Each of the world's soils is assigned to one of the twelve orders, largely on the basis of soil properties. The properties are a result of the five soil forming factors acting on the parent material over time. Emphasis for placing a soil into a soil Order is determined by the presence or absence of the diagnostic features. It is interesting to note that other countries may have different classification systems, but most are similar to the system used in the USA. The United Nations FAO Soil Map of the World, uses a different system that has 31 major soil groups.http://www.fao.org/ag/agl/agll/wrb/wrbmaps/htm/soilres.htm
A summary of characteristics, a sample profile, and the the diagnostic features present are given for each of the 12 soil orders. Be sure to look at the map for each order to see its location in the U.S.A. US SOILS MAP. You will also need to know where the orders are located in the world.World Soils Map
Entisols :
Soils that have little or slight development and properties that
reflect their parent material (ent root from the word recent).
They include soils on steep slopes, flood plains, and sand dunes. They also
form on very resistant rock or deep deposits of sand. They occur in many
environments. They have an ochric and none diagnostic features. The typical
profile sequence would be A, C, or A, Bw, C (where the Bw is not cambic due to
being too sandy). Entisol Map
Gelisols : Soils that
commonly have a dark organic surface layer and mineral layers underlain by
permafrost ( el root from the Latin word gelare for to freeze ).
These soils are commonly in the tundra regions of Alaska. A
typical horizon sequence would be: A, Bw, Cf. The alternate thawing and
freezing of ice layers results in special features in the soil, such as
frost-heaving, deformed landscape or thermo-karst features. For many soils the
slow decomposition of the organic matter due to cold temperatures results in
the formation of a peat layer at the surface. The diagnostic feature is the
presence of permafrost. Gelisols Map
Histolsols : 
Dark soils that have slightly decomposed to well decomposed
organic materials derived from the sedges, grasses, leaves, hydrophytic plants
and woody materials ( root ist from the Greek histos for tissue.
These soils dominantly are very poorly drained and occur in low-lying areas,
like swamps, coastal marshes or deltas. They occur in many environments from
Alaska to the tropical islands. In cold regions the low temperatures retard
organic matter decomposition and in warm regions the soil is saturated.
Histosols form many areas of valuable wetlands. If drained they decompose
rapidly but are valuable agronomic soils for vegetables or turf. The diagnostic
feature is the Histic epipedon. A typical horizon sequence would be Oi,
Oe, Oa. Histosol Map
Vertisols : 
Vertisols are clayey soils that shrink and develop cracks when
they dry and swell shut when they are moist ( the root ert comes from
the Latin word Vertare for turn over). The shrinking and swelling damage
buildings and roads. The mineral montmorillonite in the clay fraction causes
the soil to have these properties. The soil develops shiny surfaces in the
subsoil called slickenslides as a result of the peds rubbing against each other
as they swell (the ss designation indicates the presence of these features) The
diagnostic feature for the soil order is the high clay percentage and the
presence of montmorillonite a 2:1 clay the causes the shrinking and swelling
feature. A typical horizon sequence would be A, Bss and C.
Vertisol Map
Andisols :
Andisols form in material that has recently ejected
from volcanoes (ash) and are commonly found on land surrounding the Pacific Rim
( the root and comes from the Japanese word ando meaning black).
Since volcanoes have erupted several times during the past hundred to million
years the soils often have distinct layers. The soils have high porosity,
particle surface area and water-holding capacity. The also have a very low
bulk density or are very light in weight. On steep
slopes they are forested but on more gently slopes they are a prized agronomic
soil because of their innate fertility and water holding capacity. The
diagnostic feature is the low bulk density of volcanic ash soils. A typical
horizon sequence would be A ,Bw, C, R.Andisol
Map
Inceptisols : 
Inceptisols have altered horizons but have not weathered enough to
become illuvial horizons ( the root ept comes from the Latin word
inceptum for beginning). These are young or in the beginning stage of
soil formation. Many are shallow to bedrock, or occur on steeply sloping land.
These soils occur in a wide range of temperature and moisture environments. The
diagnostic features are the ochric epipedon and cambic horizon. A typical
horizon sequence would be A, Bw, C. Inceptisol
Map
Aridisols:
Aridisols are the dry soils of deserts ( the root
arid or id comes from the Latin aridus for dry). They do not
receive enough rain to permit normal agriculture. They may have a clay enriched
subsoil and /or cemented to non cemented deposits of salts or carbonates.
Salinization, or salt buildup is the greatest danger in using Aridisols for
irrigated agriculture. Most areas of true desert are retained as wilderness
though the margins may be used for grazing. A typical horizon sequence would be
A, Bk, C. The diagnostic features are an ochric epipedon and an argillic,
cambic, calcic or gypsic subsurface horizon. Aridisol Map
Mollisol :Mollisols have
a distinctive dark surface (mollic epipedon) that is enriched with organic
matter. The surface layer has a soft, fluffy feel. These soils formed from
nutrient-rich parent materials and are commonly in grasslands. They are
naturally fertile and generally hold large amounts of water. These soils are
prized for agriculture. They are dominantly in the Great Plains and Western
States. The diagnostic feature is the mollic epipedon.
Typical horizon sequence would be A1, A2, AB, Bw, BC, C.
Mollisol Map
Alfisols :Alfisols form
in semiarid to humid areas that have a clay enriched and nutrient enriched
subsoil (argillic horizon).
They commonly have a mixed
vegetative cover but were dominantly forested soils. Alfisols have the
potential to be very productive if conserved, but can also degrade rapidly if
eroded. The diagnostic features are the ochric epipedon and the argillic
horizon. A typical horizon sequence would be A, E, Bt, C.
Alfisol Map
Ultisol :
Ultisol are similar to Alfisols in that they have an ochric and
argillic diagnostic features, however they are highly weathered and thus low in
nutrients( the root ult comes from the Latin ultimus for ultimate). They
have lower pH and organic matter than Alfisols and often are redder in color.
With soil amendments they are productive. The surface soil usually is
coarser-textured, making it easy to till, while the clay-enriched subsoil
stores water for plant roots. These soils are extensive in the southeastern
U.S. . A typical horizon sequence would be A, E, Bt, BC, C.
Ultisol Map
Spodosol : 
Spodosols typically have a dark surface layer underlain by a light
eluvial horizon over a reddish, aluminum and /or iron enriched horizon (ochric,
albic, and spodic diagnostic features). The soil gets its name from the
bleached layer (E horizon) that looks like wood ashes or the Greek work
spodos for wood ashes. The soils are acid and generally under forest.
Although some are used for agriculture, this requires careful water and
fertility management because they are very sandy. They are common in the
northeaster U.S., the higher elevations in the Rocky Mountains, and in Florida.
They require a sandy parent material and leaching, thus they need high amounts
of rainfall. The diagnostic features required are the ochric and spodic
horizons. The typical horizon sequence would be: A, E, Bhs, Bs, C.
Spodosol Map
Oxisols :
Oxisols are only in the tropics. In the U.S.A. they have been
found in Hawaii and Puerto Rico. The root comes from the French word
oxide for oxides. They are intensely weathered, resulting in colors that
are yellow to red from the accumulation of large amounts of iron oxides. Even
if they are clayey, their ability to retain nutrients is low as the clay
fraction consists of iron oxides and kaolinite. They frequently do not have
very distinct horizons and are very deep. A variety of tropical crops can be
grown in humid areas but generally much fertilizer is needed. The diagnostic
feature is the oxic horizon. A typical horizon sequence would be A, Bo1, Bo2.
Oxisol Map
For a Review of the 12 Soil Orders this web page from the Univ. of Idaho will be most helpful 12 Soil Orders
© Regents of the University of Minnesota, 2007. The University of Minnesota is an equal opportunity educator and employer.
Prepared by tcooper@umn.edu
