Soils of the middle-latitude deserts and tropical deserts fall into two great soil groups on the basis of colour.
(1) gray desert soils, or sierozem, and
(2) red desert soils.
The gray desert soils, or sierozem, are well developed in Wyoming and in the deserts of Nevada, western Utah, and southern portions of Oregon and Idaho. This latter region is sometimes referred to as the Great Basin because of the prevalence of interior drainage systems ending in evaporating basins. It corresponds approximately with the middle-latitude desert climate and has northward extensions into the middle-latitude steppe climate.
The gray desert soils contain little humus because of the sparse growth of vegetation, such as sagebrush and bunchgrass. Colour ranges from light gray to grayish brown. Horizons are present, but being only slightly differentiated are not conspicuous. Excessive amounts of calcium carbonate are present at depths of less than one foot (0.3 m) in the form of lime crust, or caliche, a deposit of calcium carbonate or hydrous calcium sulphate. In places this deposit has the appearance of a hard rock layer and may even resist erosion in such a way as to produce small mesas or platforms. Gravels deposited by streams are often thus cemented into a conglomerate rock. Lime crust forms during prolonged dry periods when ground waters rise slowly surfaceward by capillary attraction and evaporate near the surface, leaving the salts behind in the soil.
In the more arid, hotter tropical deserts are found the red desert soils. These range form a pale reddish gray to a pronounced deep red. Humus is reduced to the minimum, there being only a scattered growth of desert shrubs. Thus the activity of plants as soil formers reaches its lowest point in the red desert soils, as does also the activity of animals. Colour is derived from small amounts of oxides of iron. Horizons are poorly developed; texture is often coarse, with many fragments of parent rock throughout the soil. Deposits of lime carbonate are present, as with the gray desert soils.
The gray and red desert soils are suitable for cultivation only where they are fine textured, as along the floodplain terraces of exotic streams and on the outer slopes of alluvial fans. Irrigation is essential, whether water is diverted from a river or obtained from wells penetrating the ground water reserves in alluvial fans.
Altitude and soils
Climate zoning in mountains tends to reproduce world climates in a series of vertical zones, the effect of increasing altitude being much the same as that of increasing latitude. Because climate is a major determining factor in soils, we might also expect increasing altitude to result in a series of great soils groups. Such a series is illustrated by soils of the Big Horn Mountains in Wyoming. Starting with gray desert soils at the lowest elevation, the series progresses through the zonal soils of dry climates through prairies soils to podzols at the high elevations.
Compare these altitude changes with which summarizes the profile changes along an imaginary traverse line from the southwestern desert of the United States, eastward across the high plains region, then north into the Lake Superior region.
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