Desert soils cover 6.1 million square miles (15.73 million square kilometers) or 12 percent of Earth’s surface. Desert soil types range from pure sand in dunes to heavy clays on playas (desert basins where run-off evaporates) to deep fertile soils in river valleys. Desert soils that developed on sediment left behind by ancient seas are salty and alkaline.
The United States Department of Agriculture (USDA) describes soils within a system of 12 orders based on soil forming processes, such as climate and plant cover. Each order is organized into suborder, great group, subgroup, family and series. Desert soils are in the Aridisol order, which includes about 2,600 minutely detailed soil series descriptions. For example: Acoma series soil from Lincoln County, Nevada, is deep, well-drained soil formed over two to 15percent sloped alluvial fan remnants of welded tuff (volcanic ash) and mixed lake deposits under 9.8 inches annual precipitation and average annual temperature of 46.4 degrees Fahrenheit.
Aridisols display an ochric epipedon (yellow surface) plus diagnostic characters in horizons (layers). Argillic (clay) horizons develop in current soil or may be remnants of ancient soil under past climates. Calcic horizon contains calcium carbonate (chalk) or calcium sulfate (gypsum). Cambic horizon is very fine sand. Petrocalcic horizon is cemented calcium or magnesium carbonate. Gypsic horizon is cemented calcium sulfate. Natric horizon has prismatic or columnar clay and a subhorizon at least 15 percent sodium. Salic horizon is at least seven inches with two to three percent soluble salts. Duripan is cemented with silicon.
Aridisol is divided into the older Argid suborder with argillic (clay) or natric (prismatic or columnar clay) horizons and the younger Orthid suborder, which lacks clay horizons. Argid soils may also have cutan skins (thin upper surfaces of clay) or desert pavement and desert varnish. Desert pavement is rock and gravel that remains in place when fine soil surface materials are deflated (blown away). Desert varnish is a coating of iron oxide or manganese oxide on the surface of rocks and gravel.
The Argid suborder includes five great groups. Durargids have a duripan below an argillic or natric clay horizon within 40 inches below the surface. Haplargids have no duripan within 40 inches depth, but they have an argillic horizon of less than 35 percent clay. Nadurargids have a columnar natric clay horizon over a duripan within 40 inches depth. Natrargids have a natric clay horizon, but no petrocalcic (hard chalk or gypsum) horizon within 40 inches depth. Paleargids have either a petrocalcic horizon within 40 inches depth or an argillic horizon greater than 35 percent clay.
The Orthid suborder includes four great groups. Calciorthids have a calcic (calcium carbonate) or gypsic (calcium sulfate) horizon within 40 inches of the surface. Durorthids have a microcrystalline silicon (such as opal) cemented duripan within 40 inches depth. Paleorthids have a petrocalcic (calcium hardpan) horizon within 40 inches depth. Salorthids have a salic (salty) horizon within 30 inches of the surface.