Oxisols are an order in USA soil taxonomy, best known for their occurrence in tropical rain forest. They are defined as soils containing at all depths no more than 10 percent weatherable minerals, and less than 10 percent base saturation. Oxisols are always red or yellowish color, due to the high concentration of iron and aluminium ions. Apart from the ions they also contain quartz, kaolin and some small amounts of organic matter.

The word "Oxisol" comes from "oxide" in reference to the dominance of oxide minerals such as bauxite. In the FAO soil classification, Oxisols are known as Ferralsols.

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.

Present-day Oxisols are found almost exclusively in tropical areas of South America and Africa, almost always on highly stable continental cratons. 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.

Fossil 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.

Oxisols are often used for tropical crops such as cocoa and rubber. In some cases, rice is grown on them. Permanent cropping of Oxisols is very difficult because of low cation exchange capacities and high phosphorus fixation.

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

Pedogenesis
Pedology (soil study)
Soil classification