Mollisols are the soils of grassland ecosystems. They are characterized by a thick, dark surface horizon. This fertile surface horizon, known as a mollic epipedon, results from the long-term addition of organic materials derived from plant roots.
Mollisols are among some of the most important and productive agricultural soils in the world and are extensively used for this purpose.
Mollisols primarily occur in the middle latitudes and are extensive in prairie regions such as the Great Plains of the US. Globally, they occupy ~6.9% of the ice-free land area. In the US, they are the most extensive soil order, accounting for ~21.5% of the land area.
Albolls - wet Mollisols with a light- colored horizon formed through Fe reduction. Most of these soils are saturated with water to or near the soil surface at some time during winter or spring in normal years. These soils developed mostly on broad, nearly level to sloping ridges, on back slopes, or in closed depressions. Most are in areas of late-Pleistocene deposits. Most developed under grass or shrub vegetation.
Aquolls - Mollisols with a water table at or near the surface for much of the year. Most of the soils have had a vegetation of grasses, sedges, and forbs, but a few have had forest vegetation. In the US, Aquolls are most extensive in glaciated areas of the midwestern states where the drift was calcareous.
Rendolls - shallow Mollisols over calcareous parent material such as limestone, chalk, drift composted of limestone or shell bars, of humid regions. These soils are not extensive in the US, but are extensive in some parts of the world and formed under forest vegetation or under grass and shrubs.
Cryolls - Mollisols of cold cimates. More or less freely drained. Moderately extensive in the high mountains of the western US. Also extensive on the plains and mountains of eastern Europe and in Asia. On the plains, these soils are in areas of late-Pleistocene or Holocene deposits. The vegetation on the Cryolls on the plains was mostly grasses. Cryolls in the mountains have either forest or grass vegetation. Cryolls in Alaska support spruce, birch and aspen trees.
Xerolls - temperate Mollisols with very dry summers and moist winters. Found in regions with a Mediterranean climate. Xerolls in the US formed mainly in late-Pleistocene loess that varies in thickness and overlays bedrock on nearly level to steep older surfaces. Tertiary lake sediments, older crystalline rocks and alluvium are common parent materials in some areas. Xerolls are not extensive in the world but are extensive in parts of Turkey, northern Africa near the Mediterranean and in some of the southern republics of the former USSR and in Washington, Oregon, Idaho, Utah, Nevada and California in the US.
Ustolls - Mollisols of semiarid and subhumid climates. Rainfall occurs mainly during a growing season, often in heavy showers, but is erratic. Drought is frequent and may be severe. During drought conditions, soil blowing becomes a problem. Extensive on the western Great Plains in the US. Most US Ustolls on the Great Plains had a grass vegetation, and some Ustolls in the mountains of the western US supported forest vegetation. The Aridic subgroups support mostly short grasses. Ustolls formed in sediments and on surfaces of varied ages from Holocene to mid Pleistocene or earlier.
Udolls - Mollisols of humid climates. Formed mainly in late-Pleistocene or Holocene deposits or on surfaces of comparable ages. In the US, the vegetation was dominantly tall grass prairie but some of the soils on Pleistocene surfaces appear to have supported boreal forests several thousand years ago. Ustolls formed in sediments and on surfaces of varied ages from Holocene to mid Pleistocene or earlier. Most occur in the eastern part of the Great Plains or are east of the Great Palins. The soils are most exentsive in Illinois, Iowa and adjacent states.