Mollisols are a soil order in USA soil taxonomy. Mollisols form in semi-arid to semi-humid areas, typically under a grassland cover. They are most commonly found latitudinally in a band of 50 degrees north of the equator, although there are some in South America, South-Eastern Australia (mainly South Australia) and South Africa. Their parent material is generally limestone, loess, or wind-blown sand. The main processes that lead to the formation of grassland mollisols are melanisation, decomposition, humification and pedoturbation.
They have deep, high organic matter, nutrient-enriched surface soil (A horizon), typically between 60-80 cm thick. This fertile surface horizon, known as a mollic epipedon, results from the long-term addition of organic materials derived from plant roots. The significance of clay in the B horizon may stem from the rainfall pattern causing chemical weathering of the parent material. They have a soft, granular, soil structure. Mollisols occur in savannahs and mountain valleys (such as Central Asia, or the North American Great Plains). It was estimated that in 2003, between 14 and 26 percent of grassland ecosystems still remained in a relatively natural state (that is, they weren't used for agriculture due to the fertility of the A horizon). Because of their productivity and abundance, the mollisols represent one of the more economically important soil orders. Mollisols are geologically by far the youngest soil order in USA Soil Taxonomy. Whereas all the other soil orders known today existed by the time of the Carboniferous Ice Age 280 million years ago, Mollisols are not known from the paleopedological record any earlier than the Eocene. Their development is very closely associated with the cooling and drying of the global climate that occurred during the Oligocene, Miocene and Pliocene
Soils which are in most ways similar to Mollisols but contain either continuous or discontinuous permafrost and are consequently affected by cryoturbation are common in the high mountain plateaux of Tibet and the Andean altiplano. Such soils are known as Molliturbels or Mollorthels and provide the best grazing land in such cold climates because they are not acidic like many other soils of very cold climates.
Other soils which have a mollic epipedon are classed as Vertisols because the presence of high shrink swell characteristics and relatively high clay contents takes precedence over the mollic epipedon. These are especially common is parts of South America in the Parana River basin that have abundant but erratic rainfall and extensive deposition of clay-rich minerals from the Andes. Mollic epipedons also occur in some Andisols but the andic properties take precedence.