Ultisols are mineral soils almost always formed in acidic parent materials. Ultisols have a distinct clay content increase in a subsoil argillic or kandic (Bt) horizon. Base saturation percentage usually decreases with depth and must be less than 35 percent by sum of cation method (CEC determined at pH 8.2) at 125 cm below the top of one of the above horizons or 180 cm below the soil surface. Ultisols are formed in all but aridic soil moisture regimes. Almost all Ultisols are on level to moderately sloping topography under woody vegetation. Most have small quantities of silt and thus relatively low plant available water holding capacity and relatively low cation exchange capacities due to limited amounts of 2:1 clays. Although limited in natural fertility extensive areas of Ultisols have supported human food production as areas of natural forest vegetation are cut and burned to provide fertile ash sufficient for a few crops. The small amount of nutrients in the ash are sufficient for only a few crops after which the sites are abandon to allow some 20 or more years of natural regrowth accumulates enough biomass to again cut, burn and plant a food crop. This slash-and-burn cultivation scenario has been and is still prevalent on many Ultisols. Sustained high crop yields are routinely harvested on Ultisols where lime to overcome the acid, aluminum toxic conditions and fertilizer to replace the nutrients exported in the harvested crops are added. http://www.tran33m.com/index.php
