ARIDISOL

one of the 12 soil orders in the U.S. Soil Taxonomy. Aridisols are dry, desertlike soils that have low organic content and are sparsely vegetated by drought- or salt-tolerant plants. (Not included in this order are soils located in polar regions or high-elevation settings.) Dry climate and low humus content limit their arability without irrigation. Covering only about half of the arid regions of the Earth, they account for 18.5 percent of the nonpolar continental land area, being second only to Inceptisols in extent. Aridisols occur extensively in the southwestern United States and Australia, northwestern Mexico, and the Sahara and across Asia south of the steppe regions. Aridisols are characterized by a surface horizon (uppermost layer) that is light in colour with very low humus content, by dry soil conditions for most of the year, and by a significant accumulation of translocated (migrated) layer silicate clay, soluble salts, or sodium ions. Soils lacking any of these characteristics are also classified as Aridisols if a layer of calcium carbonate, gypsum (hydrated calcium sulfate), or cemented silica is present below the surface horizon. As these soils mature under the slow weathering conditions of their arid environments, they tend to show subsurface clay and calcium carbonate accumulations and develop surface pebble layers (the desert pavement). Aridisols differ from Inceptisols, another soil order of low humus content, primarily by their drier (or more saline) soil condition.