ABSTRACT
Soils of arid regions possess many characteristics that distinguish them from soils of humid regions. They commonly have low biological activity and a low level of organic matter, are usually slightly acid to alkaline in reaction (pH) in the surface, and commonly contain accumulations of calcium carbonate and sometimes accumulations of soluble salts, gypsum, or free silica. Some arid soils may have properties that are also present in humid soils, such as shallow subsurface zones of clay accumulation. Arid soils show considerable variation in age, ranging from very recent (late Holocene) to early Pleistocene or older. In contrast to earlier concepts of arid soils being a rather homogeneous group, it is now recognized that they show considerable morphological variability (Jackson, 1957; Nettleton and Peterson, 1983). Modern systems of soil classification generally recognize this variability. This chapter describes the major soil classification systems that include a comprehensive classification of arid and semiarid soils. Emphasis is placed on the U.S. soil taxonomy (Soil Survey Staff, 1975). The processes leading to the formation of important features of arid and semiarid soils are also addressed.
