Oxisols are a
soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...
order in
USDA soil taxonomy
USDA soil taxonomy (ST) developed by the United States Department of Agriculture and the National Cooperative Soil Survey provides an elaborate classification of soil types according to several parameters (most commonly their properties) and in ...
, best known for their occurrence in
tropical
The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in
the Southern Hemisphere at S. The tropics are also referred to ...
rain forest within 25 degrees north and south of the
Equator
The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can al ...
. In the
World Reference Base for Soil Resources (WRB), they belong mainly to the
ferralsol
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), they belong mainly to the ferralsols ...
s, but some are
plinthosols or
nitisols. Some oxisols have been previously classified as
laterite
Laterite is both a soil and a rock type rich in iron and aluminium and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content. They develop by ...
soils.
Formation
The main processes of soil formation of oxisols are
weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movemen ...
,
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
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
(III) and
aluminium
Aluminium (aluminum in AmE, American and CanE, Canadian English) is a chemical element with the Symbol (chemistry), symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately o ...
oxides
An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the ...
and
hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water ...
s. They also contain
quartz
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...
and
kaolin, plus small amounts of other
clay minerals and
organic matter
Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...
.
Etymology
The word "oxisol" comes from ''"oxide"'' in reference to the dominance of oxide minerals such as
bauxite
Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO(O ...
. 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
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the souther ...
and
Africa
Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
, almost always on highly stable continental
cratons.
In
Southeast Asia
Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or SEA, is the geographical United Nations geoscheme for Asia#South-eastern Asia, south-eastern region of Asia, consistin ...
, oxisols are found on remnants of the
Cimmerian microcontinent, and on the
Shan–Thai Terrane. In
Thailand
Thailand ( ), historically known as Siam () and officially the Kingdom of Thailand, is a country in Southeast Asia, located at the centre of the Indochinese Peninsula, spanning , with a population of almost 70 million. The country is b ...
, rhodic ferralsols, called
Yasothon soils, are said to have formed under humid tropical conditions in the early
Tertiary
Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago.
The period began with the demise of the non- avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...
, on an extensive plain later uplifted to form the
Khorat Plateau
The Khorat Plateau ( th, ที่ราบสูงโคราช) is a plateau in the northeastern Thai region of Isan. The plateau forms a natural region, named after the short form of Nakhon Ratchasima, a historical barrier controlling acces ...
. Characterized by a bright red color, these
relict soils occur on uplands in a great semicircle around the southern rim, overlying associated
gravel horizons said to have been cleared of
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a soil texture, textur ...
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
brown soils.
[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
skeletal soils can form.
Genesis
Fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
oxisols are known from the first appearance of free
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...
in the
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. ...
about 2,200 million years ago. In warm periods like the
Mesozoic
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Creta ...
and
Paleocene
The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...
, oxisols extended to areas that now have quite cool climates, extending well into
North America and
Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located enti ...
. 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
rain forests would provide rich nutrients, but as rainfall passes through the litter on the forest floor the rain is
acidified 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
Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologica ...
.
Uses
Oxisols are often used for tropical crops such as
cocoa
Cocoa may refer to:
Chocolate
* Chocolate
* ''Theobroma cacao'', the cocoa tree
* Cocoa bean, seed of ''Theobroma cacao''
* Chocolate liquor, or cocoa liquor, pure, liquid chocolate extracted from the cocoa bean, including both cocoa butter and ...
and
rubber
Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, a ...
. In some cases,
rice
Rice is the seed of the grass species '' Oryza sativa'' (Asian rice) or less commonly '' Oryza glaberrima'' (African rice). The name wild rice is usually used for species of the genera '' Zizania'' and ''Porteresia'', both wild and domestica ...
is grown on them. Permanent cropping of oxisols in low-income areas is very difficult because of low
cation exchange
Ion exchange is a reversible interchange of one kind of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, ...
capacities and high
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...
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 fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...
). A recent example of exploitation by modern methods involves the growing of
soybeans in
Brazil
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
.
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
Southern Africa is the southernmost subregion of the African continent, south of the Congo and Tanzania. The physical location is the large part of Africa to the south of the extensive Congo River basin. Southern Africa is home to a number ...
.
* Ustox – oxisols of semiarid and subhumid climates
* Udox – oxisols of humid climates
See also
*
*
*
*
References
* Buol, Hole, and McCracken, (1997). ‘Soil Genesis and Classification.’ 4th edition. (The
Iowa State University Press,
Ames, Iowa
Ames () is a city in Story County, Iowa, United States, located approximately north of Des Moines in central Iowa. It is best known as the home of Iowa State University (ISU), with leading agriculture, design, engineering, and veterinary med ...
.)
*
*
*
Further reading
* W. Zech, P. Schad, G. Hintermaier-Erhard: Soils of the World. Springer, Berlin 2022, Chapter 10.3.1.
External links
profile photos (with classification)WRB homepage
profile photos (with classification)IUSS World of Soils
{{Soil type
Pedology
Types of soil