Hole, Francis Doan, 1913-2002 / Soils of Wisconsin
Lee, Gerhard B.
Chapter 5 classification of soils, pp. 40-46
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. 40 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
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