OXISOL

one of the 12 soil orders in the U.S. Soil Taxonomy. Oxisols form principally in humid tropical zones under rainforest, scrub and thorn forest, or savanna vegetation on flat to gently sloping uplands. They are typically found on old landscapes that have been subject to shifting cultivation for millennia. Intensive plantation agriculture is possible if lime and fertilizers are applied with careful management to prevent erosion. Oxisols occupy 7 percent of the nonpolar continental land area on Earth, mostly in the equatorial regions of South America and Africa. Oxisols are characterized by a thick subsurface layer (the oxic horizon) that contains kaolin-group clay minerals and metal oxides in a finely textured matrix with very little or no easily weathered silicates. Ferromagnesian parent materials (minerals containing both iron and magnesium) are also thought to be essential, since loss of silica and oxidation of iron are important pathways in Oxisol formation. Oxisols differ from Ultisols in not having a layer of translocated (migrated) clay accumulation and from Vertisols in not containing significant amounts of swelling clay minerals.