Oxisols are a soil order in USDA soil taxonomy, best known for their occurrence in tropical rain forest within 25 degrees north and south of the Equator. In the World Reference Base for Soil Resources (WRB),
Formation
The main processes of
soil formation of oxisols are
weathering,
humification and
pedoturbation due to animals. These processes produce the characteristic
soil profile. They are defined as soils containing
at all depths no more than ten percent weatherable
minerals, and low cation exchange capacity. Oxisols are always a red or yellowish color, due to the high concentration of
iron(III) and
aluminium oxides and
. They also contain
quartz and
kaolin, plus small amounts of other
clay minerals and
organic matter.
Etymology
The word "oxisol" comes from
"oxide" in reference to the dominance of oxide minerals such as
bauxite. In the World Reference Base for Soil Resources, oxisols are known as
ferralsols.
Occurrence
Present-day oxisols are found almost exclusively in tropical areas, in
South America and
Africa, almost always on highly stable continental
.
In Southeast Asia, oxisols are found on remnants of the Cimmerian microcontinent, and on the Shan–Thai terrane. In Thailand, rhodic ferralsols, called Yasothon soils, are said to have formed under humid tropical conditions in the early Tertiary period, on an extensive plain later uplifted to form the Khorat Plateau. Characterized by a bright red color, these relict soils occur on uplands in a great semicircle around the southern rim, overlying associated gravel Soil horizon said to have been cleared of sand by termites, in a prolonged and still on-going process of bioturbation. Xanthic ferralsols of the Khorat and Udon series, characterized by a pale yellow to brown color, developed in midlands in processes still under investigation; as are those forming lowland soils resembling European .[Lofjle, E.; Kubiniok, J. Landform development and bioturbation on the Khorat plateau, Northeast Thailand, Nat.Hist.Bull.Siam Soc. (56), 1996 ]
In Australia vast areas formerly covered in rainforest have become so dry that oxisols have formed a hard ironstone cover upon which only Orthents can form.
Genesis
Fossil oxisols are known from the first appearance of free
oxygen in the atmosphere about 2,200 million years ago. In warm periods like the
Mesozoic and
Paleocene, oxisols extended to areas that now have quite cool climates, extending well into
North America and
Europe. It is believed oxisols became vegetated later than
ultisols or
alfisols, probably because vegetation took a long time to adapt to the infertility of oxisols.
History
Scientists originally thought that the heavy vegetation of tropical
would provide rich nutrients, but as rainfall passes through the litter on the forest floor the rain is
and leaches
minerals from the above soil layers. This forces plants to get their nutrition from decaying litter as oxisols are quite infertile due to the lack of organic matter and the almost complete absence of soluble minerals leached by the wet and humid
climate.
Uses
Oxisols are often used for tropical crops such as
Cocoa bean and
rubber. In some cases,
rice is grown on them. Permanent cropping of oxisols in low-income areas is very difficult because of low
cation exchange capacities and high
phosphorus fixation on iron and aluminium oxides (ligand exchange mechanism; inner sphere complex with phosphate). However, many oxisols can be cultivated over a wide range of moisture conditions. On this account, oxisols are intensively exploited for agriculture in some regions which have enough wealth to support modern agricultural practices (including regular additions of lime and
fertilizer). A recent example of exploitation by modern methods involves the growing of
soybeans in
Brazil.
Suborders
Oxisols are divided into the following suborders:
-
Aquox – oxisols with a water table at or near the surface for much of the year
-
Perox – oxisols of continuously humid climates, where precipitation exceeds evapotranspiration in all months
-
Torrox – oxisols of arid climates. Because the present climate can never produce enough weathering to produce oxisols, torrox soils are always paleosols formed during periods of much wetter climates. They occur mainly in Southern Africa.
-
Ustox – oxisols of semiarid and subhumid climates
-
Udox – oxisols of humid climates
See also
-
Buol, Hole, and McCracken, (1997). ‘Soil Genesis and Classification.’ 4th edition. (The Iowa State University Press, Ames, Iowa.)
Further reading
-
W. Zech, P. Schad, G. Hintermaier-Erhard: Soils of the World. Springer, Berlin 2022, Chapter 10.3.1.
External links
