Oxisol
Encyclopedia
Oxisols are an order in USDA soil taxonomy
, best known for their occurrence in tropical rain forest, 15-25 degrees north and south of the Equator
. Some oxisols have been previously classified as laterite
soils.
, 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 10 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. In addition they also contain quartz
and kaolin, plus small amounts of other clay minerals
and organic matter
.
. In the FAO soil classification
, oxisols are known as ferralsols.
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 leached
by the wet and humid climate
.
and Africa
, almost always on highly stable continental craton
s.
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
, 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
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 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.
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.
oxisols are known from the first appearance of free oxygen
in the atmosphere
about 2200 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.
. 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
.
USDA soil taxonomy
USDA Soil Taxonomy developed by United States Department of Agriculture and the National Cooperative Soil Survey provides an elaborate classification of soil types according to several parameters and in several levels: Order, Suborder, Great Group, Subgroup, Family, and Series.- Example of...
, best known for their occurrence in tropical rain forest, 15-25 degrees north and south of the Equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
. Some oxisols have been previously classified as laterite
Laterite
Laterites are soil types rich in iron and aluminium, formed in hot and wet tropical areas. Nearly all laterites are rusty-red because of iron oxides. They develop by intensive and long-lasting weathering of the underlying parent rock...
soils.
Formation
The main processes of soil formation of oxisols are weatheringWeathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
, 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 10 percent weatherable minerals, and low cation exchange capacity
Cation exchange capacity
In soil science, cation-exchange capacity is the maximum quantity of total cations, of any class, that a soil is capable of holding, at a given pH value, for exchanging with the soil solution. CEC is used as a measure of fertility, nutrient retention capacity, and the capacity to protect...
. Oxisols are always a red or yellowish color, due to the high concentration of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
(III) and aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
oxides and hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
s. In addition they also contain quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
and kaolin, plus small amounts of other clay minerals
Clay minerals
Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products and low...
and organic matter
Organic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
.
Etymology
The word "oxisol" comes from "oxide" in reference to the dominance of oxide minerals such as bauxiteBauxite
Bauxite is an aluminium ore and is the main source of aluminium. This form of rock consists mostly of the minerals gibbsite Al3, boehmite γ-AlO, and diaspore α-AlO, in a mixture with the two iron oxides goethite and hematite, the clay mineral kaolinite, and small amounts of anatase TiO2...
. In the FAO soil classification
FAO soil classification
The Food and Agriculture Organization of the United Nations developed a supra-national classification, also called World Soil Classification, which offers useful generalizations about soils pedogenesis in relation to the interactions with the main soil-forming factors. It was first published in...
, oxisols are known as ferralsols.
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 acidAcid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
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 leached
Leaching (chemical science)
Leaching is the process of extracting minerals from a solid by dissolving them in a liquid, either in nature or through an industrial process. In the chemical processing industry, leaching has a variety of commercial applications, including separation of metal from ore using acid, and sugar from...
by the wet and humid climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
.
Where found
Present-day oxisols are found almost exclusively in tropical areas, in South AmericaSouth America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
and Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, almost always on highly stable continental craton
Craton
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...
s.
In Southeast Asia, oxisols are found on remnants of the Cimmerian microcontinent
Cimmeria (continent)
Cimmeria was an ancient microcontinent that existed about 200 million years ago. It rifted north from Gondwana during the Late Carboniferous and collided against eastern Laurasia during the Late Triassic together with the Chinese continents. The collision created new mountain ranges between...
, and on the Shan–Thai Terrane
Shan–Thai Terrane
The Shan–Thai terrane rifted from Australia in the Permian and collided with the Indochina terrane in the Triassic. It extends from from Malaysia, through peninsular Thailand, Myanmar, West Yunnan, to Lhasa....
. In Thailand
Geography of Thailand
Thailands 514,000 square kilometers lie in the middle of mainland Southeast Asia. The nation's axial position influenced many aspects of Thailand's society and culture—it controls the only land route from Asia to Malaysia and Singapore....
, rhodic ferralsols, called Yasothon
Yasothon Province
Yasothon is one of the provinces of Thailand, located in the North-East of Thailand on the Chi River. Neighboring provinces are Mukdahan, Amnat Charoen, Ubon Ratchathani, Sisaket and Roi Et.-Geography:...
soils, are said to have formed under humid tropical conditions in the early Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
, on an extensive plain later uplifted to form the Khorat Plateau
Khorat Plateau
The Khorat Plateau also Korat Plateau, is a plateau in the northeastern region of Thailand, named for the short form of Nakhon Ratchasima, an historical stronghold controlling access to and from the plateau.-Geography:...
. Characterized by a bright red color, these relict
Relict
A relict is a surviving remnant of a natural phenomenon.* In biology a relict is an organism that at an earlier time was abundant in a large area but now occurs at only one or a few small areas....
soils occur on uplands in a great semicircle around the southern rim, overlying associated gravel
Gravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...
horizon
Soil horizon
A soil horizon is a specific layer in the land area that is parallel to the soil surface and possesses physical characteristics which differ from the layers above and beneath. Horizon formation is a function of a range of geological, chemical, and biological processes and occurs over long time...
s said to have been cleared of sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
by termites, in a prolonged and still on-going process of bioturbation
Bioturbation
In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...
. Xanthic ferralsols of the Khorat
Nakhon Ratchasima Province
Nakhon Ratchasima or , often shortened to Korat or Khorat , is one of the north-eastern provinces of Thailand...
and Udon
Ubon Ratchathani Province
-History:The area was part of the Khmer Empire. Before the late eighteenth century, this area evidently was outside Siamese or Thai Ayutthaya Kingdom....
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.
In Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
vast areas formerly covered in rainforest have become so dry that oxisols have formed a hard ironstone
Ironstone
Ironstone is a sedimentary rock, either deposited directly as a ferruginous sediment or created by chemical repacement, that contains a substantial proportion of an iron compound from which iron either can be or once was smelted commercially. This term is customarily restricted to hard coarsely...
cover upon which only skeletal soils can form.
Genesis
FossilFossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
oxisols are known from the first appearance of free oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
in the atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
about 2200 million years ago. In warm periods like the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
and Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
, oxisols extended to areas that now have quite cool climates, extending well into North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
. It is believed oxisols became vegetated later than ultisols
Ultisols
Ultisols, commonly known as red clay soils, are one of twelve soil orders in the United States Department of Agriculture soil taxonomy. They are defined as mineral soils which contain no calcareous material anywhere within the soil, have less than 10% weatherable minerals in the extreme top layer...
or alfisols
Alfisols
Alfisols are a soil order in USDA soil taxonomy. Alfisols form in semiarid to humid areas, typically under a hardwood forest cover. They have a clay-enriched subsoil and relatively high native fertility. "Alf" refers to aluminium and iron . Because of their productivity and abundance, the...
, probably because vegetation took a long time to adapt to the infertility of oxisols.
Uses
Oxisols are often used for tropical crops such as cocoa and rubberRubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...
. In some cases, rice
Rice
Rice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...
is grown on them. Permanent cropping of oxisols in low-income areas is very difficult because of low cation exchange capacities and high phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
fixation on iron and aluminium oxides (ligand exchange mechanism; inner sphere complex
Inner sphere complex
Inner sphere complex is a type of surface complex. Formation of inner sphere complexes occurs when ions bind directly to the surface with no intervening water molecules. These types of surface complexes are restricted to ions that have a high affinity for surface sites and include specifically...
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
Lime (mineral)
Lime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
and fertilizer
Fertilizer
Fertilizer is any organic or inorganic material of natural or synthetic origin that is added to a soil to supply one or more plant nutrients essential to the growth of plants. A recent assessment found that about 40 to 60% of crop yields are attributable to commercial fertilizer use...
). A recent example of exploitation by modern methods involves the growing of soybeans in Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
.
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 AfricaSouthern AfricaSouthern Africa is the southernmost region of the African continent, variably defined by geography or geopolitics. Within the region are numerous territories, including the Republic of South Africa ; nowadays, the simpler term South Africa is generally reserved for the country in English.-UN...
. - Ustox - oxisols of semiarid and subhumid climates
- Udox - oxisols of humid climates
See also
- PedogenesisPedogenesisPedogenesis is the science and study of the processes that lead to the formation of soil ' and first explored by the Russian geologist Vasily Dokuchaev , the so called grandfather of soil science, who determined that soil formed over time as a consequence of...
- Pedology (soil study)Pedology (soil study)Pedology is the study of soils in their natural environment. It is one of two main branches of soil science, the other being edaphology...
- Soil classificationSoil classificationSoil classification deals with the systematic categorization of soils based on distinguishing characteristics as well as criteria that dictate choices in use.- Overview :...
- Terra pretaTerra pretaTerra preta is a type of very dark, fertile anthropogenic soil found in the Amazon Basin. Terra preta owes its name to its very high charcoal content, and was indeed made by adding a mixture of charcoal, bone, and manure to the otherwise relatively infertile Amazonian soil, and stays there for...