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CHAPTER    5
Classification of Soils
Gerhard B. Lee
  The purpose of soil classification is to organize our knowl-
edge of soils by grouping similar soils into manageable classes.
If intellectually possible, it would be better to deal with each
soil individually, i.e., with each pedon or polypedon; however,
individual soils are so numerous that in most cases this cannot
be done. Because it is useful to deal with soils at different levels
of generalization, classification systems have been devised that
group soils at several categorical levels. These levels range from
very general classes to very specific ones. In summary, soil
classification is simply a device by which we order, or organize,
taxonomic units of soil into usable classes.
  Soils have been classified in several ways (Bidwell and Hole,
1963). A simple classification relates to use, as illustrated by
the terms corn soils, alfalfa soils, hardwood soils, and pine
soils. Such systems, while useful for some purposes, may
change rather rapidly with changes in technology. More perma-
nent, scientific  systems of classification are based on soil
characteristics and their relationship to soil genesis. Such
systems express universal relationships that exist in nature and
enable one to understand, remember, and predict from the in-
formation obtained. Scientific systems of soil classification
have the following advantages:
  1. They enable us to identify pedons or soil individuals and
     place them in their proper class.
  2. They make it possible for us to organize our information
     about soils.
  3. They show natural relationships among soils.
  4. They allow us to make predictions, i.e., extend to other
     soils information gained by study of and experience with
     one soil.
  At the present time several systems of soil classification are in
use in the United States. The New System of Soil Taxonomy
(final version now in press) has been in official use by the
USDA since 1965. However, much published material exists in
which older systems or terms from older systems are used. The
county maps and soil survey reports published prior to the early
1930s, for example, classify soils as series and types. The soil
  1. The Land-Use Capability System of the U.S. Soil Conservation
Service is a useful classification of soil landscape units on the basis of
degrees of hazard to the soil and response of the soil under various
managements. This classification is, therefore, not based solely on soil
properties and is considered a practical system rather than a scientific
one.
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series unit dates back to the early 1900s and was first conceived
of as a textural sequence of soils from a common source. Thus
we might have Miami clay, Miami clay loam, loam, sandy
loam, etc., all derived from glacial drift. With time the series
concept narrowed. In the soil survey report for Green Lake
County (Whitson et al., 1929), soils were divided into series on
the basis of the amount of organic matter in them, topography,
drainage conditions, and other factors. 'Each series represents
a definite combination of these factors" and each series is given
a proper name. Soil types were reorganized on the basis of dif-
ferences in textural class of otherwise uniform material.
  In the early 1930s, United States systems of soil classification
changed drastically under the influence of C. F. Marbut (see
his Soils of the United States, 1935). In Marbut's concept,
adapted from the Russian approach, the soil profile was en-
visioned as the unit of study. Soils were classified according to
similarities in morphological features, such as kind and ar-
rangement of soil horizons, their color, texture, structure, con-
sistence, and depth, as well as composition and origin of the
initial geologic deposits. Marbut's system was revised in 1938
(Baldwin, Kellogg, and Thorp) and again in 1949 (Thorp and
Smith). The 1949 system was used until the "new" system was
officially adopted in 1965 and appears in many soil survey
reports and other soils publications of that period. For this
reason a brief outline of the higher categories of this system is
included (Table 5-1). As can be seen from this outline, soils in
the order and suborder categories were classified essentially on
the basis of assumed genetic factors of soil formation. At the
Great Soil Group level, however, groupings were made on the
basis of kind and arrangement of soil horizons.
  In addition to the higher categories, the 1949 system had
three lower categories: family, series, and type. An example of
how a soil such as Miami silt loam would be classified in that
system is shown in Table 5-2.
  The "new" United States soil classification system (Soil Con-
servation Service, 1960) was developed over a period of many
years, most intensively since the early 1950s. At that time it was
realized that the older systems, patterned after Marbut, had
limitations that did not allow the proper classification of all
soils. As a result a new system was developed, with greater reli-
ance on properties of soils that could be observed and mea-
sured. The new system placed less emphasis on soil genesis,
although a genetic thread runs throughout it. Since its recent
adoption, the new system has been in official use by the USDA
Soil Conservation Service.
  The new system has several unique features. One is the
nomenclature used in the higher categories. To avoid confusion
with old terms, new names, formed from elements derived
mainly from the classical languages, have been coined. In the
past, for example, dark-colored granular soils in southern Wis-
consin were called Prairie soils, because they presumably
formed under prairie vegetation. In the new system they are
called Mollisols from mollis, Latin for "soft," referring to their
good tilth, and solum. Latin for "soil," with i as connecting
vowel. Lists of the formative elements used in the higher cate-
gories of Wisconsin soils in the new system are shown in Table
5-3.
  The basis for classification of soils into classes in the new
system includes morphological features that can be observed