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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