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Classification of Soils in Soil Taxonomy

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Soil Taxonomy

Just as plants, animals, and other living things are named and classified using taxonomic systems, soils are also classified. Many nations, as well as the Fisheries and Agricultural Organization of the United Nations, have their own system to classify their own soils. The one used in the United States (as well as several other countries) is called Soil Taxonomy (Soil Survey Staff 1975). Almost every soil on earth can be classified according to this system, and it has become the standard soil classification system in scientific journals throughout the world. Soil Taxonomy is periodically updated to encompass the results of new research and studies, with the latest complete key published in 1998.

Classification of Soils in Soil Taxonomy

Every soil is classified in six hierarchical categories. From highest rank to lowest (i.e., in order of increasing differentiating characteristics), these categories are order, suborder, great group, subgroup, family, and series.

A Few of the Diagnostic Horizons Used in Soil Taxonomy
Mollic horizon is thick, dark, fertile surface horizon, usually formed in grasslands, that is high in “bases.” They are the A horizon in soils that have them.

Argillic, kandic, and natric horizons are all subsurface horizons with appreciable clay accumulations. They are all or part of the B horizon in soils that have them. These usually form in soils supporting forest vegetation.

Spodic horizons are B horizons that are subsurface accumulations of organic material mixed with aluminum and/or iron oxides. These also form in soils supporting forest vegetation.

Calcic, gypsic and salic horizons are horizons that have accumulations of calcium carbonate, gypsum or salts. These form in dry climates.

The United States system of soil taxonomy has a hierarchical structure. At the top of this hierarchy, there are currently 12 soil orders:

  1. Entisols — soils with little profile development (derived from recent). These soils may have an A horizon but no subsoil horizons. There are a number of these soils in the ACE Basin study area (e.g. Chipley series, Lakeland series, Bohicket series, etc.).
  2. Inceptisols — soils in the early stages of development, with little subsurface accumulation of any materials (derived from inception). These soils have an A horizon and sometimes a weak B horizon. There are a few of these soils in the ACE Basin study area (e.g. Dawhoo series).
  3. Alfisols — soils with argillic, kandic, or natric horizons, high in “bases” (high fertility). These are mostly forest soils. (“Alf” comes from an archaic term from soil science history.) There are a number of these soils in the ACE Basin study area (e.g. Yonges series, Edisto series, Argent series, etc.).
  4. Ultisols — soils are similar to Alfisols, except that they are low in “bases” (low fertility). These are forest soils and are the dominant soil order in the Southeastern United States. (“Ult” derived from ultimate in weathering and profile development.) Many soils in the ACE Basin study area, and South Carolina as a whole, are Ultisols (e.g., Goldsboro series, Lynchburg series, Rains series, etc.).
  5. Spodisols — unusual forest soils with B horizons that are darkened by an accumulation of humic materials and also enriched with iron and/or aluminum (a spodic horizon). Spodos is Greek for ash, which is how the E horizon of these soils appear. This soil often forms in the boreal forests of North America and Eurasia; however, a number occur in the southeast United States. These soils only form in sands (clay prohibits their formation) and coarse silts. The organic-stained B horizon (denoted Bh in this area, for humus ) is found in the zone where the water table fluctuates. (Some ACE Basin Spodosols include Baratari series, Seewee series, Ridgeland series, etc.)
  6. Oxisols — highly oxidized soils made up primarily of aluminum and iron oxides, which represent the residue of the minerals after extreme weathering. These occur in the tropics. None are found in South Carolina.
  7. Mollisols — grassland soils with deep, thick A horizons, high in fertility. They are easily worked for agricultural purposes. The name stems from the Latin mollis, meaning soft. There are three in South Carolina, but they did not form under a grassland ecosystem. ACE Basin Mollisols are wet soils that accumulated significant organic matter in their A horizons. They are high in “bases” like calcium. This is likely due to underlying marl deposits. (ACE Basin Mollisols include the Santee series.)
  8. Histisols — organic soils named from the Greek histos, meaning tissue. They must contain between 12–18% organic matter but usually contain much more. These are very wet soils (with the exception of some in the far northern latitudes) and include peat and muck. (ACE Basin Histosols include the Pungo series and the Handsboro series.)
  9. Vertisols — soils high in shrink-swell clays in areas with a periodic dry season, during which they form huge cracks, 10-12 cm (4–5 inches) wide. The dry surface crumbles or is blown into these cracks, gradually turning the soil over. Vert comes from invert. There are none in South Carolina.
  10. Aridisols — soils of arid climates. They have calcic, gypsic or salic horizons. There are none in South Carolina.
  11. Andisols — soils whose parent materials are of recent volcanic origin. They contain much fine volcanic ash (glass). The name comes from andesite, a rock of volcanic origin. There are none in South Carolina.
  12. Gelisols — soils underlain by permafrost during the warm months of the year and subject to cryoturbation . There are none in South Carolina.

These 12 soil orders are further subdivided into suborders, great groups, subgroups, families, and more than 15,000 series. A detailed explanation of soil taxonomy is not possible here, but a few pointers are in order.

With the exception of the soil series level, the taxonomic name of a soil includes elements of all the taxonomic levels above it. Each syllable of the name has meaning. The most important characteristics of the soil (the order level) are explained in the last syllable. For example, the subgroup name Aquic Paleudult indicates that it is a member of the Paleudult great group, the Udult suborder, and the Ultisol order. The name tells us that this is a weathered soil with a B horizon that has an accumulation of clay and is low in fertility (from the syllable ult), is in a humid climate (from the syllable ud) with deep profile development (from the syllable pale), and has evidence of a water table that is within three feet of the surface for part of the year (from the adjective aquic). Although the name looks cumbersome, it offers a soil scientist a tremendous amount of information. In the list of soil orders, you will note that a syllable is italicized. When this italicized syllable appears at the end of a soil’s taxonomic name, you know that it belongs to that particular soil order. The lists of syllables for suborders and great groups, as well as the adjectives used for subgroups, are quite extensive and will not be covered here. Some (like aquic) are self-explanatory.

Back to Soil Composition and Formation

Author

R. Scharf, SCDNR Land, Water, and Conservation Division


References

Soil Survey Staff. 1975. Soil taxonomy: a basic system of soil classification for making and interpreting soil surveys. US Department of Agriculture, Soil Conservation Service. U.S. Government Printing Office, Washington, DC.


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