Given often limited plant cover and meager precipitation, soil-building in deserts can be a very slow process indeed. Large expanses have only a scanty veneer of soil, commonly pale or whitish from salt or calcium deposits, or sometimes a rusty red from weathered iron-rich bedrock; tracts of bare stone and active sand dunes may lack soil altogether. Unsurprisingly, arid climate characteristics help determine the defining elements of desert soils.
Desert Soil Basics
Because of low precipitation, water doesn’t flush desert soils of salts and other soluble minerals as readily as it does in moister climate zones, which means they can accumulate significantly. That low precipitation also generally limits the amount of water within the soil – further reduced by high temperatures, which increase rates of evaporation and transpiration (the loss of water from plants) – and how deeply it penetrates, which helps determine the overall depth of the desert soil.
Wind, which can be significant in deserts, also enhances evapotranspiration – the combined water loss from evaporation and transpiration – and serves as a major agent of erosion given the typically sparse groundcover of deserts; the dust and fine sand raised by winds, once deposited, serve as soil-building inputs elsewhere.
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Common Desert Soil Types: Aridisols and Entisols
The “quintessential” desert ground soils are Aridisols, which underlie close to a fifth of the planet’s terrestrial surface. These soils tend to have an upper horizon (or soil layer) poor in organic matter and often include deposits of salt, calcite and gypsum. Even in major Aridisol zones, though – which correspond to the great tracts of subtropical and temperate deserts – you’ll find extensive examples of Entisols, which are very young soils under formation, developing, for example, atop rocky plateaus, gravel plains or patches of sand dunes colonized by grasses or other plants.
The high concentrations of calcium carbonate, silica and iron oxides frequently found in desert soils may cement together into impervious layers known as hardpans, which can impede the downward flow of water and the downward growth of plant roots. Scientists call thick calcium-carbonate hardpans caliche, widespread in the arid American Southwest and other drylands around the world. Wind or water erosion may ultimately expose the whitish, chalky caliche at the surface by wearing away overlying soil horizons; this is an example of a truncated soil.
Biological Soil Crusts
A common feature in many deserts, biological soil crusts – also called microphytic crusts – are intermingled communities of cyanobacteria, microfungi, lichen, green algae, liverworts and mosses. Cyanobacteria thread together mats of soil subsequently colonized by other organisms. Biological soil crusts may develop over thousands of years and provide many ecosystem services, including securing soil against erosion, soaking up water and converting atmospheric nitrogen into a form usable to plants. Quite inconspicuous unless you know to look for it, these crusts can easily be damaged by people walking or driving over them.
Desert Soil and Topography
The topography of desert-scapes, as anywhere, influences the layout of their soils. Alluvial fans and bajadas – fans that have merged into rubble-filled aprons – commonly edge desert mountain ranges. From their upper reaches to their toes, where they transition into the flats of desert basins, their soil ranges from gravelly and cobbly to finer and finer-textured sands, silts and clays. Low-lying desert basins that lack a drainage outlet often accumulate salt left behind from evaporated water, and the saline soils that result make a harsh environment for many plants – though certain species, such as tamarisk trees, shadscale shrubs and the aptly named saltgrass, have adapted to tolerate such salty conditions.
The Importance of Desert Soil Texture
The defining element of desert soil from an ecological point of view is its texture; that is, the relative sizes of the particles that make it up. That’s partly because texture helps determine the movement and retention (or not) of water through the soil. Water doesn’t seep down as deeply in very fine-textured clay as it does in coarser sandy soils, which in desert climates means clay soils tend to dry out more thoroughly. More water is held in the upper layer and evaporates out, while the deeper water in sandy soil holds longer. Very generally speaking, then, sandy soils in deserts tend to be more favorable for plant growth than clay-dominated ones – a different situation than in moister climates, where clay soils tend to be more productive because of greater water and nutrient retention.
Soil may play a role in the formation of other distinctive kinds of desert terrain besides caliche outcrops and biological crusts. Desert pavement – a version of the gravel desert known as reg or serir in the Sahara and gibber in Australia – describes a surface of tightly packed stones mostly barren of vegetation. While geomorphologists (scientists who study the origin of landforms) have multiple theories for how desert pavements form, one leading explanation suggests that dust deposited among the gravel by wind gradually forms a fine-textured soil horizon that essentially raises the rocks as a single layer. The surface of desert pavement commonly turns a shiny black color – “desert varnish” – derived from chemical weathering.