Climate: Oxisols occur within the range from isomesic to isohyperthermic regimes but most Oxisols develop in isotropical soil temperature regimes. Broadly speaking, they develop in climatic zones with small seasonal variation in soil temperature and no seasonal soil freezing. They occur under a wide range of soil moisture regimes from aridic to perudic. Oxisols occuring under aridic moisture regimes are often considered as relicts. It is assumed that Oxisols develop under climatic conditions where precipitation exceeds evapotranspiration for some periods of the year to favor the removal of soluble weathering products and favors the formation and residual concentration of kaolinite and sesquioxides, which are essential to form an oxic diagnostic horizon.
Vegetation: Oxisols may occur under a wide range of vegetational zones including tropical rainforest, scrub and thorn forest, deciduous forest, and savannah (e.g. in the central Brazilian plateau). The use of Oxisols is often limited to shifting cultivation, subsidence farming, low-intensity grazing. Due to amendments Oxisols can be used also for the growth of soybeans, wheat, corn, and coffee (intense plantation agriculture).
Relief: Most Oxisols occur either on relatively stable upland summit positions, relict from a previous regional erosion surface, or on preserved remnants of an old alluvial terrace, which are nearly level topography. Oxisols are not likely to occur on steep slopes.
Parent Material: Oxisols occur on highly weathered transported material, old fluvial terraces, or on high-lying old erosion surfaces. The most extensive areas of Oxisols are in sediments that have been reworked during several erosional and depositional cycles, some extending to the earliest geologic eras, although they may also form in materials which wheather rapidly.
It has been suggested that Oxisols result because of the geologic history of the parent material prior to pedogenic conditions at the present site. It can be reasoned that if the parent material consists of only quartz, 1:1 type clays, and iron and aluminum oxides and hydroxides, few pedogenic processes are possible, and a soil formed in such material will have Oxisol properties regardless of present or past climate at the site.
Time: Most of the Oxisols are formed on transported materials (erosion) where desilication (loss of silica) and intense weathering has taken place over vast expanses of times (about 50,000 y up to 100,000 y or even longer).
Oxisols may be classified into two categories: (i) formed in situ rocks or sediments, and (ii) formed on preweathered and transported sediments.
Weathering is very intense in Oxisols showing a weathering depth much greater than for most of the other soil orders - 16 m or more having been observed. Because of weathering most of the primary minerals and 2:1 type clay minerals are transformed to 1:1 type minerals such as kaolinite and gibbsite and secondary iron and aluminum oxides and hydroxides. The formation of free non-silicated alumina (e.g. gibbsite) requires the rapid and almost immediate removal of soluble weathering products (basic cations), particularly silica (desilication). These processes are supported by free drainage conditions, intense rainfall, and a position well above the water table, where ferrous ions (Fe2+ ) produced by hydrolysis are oxidized immediately and are thus eliminated from the reaction by precipitation into ferric forms (Fe 3+ ). In tropical zones hydolysis and oxidation are increased compared to temperate zones, and heavy rainfall continuously removes dissolved reaction products. When removal of silica does not reach its extreme to form gibbsite kaolinite is formed, which is typical for intertropical climate. Lower original silica contents, higher rainfall and temperatures will generally increase the gibbsite content.
The parent material determines the intensity of weathering. Acid igneous rocks (e.g. granite) weather at a slower rate as basic rocks (e.g. basalt). Granitic saprolites are often several meters thick and primary minerals may for a long time continue to weather and the slow hydrolysis of feldspars, biotites, and amphiboles supply silica to the soil solution as to favor the formation of kaolinite. Slower transformations, as in granitic saprolites, will essentially produce kaolinite. The iron content of the oxic horizon formed under free drainage and good aeration will be a function of the original compostion of the parent material.
Clay translocation is not a major pedogenetic process taking place in oxic horizons because the clays in oxic materials have a low potential mobility, i.e., dispersion of clays and subsequent migration does not occur extensively. At the oxic stage neoformation of clay is practically nil, and consequently freshly formed clays, which tend to move more readily, are absent.
Humification takes place in all Oxisols. In regions with warm or high temperatures year-around litter humifies and mineralizes rapidly. The organic matter content in Oxisols is indirectly proportional to soil temperature. In general, Oxisols are not as dark in color at similar organic matter contents as soils of the other soil orders. Organic acids provided by decomposition and humification destabilize the soil micro-aggregates and produce water dispersable clays, which are subsequently leached. In general, organic acids tend to retain silica, iron and aluminum are complexed and leached out. When water is the sole leaching agent, the process, in terms of the products formed, leads to the residual concentration of sesquioxides, which is a component of the formation of oxic horizons.
Faunal pedoturbation is a major process in most Oxisols. This process of intense disturbance and mixing of soil is due to activity by insects, particularly termites. Due to pedoturbation numerous mounds may be formed at the soil surface. Pedoturbation by treethrow does also prevent or retard soil horizonation.
Oxisols show an oxic or kandic subsurface diagnostic horizon. An oxic horizon has to be at least 30-cm thick and is sandy loam or finer. It has a high content of low-charge 1:1 clays with an effective cation exchange capacity (ECEC) of <= 12 cmol kg -1 clay and a cation exchange capacity (CEC) of <= 16 cmol kg -1 clay at pH 7. Weathering and intense leaching have removed a large part of the silica from silicate minerals in this horizon (low nutrient reserve). Although the clay content in Oxisols is often high the CEC is low. This is due to the almost complete weathering of primary minerals and 2:1 type clay minerals to 1:1 type minerals such as kaolinite and gibbsite. Those minerals are not expandable secondary minerals and their CEC is low. The permanent charge of kaolinite and gibbsite is low but they may develop a small but significant pH dependent charge due to their low crystallinity. The most common structure of oxic horizons in soils on old geomorphic surfaces is massive separating into very fine crumbs. The primary aggregates built up of individual particles are held together by clay-sized substances. Bulk densities of the oxic horizon are usually in the range from 1 to 1.3 g cm -3 . The oxic horizon contains less than 10 % weatherable minerals and has < 5 % by volume rock structure.
The oxic horizon is generally very high in clay-size particles dominated by hydrous oxides of iron and aluminum. Most of the sesquioxides are generally goethite or hematite although maghemite may also be present in soils derived from basic rocks. Most clay-size minerals found in the oxic horizon are poorly crystallized. Generally, poorly crystallized Al and Fe-oxides may form more effective bonds between particles compared to better crystallized Al and Fe-oxides, which are poor cementing agents. This indicate that the composition of sesquioxides is as important as their quantity for structural stability of a soil.
Oxisols are classified by the presence of an oxic or a kandic horizon. Kandic horizons show the same ECEC and CEC as oxic horizons but kandic horizons have a clay content increase at its upper boundary of > 1.2 x clay within a vertical distance of < 15 cm, i.e., abrupt or clear textural boundary.
A fluctuating water table (alternating oxidation - reduction) in Oxisols may form plinthite consisting of red-and-gray mottled material. In the past this material has been designated as 'laterite' or 'lateritic iron oxide crust'. If subjected to repeated wetting and drying, as in exposure by erosion of overlying material, it becomes indurated to ironstone, which may be subsequently erode and be deposited as ironstone gravel layers in alluvial fans.
Infiltration and percolation rates in Oxisols are rapid. Many Oxisols bahave like sandy textured soils with respect to their pF curves, i.e., the water holding capacity is limited.
To qualify for an Oxisol the requirements are:
Almost all Oxisols occur in South America and Africa. Few Oxisols have been reported in the continental United States.
There are 5 suborders, whereas classification is based on the soil moisture regime:
Aquox: Oxisols that have aquic conditions for some time in most years and show redoximorphic features or a histic epipedon are defined as Aquox.
Torrox: The suborder of Torrox classifies Oxisols with an aridic soil moisture regime.
Ustox: The suborder of Ustox classifies Oxisols with an ustic or xeric soil moisture regime.
Perox: The suborder of Perox classifies Oxisols with a perudic soil moisture regime.
Udox: The suborder of Udox classifies Oxisols with an udic soil moisture regime.
Oxisols are classified by 'Acr' (e.g. Acraquox, Acrustox) when the soil profile is highly weathered and the ECEC is less than 1.50 cmol(+)/kg clay and the pH value is 5.0 or more, that are soils with negligible amounts of exchangeable cations (including aluminum).
The 'Eutr' great group of Oxisols are high base status soils, i.e., base saturation of 35 percent or more in all horizons within 125 cm of the mineral soil surface. Their high base status is attributed to enrichment during brief periods of saturation by lateral subsurface flow and lateral transport of nutrients (e.g. Eutraquox, Eutrotorrox, Eutric Acrustox).
Oxisols that have plinthite forming a continuous phase within 125 cm of the mineral soil surface are designed as 'Plinthic' (e.g. Plinthaquox, Plinthic Acroperox).
Diagnostic horizons such as 'Sombric' (e.g. Sombriustox), 'Kandic' (e.g. Kandiustox), and epipedons such as 'Histic' (e.g. Histic Eutraquox) are used to classify Oxisols at the great group and subgroup level.
Oxisols that have a lithic contact within 125 cm of the mineral soil surface are classified as 'Lithic' (e.g. Lithic Sombriustox, Lithic Acrotorrox, Lithic Acroperox). Oxisols that have a petroferric contact within 125 cm of the mineral soil surface are classified as 'Petroferric' (e.g. Petroferric Sombriustox, Petroferric Acroperox). Petroferric contact is a boundary between soil and a continuous layer of indurated soil in which iron is an important cement.
Oxisols, where the oxic horizon has its lower boundary within 125 cm of the mineral soil surface are called 'Inceptic' (e.g. Inceptic Eutrustox, Inceptic Eutroperox).
Oxisols with a high organic matter content are classified as 'Humic' (e.g. Humic Sombriustox, Humic Acroperox, Humic Kandiperox), which must have 16 kg/m2 or more organic carbon between the mineral soil surface and a depth of 100 cm.
Oxisols that have a delta pH (KCl pH minus 1:1 water pH) with a 0 or net positive charge in a layer 18 cm or more thick within a depth of 125 cm of the soil surface are classified as 'Anionic' (e.g. Anionic Acrustox).
Soil moisture regime defines 'Aquic' Oxisols at subgroup level (e.g. Aquic Acrustox, Aquic Kandiperox), which show redox depletions with a color value, moist, of 4 or more and a chroma of 2 or less. Oxisols which show redox depletions with a color value, moist, of 4 or more and a chroma of 2 or less, and also aquic conditions for some time in most years are called 'Aqueptic' (e.g. Aqueptic Haplustox). Oxisols that are saturated with water, in one or more layers within 100 cm of the mineral soil surface, for 1 month or more per year in 6 or more out of 10 years are classified as 'Oxyaquic' (e.g. Oxyaquic Haplustox).
Soil color is used at subgroup level: 'Rhodic' (a hue of 2.5YR or redder; and a value moist of 3 or less) - e.g. Rhodic Acrustox; 'Aeric' , which have, directly below an epipedon, a horizon 10 cm or more thick that has 50 percent or more chroma of 3 or more (e.g. Aeric Acraquox); 'Xanthic' (a hue of 7.5YR or yellower and color value, moist, of 6 or more between 25 and 125 cm from the mineral soil surface (e.g. Xanthic Acrustox).
Many soils have an argillic or kandic horizon but do not qualify as Oxisols because they do have less than 40 % clay in the surface 18 cm and classify as Ultisols or Alfisols.
Soils with more than 10 % weatherable minerals in the sand fraction classify as Inceptisols. For example, Quartzipsamments occupy areas, where superficial materials are sandy. They are closely associated to Oxisols.
Oxisols may include many soils previously called Laterites or Latosols, which are formed in tropical or subtropical regions.
Oxisols formed on high-lying old erosion surfaces (plateaus), terraces or floodplains are associated with Ultisols, Alfisols, or Mollisols on the sideslopes. Entisols may occur on floodplains or steep rapidly eroding areas.