What is an Aridic Soil?

Aridic soil is soil that is subjected to an arid climate. However, definitions of arid [See: Classifications of Arid Land Soils] and definitions of soil [See: How Soil is Defined] vary among users. These discrepancies between definitions can lead to confusion, especially at the limits of what is and is not considered "arid" or "soil." Despite the lack of a clear definition, aridic soils do possess certain unique characteristics.

Arid and semi-arid environments occupy around 37% of the land on earth. Soils in these environments differ from those in humid zones in seven significant respects, all owing to scant moisture (Cooke, Warren, and Goudie, 1993, p. 45):

• Less weathering and leaching, which has four consequences: coarse textures, shallow soils, retention of soluble substances, slow soil formation.

• Large areas of bare rock and coarse weathered mantle.

• Great expanses of patinated surfaces (e.g., desert varnish).

• Major inputs of aeolian material.

• Low rates of erosion, especially in ancient shield deserts such as the Sahara and in Australia, permitting the survival of extensive areas of very old soils.

• Abrupt soil boundaries as a consequence of abrupt thresholds in soil behavior with respect to slope, drainage, and windblown sand and organic material.

• Lower thresholds to degradation processes than humid soils.

Sandy soils

Coarse textured or sandy soils occur throughout arid regions primarily as a result of the weathering processes common in a dry environment (salt, chemical, biochemical, mechanical, pressure release or exfoliation, wetting and drying, and insolation or thermal expansion). Rocks and gravels decompose to finer particles. The wind selectively removes the silt and clay size particles far from its source as dust (Saharan and Sahelian dust of Africa is a major contributor to Atlantic Ocean sediment). Sand remains in the area, forming plains and dunes over old erosional surfaces. Soils that form from this wind blown sediment have high water infiltration rates. While these high rates assure capture of rainfall, sandy soils hold less water for plant uptake than finer textured soils. Sandy soils pose other difficulties for vegetation. If strong winds occur shortly after seeds germinate, the seedlings are at great risk to death by sand blasting. Sand mobility and resulting dunes can threaten buildings, roads, and irrigated agriculture land.

Thin soils

Rainfall runoff and resulting erosion in sloping areas maintain shallow soil by stripping away soil as it slowly forms. Shallow, sloping soils lose much rainfall to runoff while the water retained in the soil soon evaporates. These thin soils provide a harsh environment for the very vegetation that would help protect the soil from erosion. Existing vegetation on these shallow soils is easily degraded by grazing.

Rocky soils

Stone pavement, also called desert pavement, hammada, or reg, is an armored surface composed of angular or rounded fragments, usually only one or two stones thick. These soils generally form in areas lacking vegetation through erosion of fine particles by wind and water or through upward migration of stones in the soil. Stone pavement protects the soil from erosion, but disturbence (e.g., off road vehicles) can cause accelerated erosion by channeling water runoff over unprotected soil (Cooke, Warren, and Goudie, 1993, p. 68).

Salty soils

Chemical decomposition of minerals in all soils forms salts. The meager rainfall of an arid climate, however, is generally insufficient to leach away salt from the soil. As a result, salts accumulate at the limit of the soil wetting front after each rainfall event. Other sources of salts are atmosphere and ground water. Salts can accumulate, forming a layer of soil concrete called calcrete if formed from calcium salts, silcrete from silica, and gypcrete from gypsum. Over time, erosion often exposes the cemented soil layers that appear as rock. Many areas of exposed calcrete are difficult to distinguish from limestone or marl. Salt also accumulates in basins or playas through rainfall runoff. Salty soils of arid lands pose severe obstacles to construction and agricultural production.

Oases and playas

Water is an especially important resource in arid lands and the soils associated with water sources are of equal value. These wetland and riparian areas contrast sharply with the surrounding landscapes both visually and in biomass production. Competition for grazing, agriculture, and wildlife is most acute in these areas.

Playas are often very salty as they can accumulate salt from runoff and groundwater pumping. Two major engineering problems associated with playas are foundation protection and trafficability. These problems are caused by periodic inundation, spatial and temporal variability of compressibility, corrosiveness of salts, evaporative pumping of salts to the surface, and densification of surface material (Cooke, Warren, and Goudie, 1993, p. 219).

Climate change

Soils and desert features are not always the product of the present dry climate. Often they are relics of a much more humid past,such as the extensive ferecrete deposits occuring in deserts. Ferecrete, also called laterite and ironstone, is the result of iron concentration in a hot and humid environment.

Management Problems

Among the agricultural and range management problems commonly encountered with aridic soils are salinity and salt toxicity (especially from sodium, chloride, and boron), management of expansive soils, irrigation management of sandy soils, blowing sand and wind erosion, and overgrazing and soil erosion (Balba, 1995). Engineering problems commonly encountered are deep desiccation of soils, piping, salt corrosion and weathering (hydration, crystallization, and thermal expansion), flooding, wind blown sand and dunes, collapsing soils, and expansive soils (Fookes and Parry, 1994).

In addition to soil science and engineering, geomorphology is closely implicated in the study of aridic soils (Cooke, Warren, and Goudie, 1993, p. 45) and can be of great value in predicting soil characteristics and mapping aridic soils.

Aridic Soil Myths

There are many misconceptions about aridic soils that are perpetuated by the photogenic and bizarre, but also by valid generalizations that if followed blindly can lead to poor land management. Some of them are described below.

Hot Sand

The hot, barren, sandy images of deserts presented in movies has perpetuated a myth. Although deserts are generally hot during at least one season of the year, only 20% of deserts are covered by aeolian sand and only 13% of them are truly barren. Mountains are the most common land form of deserts and the typical vegetative cover is sparse grass and shrubs (Abrahams and Parsons, 1994, p. 3). Presented as wastelands in many land use inventories, deserts support irrigated agriculture, riparian forests, and extensive rangelands.

Desert varnish

Rock varnish coatings are not unique to arid climates but are a widespread phenomenon occurring in tropical and high-altitude environments. Rock varnish is a thin coating of clay (approximately 70%) and mineral deposits of iron and manganese oxides and hydroxides admixed with silica and calcium carbonate. The orange, gray, brown, or black coatings have physio-chemical and biological origins. Desert varnish should not be confused with "desert glaze," a coating of a siliceous precipitate (Watson, 1989).

Salinity

Not all aridic soils, especially the sandy soils, are high in salts and bases. Sandy soils allow rapid infiltration of water and leaching of salts. In areas of intense rainfall leaching, acidification can be significant, especially when the microbasins that are typical of sandy plains capture all the runoff. This can result in Alfisols occurring on the microhummics and Ultisols in the microbasins, a common occurrence in the sandy plains of Sahelian Africa. Closed basins in deserts are not always salty, as in Lake Chad of Saharan Africa which has a complex biological sink that maintains low levels of salts in its water.

References

Abrahams, A.D. and A.J. Parsons (1994). Geomorphology of desert environments. New York: Chapman & Hall. 674 pp.

Balba, A. M. (1995). Management of problem soils in arid ecosystems. CRC Press. 250 p.

Cooke, R., A. Warren, and A. Goudie (1993). Desert geomorphology. London: UCL Press Limited. 526 pp.

Fookes, P.G. and R.H.G. Parry, eds. (1994). Engineering characteristics of arid soils. Proceedings of the 1st International Symposium on Engineering Characteristics of Arid Soils, London, U.K., 6-7 July 1993. Rotterdam: A.A. Balkema. 441 p.

Spangler, M.G. and R.L. Handy (1982). Soil engineering. New York: Harper & Row. 819 pp.

Watson, A. (1989). Desert crusts and rock varnish. In Arid zone geomorphology, ed. D.S.G. Thomas. pp. 25-55. London: Belhaven Press; New York: Halsted Press.

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Last revised: Last revised: 24 August 2001
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