MOLLISOL, a dark-colored soil type of grasslands and forested areas that have the world's most productive agricultural soils. Mollisols are found in the wetter areas that border desert regions and are therefore geographically associated with aridisols. The ground cover in these areas yields abundant organic matter, which decomposes within the soil. Rainfall is not heavy enough to cause extensive leaching, so the content of chemical bases upon which fertility depends remains high. These conditions give rise to a soft, well-aggregated soil structure and the characteristic "mollic" horizon--a dark surface layer more than 10 inches (25 cm) thick containing at least 1 percent organic matter. In the lands where these soils occur -- notably the Great Plains of North America, the pampas of Argentina, and the steppes of Eurasia--grazing and dry land farming of wheat and corn (maize) are the main forms of agriculture.