characteristics of both mineral and organic soils. It is divided into three parts—horizons and characteristics diagnostic for mineral soils, characteristics diagnostic for organic soils, and horizons and characteristics diagnostic for both mineral and organic soils. The four highest categories of this taxonomy, in order of decreasing rank and increasing numbers of taxa, are distinguished by the presence or absence or a variety of combinations of diagnostic horizons and characteristics. The categories themselves are described in chapter 6. The horizons and characteristics defined below are not in a key format. Some diagnostic horizons are mutually exclusive, and some are not. An umbric epipedon, for example, could not also be a mollic epipedon. A kandic horizon with clay films, however, could also meet the definition of an argillic horizon. A soil horizon is a layer that is commonly parallel to the soil surface. In some orders, such as Gelisols, Vertisols, and Spodosols, however, horizons are not always parallel to the surface. A horizon has some set of properties that have been produced by soil-forming processes, and it has some properties that are not like those of the layers directly above and beneath it (USDA, SCS, 1993). A soil horizon commonly is differentiated from the horizons adjacent to it partly by characteristics that can be seen or measured in the field, such as color, structure, texture, rupture-resistance class, and the presence or absence of carbonates. In identifying a soil horizon, however, measurements in the laboratory are sometimes required to supplement field observations. According to the criteria we use, horizons are identified partly by their own morphology and partly by properties that differ from those of the overlying and underlying horizons. Many of the layers that are differentiae for organic soils do not meet the definition of soil horizons. Unlike the layers of soil that are commonly called horizons, they are layers that formed in differing environments during the period when the materials that now constitute the soils accumulated. Some of the layers that serve as differentiae are soil horizons, but there are no operational methods that can always distinguish between “horizons” and “layers” that have similar properties. The importance of making a distinction between horizons and layers of organic soils is unknown. In the discussion that follows, the term “soil material” is commonly used as a broader term that includes both horizons and layers in organic soils. The horizon designations used in this chapter are defined in the Soil Survey Manual (USDA, SCS, 1993) and the Keys to Soil Taxonomy (USDA, NRCS, 1998). Horizons and Characteristics Diagnostic for Mineral Soils The criteria for some of the following horizons and characteristics, such as histic and folistic epipedons, can be met in organic soils. They are diagnostic, however, only for the mineral soils. Diagnostic Surface Horizons: The Epipedon The epipedon (Gr. epi, over, upon, and pedon, soil) is a horizon that forms at or near the surface and in which most of the rock structure has been destroyed. It is darkened by organic matter or shows evidence of eluviation, or both. Rock structure as used here and in other places in this taxonomy includes fine stratification (less than 5 mm) in unconsolidated sediments (eolian, alluvial, lacustrine, or marine) and saprolite derived from consolidated rocks in which the unweathered minerals and pseudomorphs of weathered minerals retain their relative positions to each other. Any horizon may be at the surface of a truncated soil. The following section, however, is concerned with eight diagnostic horizons that have formed at or near the soil surface. These horizons can be covered by a surface mantle of new soil material. If the surface mantle has rock structure, the top of the epipedon is considered the soil surface unless the mantle meets the definition of buried soils in chapter 1. If the soil includes a buried soil, the epipedon, if any, is at the soil surface and the epipedon of the buried soil is considered a buried epipedon and is not considered in selecting taxa unless the keys specifically indicate buried horizons, such as those in Thapto-Histic subgroups. A soil with a mantle thick enough to have a buried soil has no epipedon if the soil has rock structure to the surface or has an Ap horizon less than 25 cm thick that is underlain by soil material with rock structure. The melanic epipedon (defined below) is unique among epipedons. It forms commonly in volcanic deposits and can receive fresh deposits of ash. Therefore, this horizon is permitted to have layers within and above the epipedon that are not part of the melanic epipedon.
20. 20. 22 A recent alluvial or eolian deposit that retains stratifications (5 mm or less thick) or an Ap horizon directly underlain by such stratified material is not included in the concept of the epipedon because time has not been sufficient for soil-forming processes to erase these transient marks of deposition and for diagnostic and accessory properties to develop. An epipedon is not the same as an A horizon. It may include part or all of an illuvial B horizon if the darkening by organic matter extends from the soil surface into or through the B horizon. Anthropic Epipedon The anthropic epipedon has the same limits as the mollic epipedon in color, structure, and organic-carbon content. It formed during long-continued use of the soil by humans, either as a place of residence or as a site for growing irrigated crops. In the former case, disposal of bones and shells has supplied calcium and phosphorus and the level of phosphorus in the epipedon is too high for a mollic epipedon. Such epipedons occur in the humid parts of Europe, the United States, and South America and probably in other parts of the world, mostly in kitchen middens. The high level of phosphorus in the anthropic epipedons is not everywhere accompanied by a base saturation of 50 percent or more, but it is accompanied by a relatively high base saturation if compared with the adjacent soils. In arid regions some long-irrigated soils have an epipedon that is like the mollic epipedon in most chemical and physical properties. The properties of the epipedon in these areas are clearly the consequence of irrigation by humans. Such an epipedon is grouped with the anthropic epipedons, which developed under human habitation. If not irrigated, such an epipedon is dry in all its parts for more than 9 months in normal years. Additional data about anthropic epipedons from several parts of the world may permit future improvements in this definition. Required Characteristics In summary, the anthropic epipedon shows some evidence of disturbance by human activity and meets all of the requirements for a mollic epipedon, except for one or both of the following: 1. 1,500 milligrams per kilogram or more P2O5 soluble in 1 percent citric acid and a regular decrease in P2O5 to a depth of 125 cm; or 2. If the soil is not irrigated, all parts of the epipedon are dry for 9 months or more in normal years. Soil Taxonomy normally is at the soil surface, although it can be buried. If the soil has been plowed, the organic-carbon requirements are lower than the requirements for organic soil material because of the need to accommodate the oxidation that occurs when the soil is plowed. Folistic epipedons occur primarily in cool, humid regions of the world. They differ from histic epipedons because they are saturated with water for less than 30 days (cumulative) in normal years (and are not artificially drained). Taxa for soils with folistic epipedons above the series level are not currently recognized in this taxonomy. The folistic epipedon is used only with mineral soils. Required Characteristics The folistic epipedon is defined as a layer (one or more horizons) that is saturated for less than 30 days (cumulative) in normal years (and is not artificially drained) and either: 1. Consists of organic soil material that: a. Is 20 cm or more thick and either contains 75 percent or more (by volume) Sphagnum fibers or has a bulk density, moist, of less than 0.1; or b. Is 15 cm or more thick; or 2. Is an Ap horizon that, when mixed to a depth of 25 cm, has an organic-carbon content (by weight) of: a. 16 percent or more if the mineral fraction contains 60 percent or more clay; or b. 8 percent or more if the mineral fraction contains no clay; or c. 8 + (clay percentage divided by 7.5) percent or more if the mineral fraction contains less than 60 percent clay. Most folistic epipedons consist of organic soil material (defined in chapter 3). Item 2 provides for a folistic epipedon that is an Ap horizon consisting of mineral soil material. Histic Epipedon The histic epipedon consists of organic soil material (peat or muck) if the soil has not been plowed. If the soil has been plowed, the epipedon normally has a high content of organic matter that results from mixing organic soil material with some mineral material. The histic epipedon either is characterized by saturation and reduction for some time in normal years or has been artificially drained. It is normally at the soil surface, although it can be buried. Photo 4 shows a very dark histic epipedon that is saturated for long periods and meets criterion 1 below. Folistic Epipedon Required Characteristics The folistic epipedon consists of organic material (defined in chapter 3), unless the soil has been plowed. This epipedon The histic epipedon is a layer (one or more horizons) that is characterized by saturation (for 30 days or more, cumulative)
21. 21. Horizons and Characteristics Diagnostic for the Higher Categories 23 and reduction for some time during normal years (or is artificially drained) and either: designations) of 2 or less throughout and a melanic index of 1.70 or less throughout; and 1. c. 6 percent or more organic carbon as a weighted average and 4 percent or more organic carbon in all layers. Consists of organic soil material that: a. Is 20 to 60 cm thick and either contains 75 percent or more (by volume) Sphagnum fibers or has a bulk density, moist, of less than 0.1; or b. Is 20 to 40 cm thick; or 2. Is an Ap horizon that, when mixed to a depth of 25 cm, has an organic-carbon content (by weight) of: a. 16 percent or more if the mineral fraction contains 60 percent or more clay; or b. 8 percent or more if the mineral fraction contains no clay; or c. 8 + (clay percentage divided by 7.5) percent or more if the mineral fraction contains less than 60 percent clay. Most histic epipedons consist of organic soil material (defined in chapter 3). Item 2 provides for a histic epipedon that is an Ap horizon consisting of mineral soil material. A histic epipedon consisting of mineral soil material can also be part of a mollic or umbric epipedon. Melanic Epipedon The melanic epipedon is a thick, dark colored (commonly black) horizon at or near the soil surface (photo 5). It has high concentrations of organic carbon, generally associated with short-range-order minerals or aluminum-humus complexes. The intense dark colors are attributed to the accumulation of organic matter from which “Type A” humic acids are extracted. This organic matter is thought to result from large amounts of root residues supplied by a gramineous vegetation and can be distinguished from organic matter formed under forest vegetation by the melanic index. The suite of secondary minerals generally is dominated by allophane, and the soil material has a low bulk density and a high anion adsorption capacity. Required Characteristics The melanic epipedon has both of the following: 1. An upper boundary at, or within 30 cm of, either the mineral soil surface or the upper boundary of an organic layer with andic soil properties (defined below), whichever is shallower; and 2. In layers with a cumulative thickness of 30 cm or more within a total thickness of 40 cm, all of the following: a. Andic soil properties throughout; and b. A color value, moist, and chroma (Munsell Mollic Epipedon The mollic epipedon is a relatively thick, dark colored, humus-rich surface horizon (or horizons) in which bivalent cations are dominant on the exchange complex and the grade of structure is weak to strong (photos 6 and 7). These properties are common in the soils of the steppes in the Americas, Europe, and Asia. Properties The mollic epipedon is defined in terms of its morphology rather than its genesis. It consists of mineral soil material and is at the soil surface, unless it underlies a histic epipedon or thin surface mantle, as explained earlier in this chapter. If the surface layer of organic material is so thick that the soil is recognized as a Histosol (defined below), the horizon that at one time was a mollic epipedon is considered to be buried and no longer meets the definition of an epipedon. The mollic epipedon has soil structure strong enough that less than one-half of the volume of all parts has rock structure and one-half or more of the horizon is not both hard, very hard, or harder and massive when dry. In this definition very coarse prisms, with a diameter of 30 cm or more, are treated as if they were the same as massive unless there is secondary structure within the prisms. The restriction against hardness and structure applies only to those epipedons that become dry. A mollic epipedon can directly overlie deposits with rock structure, including fine stratifications, if the epipedon is 25 cm or more thick. The epipedon does not include any layer in which one-half or more of the volume has rock structure, including fine stratifications. The mollic epipedon has dark color and low chroma in 50 percent or more of its matrix. It typically has a Munsell color value of 3 or less when moist and of 5 or less when dry and chroma of 3 or less when moist. If its structure is fine granular or fine blocky, the sample, when broken, may show only the color of the coatings of peds. The color of the matrix in such situations can be determined only by crushing or briefly rubbing the sample. Prolonged rubbing should be avoided because it may cause darkening of a sample if soft ironmanganese concretions are present. Crushing should be just sufficient to mix the coatings with the matrix. The dry color value should be determined after the crushed sample is dry enough for continued drying to produce no further change and the sample has been smoothed to eliminate shadows. Normally, the color value is at least 1 Munsell unit lower or the chroma at least 2 units lower (both moist and dry) than that of the 1C horizon (if one occurs). Some parent materials, such
22. 22. 24 as loess, cinders, basalt, or carbonaceous shale, can also have dark color and low chroma. Soils that formed in such materials can accumulate appreciable amounts of organic matter but commonly have no visible darkening in the epipedon. In these dark colored materials, the requirement that the mollic epipedon have a lower color value or chroma than the C horizon is waived if the surface horizon(s) meets all of the other requirements for a mollic epipedon and, in addition, has at least 0.6 percent more organic carbon than the C horizon. Finely divided CaCO3 acts as a white pigment and causes soils to have a high color value, especially when dry. To compensate for the color of the carbonates, the mollic epipedon is allowed to have lighter color than normal if the epipedon averages more than 15 percent carbonates. If the fine-earth fraction has a calcium carbonate equivalent of 15 to 40 percent, the limit for the dry color value is waived. If it has a calcium carbonate equivalent of 40 percent or more, the limit for the dry color value is waived and the moist color value is 5 or less. The mollic epipedon forms in the presence of bivalent cations, particularly calcium. The base saturation by the NH4OAc method is required to be 50 percent or more throughout the epipedon. The mollic epipedon is thought to be formed mainly through the underground decomposition of organic residues in the presence of these cations. The residues that are decomposed are partly roots and partly organic residues from the surface that have been taken underground by animals. Accumulation and turnover of the organic matter in the mollic epipedon probably are rapid. The radiocarbon age (mean residence time) of the organic carbon is mostly 100 to 1,000 years. A high percentage of the organic matter is so-called “humic acid.” The minimum organic-carbon content throughout the thickness of the mollic epipedon is 0.6 percent in most mollic epipedons. Exceptions are (1) a minimum of 2.5 percent organic carbon in epipedons that have a color value, moist, of 4 or 5 and a fine-earth fraction with a calcium carbonate equivalent of 40 percent or more and (2) a minimum of 0.6 percent more organic carbon than in the C horizon in epipedons in which the C horizon has a color as dark as or darker than the color of the epipedon. The maximum organic-carbon content of a mollic epipedon is the same as for mineral soil material. Some Ap horizons that approach the lower limit of a histic epipedon can be part of the mollic epipedon. The minimum thickness of the mollic epipedon depends on the depth and texture of the soil. The minimum thickness is for soils with an epipedon that is loamy very fine sand or finer and that is directly above a densic, lithic, or paralithic contact, a petrocalcic horizon, or a duripan. These soils have a minimum thickness of 10 cm. Soils that are 10 to 18 cm deep have a mollic epipedon if the whole soil meets all of the criteria for a mollic epipedon when mixed. The minimum thickness is 25 cm for: (1) all soils with a texture throughout the epipedon of loamy fine sand or coarser; Soil Taxonomy (2) all soils that have no diagnostic horizons or features below the epipedon; and (3) soils that are 75 cm or more deep to a densic, lithic, or paralithic contact, a petrocalcic horizon, or a duripan, are more than 75 cm deep to the upper boundary of any identifiable secondary carbonates, and are more than 75 cm deep to the lower boundary of any argillic, cambic, kandic, natric, oxic, or spodic horizon (all defined below). The minimum thickness is one-third of the thickness from the mineral soil surface to any of the features described in the paragraph above if (1) the texture throughout the epipedon is loamy very fine sand or finer and (2) depth to the feature described in the paragraph above is between 54 and 75 cm. The minimum thickness is 18 cm for all other soils. The mollic epipedon has less than 1,500 milligrams per kilogram of P2O5 soluble in 1 percent citric acid or has an irregular decrease in the amounts of P2O5 with increasing depth below the epipedon, or there are phosphate nodules within the epipedon. This restriction is intended to exclude plow layers of very old arable soils and kitchen middens that, under use, have acquired the properties of a mollic epipedon and to include the epipedon of a soil developed in highly phosphatic parent material. Some part of the epipedon is moist for 90 days or more (cumulative) in normal years during times when the soil temperature at a depth of 50 cm is 5 oC or higher and the soil is not irrigated. Sediments that have been continuously under water since they were deposited have a very high water content and are unable to support livestock. Although many soils that have a mollic epipedon are very poorly drained, the mollic epipedon is required to have an n value (defined below) of less than 0.7. Several accessory properties are common in soils that have a mollic epipedon. Most natural environments (not made by humans) that produce a mollic epipedon also produce 2:1 lattice clays from minerals that can be altered, preclude serious toxicity from aluminum or manganese, and ensure a reasonable reserve of calcium, magnesium, and potassium and of nitrogen if the soil has not been cultivated for a long time. These are accessory properties that are important to plant growth. Permeability is another accessory property important to most uses of the soil. The structure of the mollic epipedon facilitates the movement of moisture and air whenever the soil is not saturated with water. The content of organic matter indicates that the soil has received enough moisture to support fair to luxuriant plant growth in normal years. The mollic epipedon must be moist in at least some part for 90 days or more (cumulative) in normal years at times when the soil temperature is 5 oC or higher at a depth of 50 cm and when the soil is not irrigated. Although the mollic epipedon is a surface horizon that can be truncated by erosion, its many important accessory properties suggest its use as a diagnostic horizon at a high categoric level. Some soils have eroded to the extent that the epipedon is no longer thick enough to meet the requirements
23. 23. Horizons and Characteristics Diagnostic for the Higher Categories 25 for a mollic epipedon. In this case human activities have altered the surface horizon, changing a mollic epipedon into an ochric epipedon (defined below). a. 10 cm or the depth of the noncemented soil if the epipedon is loamy very fine sand or finer and is directly above a densic, lithic, or paralithic contact, a petrocalcic horizon, or a duripan that is within 18 cm of the mineral soil surface; or Required Characteristics The mollic epipedon consists of mineral soil materials and has the following properties: 1. b. 25 cm or more if the epipedon is loamy fine sand or coarser throughout or if there are no underlying diagnostic horizons (defined below) and the organic-carbon content of the underlying materials decreases irregularly with increasing depth; or When dry, either or both: a. Structural units with a diameter of 30 cm or less or secondary structure with a diameter of 30 cm or less; or c. 25 cm or more if all of the following are 75 cm or more below the mineral soil surface: b. A moderately hard or softer rupture-resistance class; and (1) The upper boundary of any pedogenic lime that is present as filaments, soft coatings, or soft nodules; and 2. Rock structure, including fine (less than 5 mm) stratifications, in less than one-half of the volume of all parts; and 3. (2) The lower boundary of any argillic, cambic, natric, oxic, or spodic horizon (defined below); and One of the following: (3) The upper boundary of any petrocalcic horizon, duripan, or fragipan; or a. All of the following: d. 18 cm if the epipedon is loamy very fine sand or finer in some part and one-third or more of the total thickness between the top of the epipedon and the shallowest of any features listed in item 6-c is less than 75 cm below the mineral soil surface; or (1) Colors with a value of 3 or less, moist, and of 5 or less, dry; and (2) Colors with chroma of 3 or less, moist; and (3) If the soil has a C horizon, the mollic epipedon has a color value at least 1 Munsell unit lower or chroma at least 2 units lower (both moist and dry) than that of the C horizon or the epipedon has at least 0.6 percent more organic carbon than the C horizon; or e. 18 cm or more if none of the above conditions apply; and 7. a. Content less than 1,500 milligrams per kilogram soluble in 1 percent citric acid; or b. A fine-earth fraction that has a calcium carbonate equivalent of 15 to 40 percent and colors with a value and chroma of 3 or less, moist; or c. A fine-earth fraction that has a calcium carbonate equivalent of 40 percent or more and a color value, moist, of 5 or less; and 4. A base saturation (by NH4OAc) of 50 percent or more; and 5. An organic-carbon content of: a. 2.5 percent or more if the epipedon has a color value, moist, of 4 or 5; or b. 0.6 percent more than that of the C horizon (if one occurs) if the mollic epipedon has a color value less than 1 Munsell unit lower or chroma less than 2 units lower (both moist and dry) than the C horizon; or c. 0.6 percent or more; and 6. After mixing of the upper 18 cm of the mineral soil or of the whole mineral soil if its depth to a densic, lithic, or paralithic contact, petrocalcic horizon, or duripan (all defined below) is less than 18 cm, the minimum thickness of the epipedon is as follows: Phosphate: b. Content decreasing irregularly with increasing depth below the epipedon; or c. Nodules are within the epipedon; and 8. Some part of the epipedon is moist for 90 days or more (cumulative) in normal years during times when the soil temperature at a depth of 50 cm is 5 oC or higher, if the soil is not irrigated; and 9. The n value (defined below) is less than 0.7. Ochric Epipedon The ochric epipedon fails to meet the definitions for any of the other seven epipedons because it is too thin or too dry, has too high a color value or chroma, contains too little organic carbon, has too high an n value or melanic index, or is both massive and hard or harder when dry (photos 8 and 9). Many ochric epipedons have either a Munsell color value of 4 or more, moist, and 6 or more, dry, or chroma of 4 or more, or they include an A or Ap horizon that has both low color values and low chroma but is too thin to be recognized as a mollic or
24. 24. 26 umbric epipedon (and has less than 15 percent calcium carbonate equivalent in the fine-earth fraction). Ochric epipedons also include horizons of organic materials that are too thin to meet the requirements for a histic or folistic epipedon. The ochric epipedon includes eluvial horizons that are at or near the soil surface, and it extends to the first underlying diagnostic illuvial horizon (defined below as an argillic, kandic, natric, or spodic horizon). If the underlying horizon is a B horizon of alteration (defined below as a cambic or oxic horizon) and there is no surface horizon that is appreciably darkened by humus, the lower limit of the ochric epipedon is the lower boundary of the plow layer or an equivalent depth (18 cm) in a soil that has not been plowed. Actually, the same horizon in an unplowed soil may be both part of the epipedon and part of the cambic horizon; the ochric epipedon and the subsurface diagnostic horizons are not all mutually exclusive. The ochric epipedon does not have rock structure and does not include finely stratified fresh sediments, nor can it be an Ap horizon directly overlying such deposits. The ochric epipedon by itself has few or no accessory characteristics, but an ochric epipedon in combination with other diagnostic horizons and features has many accessory characteristics. For example, if there is an underlying horizon in which clay has accumulated (defined later as an argillic horizon) and if the epipedon is seldom or never dry, carbonates are absent and base saturation is moderate or low in the major part of the epipedon unless the soil has been limed. If the texture is loamy, the structure breaks down easily when the soil is cultivated. Plaggen Epipedon The plaggen epipedon is a human-made surface layer 50 cm or more thick that has been produced by long-continued manuring (photo 10). In medieval times, sod or other materials commonly were used for bedding livestock and the manure was spread on fields being cultivated. The mineral materials brought in by this kind of manuring eventually produced an appreciably thickened Ap horizon (as much as 1 m or more thick). In northwestern Europe this custom was associated with the poorly fertile, sandy Spodosols. The practice more or less ceased at the turn of the 19th century, when fertilizers became available. The color of a plaggen epipedon and its organic-carbon content depend on the materials used for bedding. If the sod was cut from the heath, the plaggen epipedon tends to be black or very dark gray, to be rich in organic matter, and to have a wide carbon-nitrogen ratio. If the sod came from forested soils, the plaggen epipedon tends to be brown, to have less organic matter, and to have a narrower carbon-nitrogen ratio. Commonly, the organic-carbon content ranges from 1.5 to 4 percent. Values commonly range from 1 to 4, moist, and chromas are 2 or less. Soil Taxonomy A plaggen epipedon can be identified by several means. Commonly, it contains artifacts, such as bits of brick and pottery, throughout its depth. There may be chunks of diverse materials, such as black sand and light gray sand, as large as the size held by a spade. The plaggen epipedon normally shows spade marks throughout its depth and also remnants of thin stratified beds of sand that were probably produced on the soil surface by beating rains and were later buried by spading. A map unit delineation of soils with plaggen epipedons would tend to have straight-sided rectangular bodies that are higher than the adjacent soils by as much as or more than the thickness of the plaggen epipedon. Umbric Epipedon The umbric epipedon is a relatively thick, dark colored, humus-rich surface horizon or horizons (photo 11). It cannot be distinguished by the eye from a mollic epipedon, but laboratory studies show that the base saturation is less than 50 percent (by NH4OAc) in some or all parts. The umbric epipedon is used for defining taxa at different levels. For those soils in which the content of organic matter is roughly proportional to the darkness of the color, the most satisfactory groupings appear to be those that assign soils with a thick, dark colored surface horizon and soils with a light colored or thin surface horizon to different suborders. Structure, bulk density, cation-exchange capacity, and other properties are related to the amount and type of organic matter in these soils. In those kinds of soil where dark color is not related to the content of organic matter, the soils that have light colored epipedons are separated from the soils that have dark colored epipedons only at lower categoric levels, if at all. Properties The umbric epipedon consists of mineral soil material and is at the soil surface, unless it underlies either a recent deposit that is less than 50 cm thick and has fine stratification if not plowed or a thin layer of organic soil material. If the surface layer of organic material is so thick that the soil is recognized as a Histosol (defined below), the umbric epipedon is considered to be buried. The umbric epipedon has soil structure strong enough so that one-half or more of the horizon is not both hard, very hard, or harder and massive when dry. Very coarse prisms, with a diameter of 30 cm or more, are treated as if they were the same as massive if there is no secondary structure within the prisms. The restriction against massive and hardness applies only to those epipedons that become dry. The umbric epipedon has dark color and low chroma in 50 percent or more of its matrix. It has a Munsell color value of 3 or less, moist, and of 5 or less, dry, and chroma of 3 or less. If its structure is fine granular or fine blocky, the sample when broken may show only the color of the coatings of peds. The color of the matrix in such situations can be determined only
25. 25. Horizons and Characteristics Diagnostic for the Higher Categories by crushing or briefly rubbing the sample. Prolonged rubbing should be avoided because it may cause darkening of a sample if soft iron-manganese concretions are present. Crushing should be just sufficient to mix the coatings with the matrix. The dry color value should be determined after the crushed sample is dry enough for continued drying to produce no further change and the sample has been smoothed to eliminate shadows. Normally, the color value is at least 1 Munsell unit lower or the chroma at least 2 units lower (both moist and dry) than that of the C horizon (if present). Some parent materials, such as loess, cinders, alluvium, or shale, can also have dark color and low chroma. Soils that formed in such materials can accumulate appreciable amounts of organic matter but commonly show no visible darkening in the epipedon. In these dark colored materials, the requirement that the umbric epipedon have a lower color value or chroma than the C horizon is waived if the surface horizon(s) meets all of the other requirements for an umbric epipedon and, in addition, has at least 0.6 percent more organic carbon than the C horizon. Base saturation by the NH4OAc method is required to be less than 50 percent in some or all parts of the epipedon. The umbric epipedon is thought to be formed mainly by the decomposition of organic residues. The residues that are decomposed are partly roots and partly organic residues from the surface that have been taken underground by animals. Accumulation and turnover of the organic matter in the umbric epipedon probably are slower than in the mollic epipedon. The aluminum ions may be somewhat toxic to some kinds of soil micro-organisms. The minimum organic-carbon content throughout the thickness of the umbric epipedon is 0.6 percent. The minimum thickness of the umbric epipedon is dependent on the depth and texture of the soil. The minimum thickness is for soils with an epipedon that is loamy very fine sand or finer (when mixed) and that is directly above a densic, lithic, or paralithic contact, a petrocalcic horizon, or a duripan. These soils have a minimum thickness of 10 cm. Soils that are 10 to 18 cm deep have an umbric epipedon if the whole soil meets all of the criteria for an umbric epipedon when mixed. The minimum thickness is 25 cm for (1) all soils with a texture throughout the epipedon of loamy fine sand or coarser; (2) all soils that have no diagnostic horizons or features below the epipedon; and (3) soils that are 75 cm or more deep to a densic, lithic, or paralithic contact or a duripan and are more than 75 cm deep to the lower boundary of any argillic, cambic, kandic, natric, oxic, or spodic horizon (all defined below). The minimum thickness is one-third of the thickness from the mineral soil surface to any of the features in the paragraph above if (1) the texture in some or all parts of the epipedon is loamy very fine sand or finer and (2) depth to the feature listed in the paragraph above is between 54 and 75 cm below the mineral soil surface. 27 The minimum thickness is 18 cm for all other soils. The umbric epipedon has less than 1,500 milligrams per kilogram of P2O5 soluble in 1 percent citric acid or has an irregular decrease in the amounts of P2O5 with increasing depth below the epipedon, or there are phosphate nodules within the epipedon. This restriction is intended to exclude plow layers of very old arable soils and kitchen middens that, under use, have acquired the properties of an umbric epipedon and to include the epipedon of a soil developed in highly phosphatic parent material. Some part of the epipedon is moist for 90 days or more (cumulative) in normal years during times when the soil temperature at a depth of 50 cm is 5 oC or higher and the soil is not irrigated. Sediments that have been continuously under water since deposition have a very high water content and are unable to support livestock. Although some soils that have an umbric epipedon are very poorly drained, the umbric epipedon is required to have an n value (defined below) of less than 0.7. Several accessory properties are common in soils that have an umbric epipedon. These soils have the potential for toxicity from aluminum, and they are commonly low in calcium, magnesium, and potassium if lime and fertilizer have not been applied. These are accessory properties important to plant growth. The structure of the umbric epipedon facilitates the movement of moisture and air whenever the soil is not saturated with water. The content of organic matter indicates that the soil has received enough moisture to support fair to luxuriant plant growth in normal years. The umbric epipedon must be moist in at least some part for 3 months or more (cumulative) in normal years at times when the soil temperature is 5 oC or higher at a depth of 50 cm and when the soil is not irrigated. Although the umbric epipedon is a surface horizon that can be truncated by erosion, its many important accessory properties suggest its use as a diagnostic horizon at a high categoric level. Some plaggen epipedons meet all of the requirements for an umbric epipedon but also show evidence of a gradual addition of materials during cultivation, whereas the umbric epipedon does not have the artifacts, spade marks, and raised surfaces that are characteristic of the plaggen epipedon. Required Characteristics The umbric epipedon consists of mineral soil materials and has the following properties: 1. When dry, either or both: a. Structural units with a diameter of 30 cm or less or secondary structure with a diameter of 30 cm or less; or b. A moderately hard or softer rupture-resistance class
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