Dominant Soil Orders1

ALFISOLS
Dominant Soil Orders 1
US Distribution maps: http://soils.usda.gov/technical/classification/orders/
Alfisols (‘al’ for aluminum, ‘fi’ for ferric=iron) occur in areas with sufficient precipitation
to move clay and aluminum and iron ions downward, accumulating in the B horizon. Often
the clay accumulation layer makes the soil unfavorable for plant growth. They may have a
hardpan horizon. Found in humid and subhumid environments with tall grasslands and
savanas, from subtropical to cold temperature regions, with many of these soils now
supporting croplands.
ULTISOLS
Ultisols have a horizon that contains an appreciable amount of
translocated silicate clay and few cations (basic ions). Develop in
humid subtropical-tropical climates, under forest or savanas. Now
commonly used as cropland or pasture.
OXISOLS
Oxisols are the most intensely weathered of all soils. They occur in
tropical and subtropical regions. They have gentle slopes on
surfaces of great age. They are mixtures of quartz, kaolin, free
oxides, and organic matter. For the most part they are nearly
featureless soils without clearly marked horizons. Differences in
properties with depth are so gradual that horizon boundaries are
generally arbitrary. Most are infertile. The natural vegetation
ranges from tropical rain forests to desert savannas.
INCEPTISOLS
Inceptisols are weakly developed soils of humid and subhumid
regions. They have altered upper horizons (A horizons) that have
lost bases or iron and aluminum but retain some weatherable
minerals. Yet have no strong illuvial (B) horizons (where minerals
leached from upper layers are deposited) – in contrast to well-
developed soils.
ARIDISOLS
Aridisols have well developed, but subtle soil horizons and develop
in regions with low precipitation. What leaching that does occur
translocates calcium carbonate and other salts, sometimes forming a
hardpan (duripans or caliche). They are found from subtropical to
cold temperate desert shrublands and arid grasslands.
ANDISOLS
Andisols are young soils generally developed from deposits of
volcanic erecta (e.g., ash, cinders), with volcanic glass grains a
defining character. Profiles weakly weathered, with high organic
matter content.

MOLLISOLS
Mollisols have a dark-colored, organic-rich surface horizon (A horizon) and are found
largely in grasslands. They occur from tropical to cold temperate, and from subhumid
to semiarid environments.
SPODOSOLS
Spodosols have moderately acidic sandy profiles with an ashywhite
upper horizon. In these soils, amorphous mixtures of
organic matter and aluminum, with or without iron, have
accumulated. They are typical of sandy soils under cool, wet
conditions under conifer and hardwood forests such as in the
boreal/cold temperate forests of Canada.
GELISOLS
Gelisols are soils that have permafrost within 100 cm of the soil
surface and/or have gelic materials within 100 cm of the soil
surface and have permafrost within 200 cm. Gelic materials are
mineral or organic soil materials that have evidence of
cryoturbation (frost churning) and/or ice segeration in the active
layer (seasonal thaw layer) and/or the upper part of the permafrost.
Occur in arctic and subarctic regions. Permafrost influences
pedogenesis through its effect on the downward movement of the
soil solution and on the soil forming processes.
HISTOSOLS
Histosols are dominantly organic. They are mostly soils that are
commonly called bogs, moors, or peats and mucks. A soil is
classified as Histosols if it does not have permafrost and is
dominated by organic soil materials.
ENTISOLS
Entisols are recent soils with little or no profile development. At
the most, have a weak organic matter surface layer (O horizon).
Include deep, uniform sandy aeolian deposits (pictured here) to
complex fluvial deposits. Some Entisols have steep, actively
eroding slopes, and others are on flood plains or glacial outwash
plains that frequently receive new deposits of alluvium.
VERTISOLS
Vertisols have a high content of expending clay and that have at
some time of the year deep wide cracks. They shrink when drying
and swell when they become wetter.
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1 Descriptions and images from–
- NRCS http://soils.usda.gov/technical/classification/orders/ (Accessed 6/13/04)
- Brady NC. RR Weil. 2008. The Nature and Properties of Soils. Prentice-Hall. Ch 3.
- Donahue, RL, R Miller, J Shickluna. 1983. Soils: An Introduction to Soils and Plant
Growth. 5th Ed. Prentice-Hall.
Other resources –
http://soils.cals.uidaho.edu/soilorders/orders.htm
http://soils.ifas.ufl.edu/faculty/grunwald/teaching/eSoilScience/soil_orders.shtml
T.Kittel 604, 211