Classification of Soils in Soil
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
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
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
The United States system of soil taxonomy
has a hierarchical structure. At
the top of this hierarchy, there are currently 12 soil orders:
- 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.).
- 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).
- 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.).
- 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.).
- 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
- 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.
- 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
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.)
- Histisols organic soils named from the Greek histos,
meaning tissue. They must contain between 1218% 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.)
- Vertisols soils high in shrink-swell clays in areas with a
periodic dry season, during which they form huge cracks, 10-12 cm (45
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
- Aridisols soils of arid climates. They have calcic, gypsic or
salic horizons. There are none in South Carolina.
- 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.
- Gelisols soils underlain by permafrost during the warm months
of the year and subject to cryoturbation
. There are none in South
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 soils
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
R. Scharf, SCDNR Land, Water, and Conservation Division
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.