Deserts, which cover about one-fifth of the earth's surface, are defined by rainfall less than 20 inches a year. Most deserts are hot and frequently windy, although there are cold deserts. The soils of hot, arid interior and coastal deserts are remarkably similar; they are sandy or rocky and have abundant nutrients, although they contain little or no organic matter.
Soils of Hot, Dry Deserts
These deserts include the Chijuajua, Great Basin, Mojave and Sonora deserts of North America, the Arabian and other deserts of South Asia, as well as deserts in Ethiopia and Australia. The wind has blown away fine dust and particles in these deserts, leaving coarse-textured sand. Since there is little or no rain, there is less chemical weathering. The soils are shallow, sandy, gravelly or rocky. They drain quickly and there is no water immediately below the surface. Since few plants grow in these desert soils, they contain little organic matter. This means that micro-organisms have nothing to convert into hummus. Since these soils have little organic matter, they can't retain water. Above the flat areas of the Sonora Desert, bajadas--shallow slopes of rocky hills--contain a mixture of small rocks, stones and gravel. In some areas of the Sonora, reddish-brown clay soils have washed down from higher elevations. They gather above layers of dissolved limestone. These are often turned into adobe brick.
Soils of Semiarid Deserts
These deserts include the sagebrush land of the Great Basin, Montana and Utah. Similar deserts are found in the Nearctic ecozones of Greenland, Newfoundland, Europe, Northern Asia and Russia. The summers of these deserts are long and dry, but the winters bring some rainfall. The soil is ordinarily coarse and rocky or gravelly on mountain slopes and sandy and fine-textured in bottom land. There is no subsurface water and the soil is usually alkaline. The sandy soils are similar to the hot, dry deserts with the difference that there is more moisture here.
Soils of Coastal Deserts
Coastal deserts such as the Atacama desert of Chile and some deserts of Australia are found in both cool and warm areas. The intense evaporation of water brings dissolved salts to the surface of sandy soils. Sodium and calcium on the surface can dissolve salts, bringing saltpans where nothing can grow. The soil is ordinarily porous and well-drained.
Soils of Cold Deserts
Cold deserts have snow in the winter and periods of high rainfall. These are deserts in Antarctica, Greenland, and the Nearctic ecological zones. The soils are very different from those in hot, arid or semi-arid deserts. They are heavy, full of silt, and they are salty. There is less salt where the soils are able to drain and the salt is leached out.