Editing Oxisol

{{short description|Soil type known for occurring in tropical rain forests}}
{{refimprove|date=July 2022}}
{{Infobox soil
|name=Oxisol
|alternative_name=
|image=Oxisol.jpg
|image_size=200px
|image_caption=Oxisol profile
|classification_system= [[USDA soil taxonomy]]
|profile= OABC
|parent_material=
|process=[[leaching (pedology)|leaching]], [[weathering]]
|climate=[[tropical climate|tropical]]
}}

[[File:OxisolWorld.jpg|thumb|upright=1.6|Oxisols of the world]]
[[File:River in the Amazon rainforest.jpg|thumb|upright=1.6|Oxisols are generally associated with the [[equatorial forest]] biome]]

'''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),<ref>{{Cite web|url = https://wrb.isric.org/files/WRB_fourth_edition_2022-12-18.pdf|title = World Reference Base for Soil Resources, fourth edition|author=IUSS Working Group WRB|year = 2022|publisher = International Union of Soil Sciences, Vienna}}</ref> they belong mainly to the [[ferralsol]]s, but some are [[plinthosol]]s or [[nitisol]]s. Some oxisols have been previously classified as [[laterite]] soils.

==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 [[hydroxide]]s. 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 [[craton]]s.

In [[Southeast Asia#Geography|Southeast Asia]], oxisols are found on remnants of the [[Cimmeria (continent)|Cimmerian microcontinent]], and on the [[Shan–Thai terrane]]. In [[Geography of Thailand|Thailand]], rhodic ferralsols, called [[Yasothon Province|Yasothon]] soils, are said to have formed under humid tropical conditions in the early [[Tertiary]], on an extensive plain later uplifted to form the [[Khorat Plateau]]. Characterized by a bright red color, these [[relict (geology)|relict]] soils occur on uplands in a great semicircle around the southern rim, overlying associated [[gravel]] [[Soil horizon|horizon]]s said to have been cleared of [[sand]] by [[termites]], in a prolonged and still on-going process of [[bioturbation]].  Xanthic ferralsols of the [[Nakhon Ratchasima Province|Khorat]] and [[Ubon Ratchathani Province|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 soil]]s.<ref>Lofjle, E.; Kubiniok, J. ''Landform development and bioturbation on the Khorat plateau, Northeast Thailand'', Nat.Hist.Bull.Siam Soc. (56), 1996 {{cite web |url=http://www.aseanbiodiversity.info/Abstract/53002297.pdf |title=Archived copy |access-date=2010-12-23 |url-status=usurped |archive-url=https://web.archive.org/web/20110721184927/http://www.aseanbiodiversity.info/Abstract/53002297.pdf |archive-date=2011-07-21 }}</ref>

In [[Australia]] vast areas formerly covered in rainforest have become so dry that oxisols have formed a hard [[ironstone]] cover upon which only [[Orthents|skeletal soils]] can form.

==Genesis==
[[Fossil]] oxisols are known from the first appearance of free [[oxygen]] in the [[Earth's atmosphere|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 [[rain forest]]s would provide rich nutrients, but as rainfall passes through the litter on the forest floor the rain is [[acid]]ified 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 [[Leaching (chemical science)|leached]] by the wet and humid [[climate]].

==Uses==
Oxisols are often used for tropical crops such as [[Cocoa bean|cocoa]] 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 (mineral)|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 ==
*{{annotated link|Pedogenesis}}
*{{annotated link|Pedology}}
*{{annotated link|Soil classification}}
*{{annotated link|Terra preta}}

== References ==
{{reflist}}

* Buol, Hole, and McCracken, (1997). ‘Soil Genesis and Classification.’ 4th edition.  (The [[Iowa State University|Iowa State University Press]], [[Ames, Iowa]].)
* {{cite web| url =http://soils.usda.gov/technical/classification/orders/oxisols.html| title =Oxisols| publisher =USDA-NRCS| access-date =2006-05-14| url-status =dead| archive-url =https://web.archive.org/web/20060206224026/http://soils.usda.gov/technical/classification/orders/oxisols.html| archive-date =2006-02-06}}
* {{cite web|url=http://grunwald.ifas.ufl.edu/Nat_resources/soil_orders/oxisols.htm |title=Oxisols |publisher=University of Florida |access-date=2006-05-14 |url-status=dead |archive-url=https://web.archive.org/web/20060916044316/http://grunwald.ifas.ufl.edu/Nat_resources/soil_orders/oxisols.htm |archive-date=September 16, 2006 }}
* {{cite web| url =http://soils.ag.uidaho.edu/soilorders/oxisols.htm| title =Oxisols| publisher =University of Idaho| access-date =2006-05-14| archive-url =https://web.archive.org/web/20060604031120/http://soils.ag.uidaho.edu/soilorders/oxisols.htm| archive-date =2006-06-04| url-status =dead}}

==Further reading==
* W. Zech, P. Schad, G. Hintermaier-Erhard: Soils of the World. Springer, Berlin 2022, Chapter 10.3.1. {{ISBN|978-3-540-30460-9}}

== External links==
* [https://wrb.isric.org/picture-gallery/ profile photos (with classification)] WRB homepage
* [https://www.iuss.org/index.php?article_id=73|title=IUSS profile photos (with classification)] IUSS World of Soils
{{Soil type}}

[[Category:Pedology]]
[[Category:Types of soil]]