The word "Ultisol" is derived from "ultimate", because Ultisols were seen as the ultimate product of continuous weathering of minerals in a humid temperate climate without new soil formation via glaciation.
Typically Ultisols are red to yellow in color and are quite acidic, often having a pH of less than 5. The red and yellow colors result from the accumulation of iron oxide which is highly insoluble in water. Major nutrients, such as calcium and potassium, are typically deficient in Ultisols, which means they generally cannot be used for sedentary agriculture without the aid of lime and other fertilizers such as superphosphate.
Ultisols are strongly leached, acid forest soils with relatively low native fertility. They are found primarily in humid temperate and tropical area of the world, typically on older, stable landscapes. Ultisols have a subsurface horizon in which clays have accumulated, often with strong yellowish or reddish colors resulting from the presence of Fe oxides. The 'red clay' soils of the southeastern United States are examples of Ultisols.
Because of the favorable cimate regimes in which they are typically found, Ultisols often support productive forests. They are poorly suited for continuous agriculture without the use of fertilizer and lime, but can be very productive with these inputs.
Ultisols occupy ~8.5% of the global ice-free land area and support 18% of the world's population. They are the dominant soils of much of the southeastern US and occupy ~9.2% of the total US land area.
Aquults - Ultisols with a water table at or near the surface for much of the year. Extensive on the coastal plains of the US, particularily on the Atlantic and Gulf of Mexico. Most of the soils had or continue to have forest vegetation.
Humults - well-drained Ultisols that have high organic matter content. In the US mainly formed in basic country rock on surfaces that are late Pleistocene or older. Natural vegetation consisted mostly of coniferous forest plants at mid latitudes and rain forest plants at low latitudes.
Udults - Ultisols of humid climates. Most receive well distributed rainfall. Developed in sediments and on surfaces that range from late Pleistocene to Pliocene or possibly older. Most have or had a forest vegetation, but some have a savanna.
Ustults - Ultisols of semiarid and subhumid climates and have a relatively low content of organic carbon. The vegetation commonly consists of forest or savanna plants. These soils are of small extent in the US.
Xerults - temperate Ultisols with very dry summers and moist winters typically of Mediterranean climates. Not extensive in the US except locally in California and Oregon in the mountains. Natural vegetation consisted mostly of coniferous forest plants.