Soil classification

Soil classification ideals with the systematic categorization of soils based on distinguishing characteristics as well as criteria that dictate choices in use.

Overview

Soil classification is a contentious subject, from the structure of the system itself, to the definitions of classes, and finally in the application in the field. The essential problem is that soils do not reproduce or have DNA like living organisms, so no "objective" criteria can be used to choose among classifications. The most qualified specialists in the world can and do spend hours during a field trip at each soil profile, arguing about the classification. Yet, in a well-constructed system, the different names refer to similar concepts, so that interpretations do not vary widely.

Types by discipline

Soil science

Marlin Cline ("Basic principles of soil classification", Soil Science, 2:81–91, 1949) stated the basic rationale behind utilitarian classification:

"The purpose of any classification is so to organize our knowledge that the properties of objects may be remembered and their relationships may be understood most easily for a specific objective. The process involves formation of classes by grouping the objects on the basis of their common properties. In any system of classification, groups about which the greatest number, most precise, and most important statements can be made for the objective serve the purpose best."

For soils, experience has shown that a natural system, i.e. grouping soils by their intrinsic property, behaviour, or genesis, results in classes that can be interpreted for many diverse uses. This is in contrast to a technical classification (such as the Fertility Capability Classification), where soils are grouped according to their fitness for a specific use. Natural systems are based strictly on presumed soil genesis, but modern hierarchical systems such as USA soil taxonomy and the World Reference Base for Soil Resources use objective criteria (both field morphology and laboratory tests) as far as possible, to reduce disagreements among classifiers.

Another approach is numerical classification, also called ordination, where soil individuals are grouped by multivariate statistical methods such as cluster analysis. This is supposed to create natural groupings without requiring any inference about soil genesis.

In soil survey, as practiced in the United States, soil classification usually means criteria based on soil morphology in addition to characteristics developed during soil formation. Criteria are designed to guide choices in land use and soil management. As indicated, this is a hierarchical system that is a hybrid of both natural and objective criteria. USDA soil taxonomy provides the core criteria for differentiating soil map units. This is a substantial revision of the 1938 USDA soil taxonomy which was a strictly natural system. Soil taxonomy based soil map units are additionally sorted into classes based on technical classification systems. Land Capability Classes, hydric soil, and prime farmland are some examples.

In addition to scientific soil classification systems, there are also vernacular soil classification systems. Vernacular (descriptive) systems have been used for millennia, while scientifically based systems are relatively recent developments. Soil classification systems. Url last accessed 2006-04-18

Engineering

Civil engineers classify "soils", or more properly earthy materials, for their properties relative to foundation support or use as building material. These technical systems are designed to predict some of the engineering properties and behavior of a soil based on a few simple laboratory or field tests. The most common is the Unified Soil Classification System, with three major groups: (1) coarse-grained, sands and gravels, (2) fine-grained, silts and clays, and (3) highly organic soils (referred to as peat even when the soil is not truly a peat). The first two groups are then subdivided as follows:

Coarse grain: gravels, sands, based on the grain-size of the coarse-grained fraction.
Fine grain: silts, clays, organics silts & clays, based on plasticity and organic content.

Fine-grained soils are then subdivided according to their plasticity (for fine soils and coarse soils with some fines), while coarse-grained soils are subdivided by the properies of the fines or their grain-size homogeneity.

A formal geotechnical engineering description will include other properties of the soil, including color, in-situ moisture content, in-situ strength, and somewhat more detail about the material properties of the soil than is provided by the USCS description.

Notes

External links

References

Overviews

Eswaran, H., Rice, T., Ahrens, R., & Stewart, B. A. (Eds.). (2002). Soil classification : a global desk reference. Boca Raton, Fla.: CRC Press.

Current international system

Buol, S.W., F.D. Hole, R.J. McCracken, and R.J. Southard. 1997. Soil Genesis and Classification, 4th Edition. Iowa State Univ. Press, Ames, IA.
Driessen, P., Deckers, J., Spaargaren, O., & Nachtergaele, F. (Eds.). (2001). Lecture notes on the major soils of the world. Rome: FAO.
FAO. (1998). World Reference Base for Soil Resources. Rome: Food and Agriculture Organization of the United Nations.

Current natural systems

Agriculture Canada Expert Committee on Soil Survey. (1987). The Canadian system of soil classification (2nd ed.). Ottawa: Canadian Government Publishing Centre.
Avery, B. W. (1980). Soil classification for England and Wales: higher categories. Cranfield, England: Cranfield University, Soil Survey & Land Research Centre.
Baize, D., & Girard, M. C. (Eds.). (1995). Référentiel pédologique 1995. Paris: Institut National de la Recherche Agronomique.
Baize, D., & Girard, M. C. (Eds.). (1998). A sound reference base for soils: The "Référentiel Pédologique" (English translation by Hodgson J.M., Eskenazi N.R., & Baize D. ed.). Paris: Institut National de la Recherche Agronomique.
Hewitt, A. E. (1992). Soil classification in New Zealand: legacy and lessons. Australian Journal of Soil Research, 30, 843-854.
Isbell, R. F. (1996). The Australian soil classification. Collingwood, Victoria, Australia: CSIRO.
Soil Classification Working Group. (1991). Soil classification: a taxonomic system for South Africa. Pretoria: Department of Agricultural Development, RSA.
Soil Survey Staff. (1999). Soil taxonomy: a basic system of soil classification for making and interpreting soil surveys (2nd ed.). Washington, DC: US Department of Agriculture Soil Conservation Service.

Current technical systems

Boorman, D. B., Hollis, J. M., & Lilly, A. (1995). Hydrology of soil types: a hydrologically-based classification of the soils of the United Kingdom (No. 126): UK Institute of Hydrology.
Klingebiel, A. A., & Montgomery, P. H. (1961). Land capability classification. Washington, DC: US Government Printing Office.
Sanchez, P. A., Palm, C. A., & Buol, S. W. (2003). Fertility capability soil classification: a tool to help assess soil quality in the tropics. Geoderma, 114(3-4), 157-185.

Earlier systems, of historical interest

Baldwin, M., Kellogg, C. E., & Thorp, J. (1938). Soil classification. In Soils and men: Yearbook of agriculture (pp. 979-1001). Washington, DC: U.S. Department of Agriculture.
Simonson, R. W. (1989). Historical aspects of soil survey and soil classification with emphasis on the United States, 1899-1970. Wageningen, NL: International Soil Reference and Information Centre (ISRIC).

Principles

Butler, B. E. (1980). Soil classification for soil survey. Oxford: Oxford Science Publications.Science, 96,
Cline, M. G. (1949). Basic principles of soil classification. Soil Science, 67(2), 81-91.
Cline, M. G. (1963). Logic of the new system of soil classification. Soil 17-22.
Webster, R. (1968). Fundamental objections to the 7th approximation. Journal of Soil Science, 19, 354-366.

Numerical classification

McBratney, A. B., & de Gruijter, J. J. (1992). A continuum approach to soil classification by modified fuzzy k-means with extragrades. Journal of Soil Science, 43(1), 159-175.

Pedology

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