<P> Mollisols are a soil order in USDA soil taxonomy . Mollisols form in semi-arid to semi-humid areas, typically under a grassland cover . They are most commonly found in the mid-latitudes, namely in North America, mostly east of the Rocky Mountains, in South America in Argentina (Pampas) and Brazil, and in Asia in Mongolia and the Russian Steppes . Their parent material is typically base - rich and calcareous and include limestone, loess, or wind - blown sand . The main processes that lead to the formation of grassland Mollisols are melanisation, decomposition, humification and pedoturbation . </P> <P> Mollisols have deep, high organic matter, nutrient - enriched surface soil (A horizon), typically between 60--80 cm in depth . This fertile surface horizon, known as a mollic epipedon, is the defining diagnostic feature of Mollisols . Mollic epipedons result from the long - term addition of organic materials derived from plant roots, and typically have soft, granular, soil structure . </P> <P> Mollisols occur in savannahs and mountain valleys (such as Central Asia, or the North American Great Plains). These environments have historically been strongly influenced by fire and abundant pedoturbation from organisms such as ants and earthworms . It was estimated that in 2003, only 14 to 26 percent of grassland ecosystems still remained in a relatively natural state (that is, they were not used for agriculture due to the fertility of the A horizon). Globally, they represent ~ 7% of ice - free land area . As the world's most agriculturally productive soil order, the Mollisols represent one of the more economically important soil orders . </P> <P> Though most of 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 . </P>

What part of the country are you likely to find the majority of the mollisols