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Soil Systems
Soil Orders
The are several different ways in which soils
have been classified. The brief discussion below reflects the
way soils are classified in the United States. Soils are
classified on the basis of diagnostic
horizons, that are
different from the O, A, B, C horizons. A diagnostic
horizon has a unique feature that reflects the soil development
processes acting at a site. For instance, a mollic epipedon (an
epipedon is a type of diagnostic horizon) is a organic-rich horizon typical of a mollisol soil. The following material
is intended to give you a basic understanding of the major
categories of soils called soil orders and the
environments under which they form. For a comprehensive description
of how soils are classified in the United States, see the Soil
Taxonomy (only for those of you dying to learn more about
soil classification than you ever wanted to know!)
Figure SS.20 Global Soil Regions
Click image to enlarge
(Courtesy NRCS)
 Entisols, like this one found in southwestern
Wisconsin, are soils lacking horizons because their parent material has only recently
accumulated. Entisols also form where the parent material is quartz sand, in which
horizons do not easily form. They have a wide geographic distribution and can be found in
any climate and under any vegetation. Entisols and Inceptisols are often found on
floodplains, delta deposits, or steep slopes where parent material has difficulty
accumulating.
Figure SS.21 Entisol
Click image to enlarge (Image Source:
USDA NRCS Used with permission)
Inceptisols are soils just starting to show
horizon development because the soil is quite young. You can see the differentiation of
layers in an inceptisol formed on colluvium in West Virginia on the right. Inceptisols,
like Entisols, are found in any type of environment and
are commonly found forming in alluvium on
floodplains and delta deposits.
Figure SS.22 Inceptisol
Click image to enlarge (Image Source:
USDA NRCS Used with permission)
 Histosols have a very high content of organic matter in
the dark upper layer of the profile. Found in many different environments from the tundra
to the tropics, Histosols form in places where organic matter is slow to decompose and
thus accumulates over time such as bogs and swamps. They are often
"mined" for peat which is dried and burned as fuel.
Figure SS.23 Histosol
Click image to enlarge (Image Source:
USDA NRCS Used with permission)
Aridisols are soils of arid and semiarid environments
where moisture is scarce. They are typically light in color as there is little
vegetation to add organic matter to the soil profile. A negative moisture balance in these soils inhibits eluviation. Calcification and salinization are important soil forming
processes acting in these soils. Soil horizons are weakly developed and sodium is often
high in concentration making them alkaline. The coarse texture of aridisols makes it
difficult to retain much moisture. Aridisols can be quite fertile soil if irrigation is properly used. Used
improperly, a salt crust can form on the soil. Most aridisols are used for grazing.
Figure SS.24 Aridisol Soil
Click image to enlarge (Image Source:
USDA NRCS Used with permission)
 Andisols
are soils developing in parent material containing at least fifty percent volcanic ash. The layers
of ash can be seen in this Andisol from Hawaii. Naturally fertile soils, they support a
dense natural cover in moist climates. Andisols occur around individual volcanoes created
from andesite-rich magma. They are common on the volcanic islands and mountains of
"The Ring of Fire", that encircles the Pacific Ocean from North America through
Japan.
Figure SS.25 Andisol
Click image to enlarge (Image Source:
USDA NRCS Used with permission)
Vertisols are dark black soils rich in
expandable clay minerals. The clay readily swells upon wetting and shrinks when dried.
Though found in every type of climate, they are often found in steppe and wet/dry tropical
climates where the soil develops deep cracks as it dries. Surface fragments fall into the
cracks and are "swallowed" when the soil swells upon wetting. The
soil then
develops an
"inverted profile" with organic material that is typically located
near the surface of the
profile is now found at depth.
Figure SS.26 Vertisol
Click image to enlarge (Image Source: Soil Science Division
at the University of Idaho Used with permission)
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