Oxisols are very highly weathered soils that are found primarily in the intertropical regions of the world. These soils contain few weatherable minerals and are often rich in Fe and Al oxide minerals.
Most of these soils are characterized by extremely low native fertility, resulting from very low nutrient reserves, high phosphorus retention by oxide minerals, and low cation exchange capacity (CEC). Most nutrients in Oxisol ecosystems are contained in the standing vegetation and decomposing plant material. Despite low fertility, Oxisols can be quite productive with inputs of lime and fertilizers.
Oxisols occupy ~7.5% of the global ice-free land area. In the US, they only occupy ~0.02% of the land area and are restricted to Hawaii.
Aquox - Oxisols with a water table at or near the surface for much of the year in shallow depressions and in seepage areas at the base of slopes. There is a tendency to accumulate iron in the form of secondary nodules, concretions, and plinthite. Most areas of these soils are small.
Torrox - Oxisols of arid climates. Torrox are excellent soils for a variety of crops if water and fertilizer are applied. They occur mainly in Southern Africa and in the US, they are known to occur only in Hawaii and perhaps in some areas of Australia.
Ustox - Oxisols of semiarid and subhumid climates. Crops are not grown continuously because there is usually inadequate moisture for at least 90 days in normal years. Occurs over a large portion of the interior of South America and in extensive areas of Africa. The range of natural rainfall with the regions holding Ustox may provide for two crops to be grown in some areas, but usually only one crop can be supported without supplimental irrigation.
Perox - Oxisols of continuously humid climates, where precipitation exceeds evapotranspiration in all months. Clearing and burning are difficult because of atmospheric wetness. Curing many seed crops and storing produce are also difficult.
Udox - Oxisols of humid climates. There is usually adaquate rainfall in normal years to allow for continuous crop growing. In local terms there are 1 - 3 months that are considered "dry". Udox occur mostly in South America and in parts of Africa and Asia.