Soil classification is the branch of soil science which deals with the systematic categorization of soils based on distinguishing characteristics developed during soil formation as well as criteria that dictate choices in land use and soil management.

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 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.

Civil engineers classify "soils", or more properly earthy materials, as foundations or building material. These technical systems are designed to predict 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, with three major groups: (1) coarse grain, (2) fine grain, and (3) highly organic. The first two groups are then subdivided as follows:

1. Coarse grain: gravels, sands
2. Fine grain: silts, clays, organics silts & clays

These are then subdivided according to their plasticity (for fine soils and coarse soils with some fines) and homogeneity (for coarse soils).

Soil classification remains 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.

Related articles

• World Reference Base for Soil Resources
• USA soil taxonomy
• FAO soil classification
• International Committee on Anthropogenic Soils (ICOMANTH)

External links

• A Compendium of On-Line Soil Survey Information - Soil Classification for Soil Survey by D. G. Rossiter
• Engineering soil description and classification

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

• 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.