Aridisols are characterized by being dry most of the year. In these soils, there is no consecutive 90 day period with adaquate soil moisture for plant growth. Aridisols dominate the deserts and xeric shrublands which occupy about one third of the Earth's land surface. Aridisols have a very poor concentration of organic matter (In this case decomposed material, humus). Imperfect leaching in these soils often results in one or more subsurface soil horizons in which suspended or dissolved minerals have been deposited; such as clays, calcium carbonate, silica, salts, and/or gypsum accumulations. Accumulation of salts on the surface can result in salinization.
Aridisols are used mainly for range, wildlife, and recreation. Because of the dry climate in which they are found, they are not used for agricultural production unless irrigation water is available.
Aridisols occupy ~12.0% of the Earth's ice-free land area and ~8.3% of the US.
Cryids - Aridisols of cold climates. Characteristically at high elevations, dominantly in the mountain and basin areas of the US and Asia and other parts of the world. The cold deserts have undergone alternation periods of cold and warm climates, which result in the expansion and contraction of alpine glaciers in adjacent mountains, resulting in variations in sediment loads carried by mountain streams. Cryids commonly show evidence of periglacial features.
Salids - Aridisols with soluble salt accumulations. Most common in depressions in the deserts or in closed basins in wetter areas bordering deserts. Accumulation of salts commonly occur when there is a supply of salts and a net upward movement of water in the soil. In some areas a salic horizon has formed in salty parent materials without the presence of ground water. The concept of Salids is one of accumulation of an excessive amount of salts that are more soluble than gypsum. The most common form is sodium chloride (halite) but sulfates (thenardite, mirabilite, and hexahydrite) and others may also occur. Two great groups are recongnized, Aquasalids, which are saturated with water for 1 month or more during the year, and Haplosalids, which are drier.
Durids - These soils occur dominantly on gentle slopes and formed in sediments that contain pyroclastics. The duripan is cemented partly with opal or chalcendony. The soils commonly have calcium carbonate. The duripan is a barrier to both roots and water. These soils occur in the western part of the USA particularily in Nevada. They are not known to occur outside of the USA.
Gypsids -Aridisols with gypsum accumulation. These occur in Iraq, Syria, Saudi Arabia, Iran, Somolia, West Asia and in some of the most arid regions of the USA. Accumulation of Gypsum takes place initially as crystal aggregates in voids in the soils. These aggregates grow by accretion, displacing the enclosing soil material. hen the gypsic horizion occurs as a cemented impermeable layer, it is recognized as the petrogypsic horizon. Some Gypsids have calcic or related horizons that overlie the gypsic horizon.
Argids - Aridisols with clay accumulation. These soils have an argillic or natric horizon. The presence of an argillic horizon is commonly attributed to a moister paleoclimate, although there is evidence clay illuviation occured during the Holocene in arid soils. Most of the Argids occur in North America with a few recongnized in the deserts of north Africa, or the Near East.
Calcids - Aridisols with calcium carbonate in the parent materials or was added as dust or both. Precipitation is inadequate to leach or move the carbonates to great depths. These soils are extensive in the western USA and other arid regions of the world.
Cambrids - Aridisols with a weakly developed B horizon and least degree of soild development. These soils are the most common Aridisol in the USA.