Click on the image for a high-resolution version.
Mollisols on the Great Plains and in the Western States. Nearly all Mollisols have a mollic epipedon. Most have supported grass vegetation at some time, although many apparently have been forested at times. Mollisols are used mainly as cropland. Generally, grains and sorghum are grown in the drier regions and maize (corn) and soybeans in the warmer, humid regions.
Albolls are the Mollisols that have both an albic horizon and fluctuating ground water. They are mostly on the Great Plains, in eastern Washington, and in Oregon and Idaho. Most supported grass or grass and shrub vegetation. In early stages of development, some are thought to have supported forest vegetation that was later succeeded by grass. Most of the soils have gentle slopes and are used as cropland.
Aquolls are the wet Mollisols. They are most extensive in glaciated areas of the Midwestern States, mainly Iowa, Minnesota, North Dakota, and Indiana. Most have supported vegetation of grasses, sedges, and forbs, but some supported forest vegetation. Most have been artificially drained, and are used as cropland.
Cryolls are the cool or cold, more or less freely drained Mollisols. Most are in the high mountains of the Western States. Some are in Alaska. They support forest, grass, or grass and shrub vegetation. In Alaska forests of spruce, birch, and aspen trees are common. Cryolls are used mostly as rangeland. Some are used as forest and some as pasture.
Rendolls are the Mollisols that are of humid regions and formed in highly calcareous parent materials, such as limestone, chalk, and drift composed mainly of limestone, or on shell bars. They are in Florida, on tropical islands, and in a few areas in the mountains of the Western States. They formed mostly under forest vegetation or under grass and shrubs. Most are used as cropland or pasture, but some are used as rangeland or forest.
Udolls are the more or less freely drained Mollisols of humid climates. They are in the eastern part of the Great Plains and east of the Great Plains. They are most extensive in Minnesota, Illinois, Iowa, and North Dakota. The vegetation at the time of settlement was dominantly tall grass prairie. Where slopes are not too steep, nearly all of these soils are used as cropland. Some are used as pasture or rangeland.
Ustolls are the more or less freely drained Mollisols of subhumid to semiarid climates. They are mostly on the western Great Plains. They also are common throughout the Rocky Mountain States. Most of the Ustolls on the Great Plains supported grass vegetation when the country was settled. Some Ustolls, mostly those in the mountains of the Western States, supported forest vegetation. Rainfall comes mainly during a growing season, often in heavy showers, but is erratic. Drought is frequent and may be severe. Without irrigation, the low supply of moisture usually limits crop yields. During a drought, soil blowing can be a problem. Most of these soils are used as cropland or rangeland.
Xerolls are the more or less freely drained Mollisols of regions that have Mediterranean climates. They are in the Pacific Northwest, California, Idaho, Nevada, and western Utah. Xerolls are dry for extended periods in summer, but moisture moves through most of the soils in winter and is stored above the deep layers or above bedrock in normal years. The vegetation at the time of settlement was dominantly bunchgrass and shrubs or trees in the areas that have a mesic or frigid temperature regime, a savanna of perennial grasses and oak and Douglas-fir in the Willamette Valley of Oregon, and a savanna of annual grasses and oak species on the Xerolls in California that have a thermic temperature regime. Many irrigated crops are grown on the Xerolls in the United States, especially where the temperature regime is thermic or mesic. Most of the soils that have gentle or moderate slopes are used as cropland. The very steep soils are used mainly as rangeland, but some are used as forest.