Soil Genesis and Classification, Sixth Edition. S. W. Buol, R. J. Southard, R. C. Graham and P. A. McDaniel. 2011 John Wiley & Sons, Inc. Published 2011 by John Wiley & Sons, Inc.
Aridisols: Soils of Dry Regions
Aridisols occur in both cool temperate deserts (between latitudes 35 and 55) and warm deserts at lower latitudes (Cameron 1969). Soils with permanently frozen subsoils in dry polar regions (Campbell and Claridge 1990) are classified in the Gelisol order (Chapter 12).
SettingDeserts occupy about one-third of the areas of Africa and Australia, 11% of Asia, and only about 8% of the Americas. Only a small proportion of the deserts of the planet consist of barren sand dunes and rock land, movie sets for the Arabian Nights notwithstanding. Rather, most of these areas are surprisingly well vegetated with scattered plants (Figure 10.1), the root systems of which extend considerable distances both laterally and vertically from each plant. Various species of cactus (Cactaceae), mesquite (Prosopis), creosotebush (Larrea), yucca (Yucca), sagebrush (Artemisia), shadscale (Atriplex), hopsage (Grayia), and muhlygrass (Muhlenbergia) are common. Microbial populations are low. Low carbon to nitrogen ratios in the sparse soil organic matter are probably attained by action of nitrifying bacteria and/or nitrogen-fixing blue-green algae that form a crust on some of these soils (Mayland etal. 1966; Evans and Johansen 1999; Johnson etal. 2007).
Arid regions occupy 36% of the earths land surface on the basis of climate and 35% on the basis of natural vegetation (Shantz 1956). Aridisols do not conform to all the parameters of either climatic or vegetative zones (Buol 1965; Dregne 1976; Southard 2000). The moisture control section of these soils is dry in all parts more than 50% of most years and not moist in any part as much as 90 consecutive days when the soils are warm enough (>8C) for plant growth. In an aridic soil moisture regime, potential evapotranspiration greatly exceeds precipitation during most of the year, and in most years no water percolates through the soil. Figure 10.2 (Buol 1964) shows a characteristic aridic water balance where about 1.3 cm (0.5 in.) of water is recharged (R) into the soils during December, January, and February. This stored water is utilized (U) during March, and the soils are deficient (D) in water throughout most of the year. Even though leaching is limited, many Aridisols have morphologi-cally distinct horizons due to the concentration of the limited water in a relatively small volume of soil.
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Figure 10.1. Schematic cross-section diagrams of the microtopographic positions and associated surfacesoil morphological types of loess-mantled Argids of the Humboldt loess belt of Nevada, with big sagebrush (Artemisia tridentata var. wyomingensis) plant communities. Positions are C = coppice, B = coppice bench, M = intercoppice microplain, P = playette. M and P can be wider than shown; several coppices may be linked together. Vertical lines below the soil surface show sides of surface crust polygons (A1 horizon) that extend into the underlying A2 horizon with compound weak prismatic and moderate platy structure. Type I is weakly crusted and litter covered. Type II is pinnacled. Only Types III and IV are significantly crusted. Circles indicate vesicular pores in the A1 horizons of Types III and IV. (Courtesy, Society for Range Management, Denver, Colorado, and modified from Eckert etal. 1978)
Soil-vegetation patterns of arid regions are recognized at many scales. Satellite imagery shows clearly demarcated, interspersed bright areas, which are sandy and saltation-prone under the impact of wind, and relatively dark areas, which are stable, crusted, and immune to wind erosion (Otterman and Gornitz 1983). Figure 10.1 shows fine-scale patterns of vegetation and soils as observed in Nevada (Eckert etal. 1986). Patterns of runoff and run-on are intricate and important in determining distribution and condition of desert vegetation (Schlesinger and Jones 1984; McAuliffe 1999), nutrient cycling, and leaching of soluble salts (Reid etal. 1993; Wood etal. 2005; Graham etal. 2008). Playas and other utterly barren sites are considered by some soil scientists to be without soils.
Aridisols occupy about 12% of the global land area. Only about one-third of the arid terrains are occupied by Aridisols. Other soils present are torric, ustic, and xeric great groups of Entisols, Mollisols, and other orders (Figures 10.3 and 10.4). On a transect from arid to adjacent semiarid areas (left to right, Figure 10.3), ochric epipedons darken, with increasing organic matter content (as precipitation increases) and merge into mollic epipedons, preferentially in fine-textured and/or calcareous parent materials. Alfisols appear on adjacent forestlands and on some grassland. Vertisols are present on bodies of lithogenic or pedogenic smectite clays.
Evidence of leaching below the average depth of water storage is often observed in Aridisols and attributed to pluvial times associated with Quaternary climatic
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Figure 10.3. Block diagram showing positions of some major kinds of Aridisols and their associates.
Figure 10.2. Aridic soil moisture regime for soils at Phoenix, Arizona. Note that the soil moisture control section is dry in all parts more than half of the time in an average year.
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Figure 10.4. Relationships between suborders and great groups of arid-land and semiarid-land soils. (After Flach and Smith 1969)
changes (Smith 1965; Wells et al. 1987). Another explanation is that the typically erratic distribution of the rainfall causes occasional periods of relatively high precipi-tation in the winter months, during which deeper leaching takes place. In this case, the depth of leaching may reflect the rainfall of the extreme years, rather than that of average years.
Pedogenic ProcessesPedogenic processes in desert regions have produced numerous soil features. Of special interest are (1) physical surface crusts, (2) biological soil crusts, (3) vesicular horizons, (4) desert pavement, (5) cambic horizons, (6) argillic and natric horizons, (7) pedogenic calcium carbonate accumulations (calcic and petrocalcic horizons), (8) duripans, and (9) gypsic and salic horizons (Nettleton and Peterson 1983).
A physical soil crust (Figure 10.5A), widespread in arid regions, is a thin surficial layer of uncemented fine earth that is coherent when dry and can be broken free from underlying soil material. Its strength arises from physical cohesion rather than biological influences. The crust is generally less than 10- to 20-mm thick and is often massive. Such crusts may grade into vesicular horizons as described below. Where present, physical crusts can impede infiltration and seedling germination.
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Biological soil crusts (formerly called microbiotic or cryptogamic crusts) (Figure 10.5B) are dominant features of many arid ecosystems. They are actually a crust of living organismsintricately intertwined lichens, cyanobacteria, mosses, and algaegrowing on the soil surface. Low water availability in arid environments results in discontinuous plant cover with soil exposed to direct sunlight in the inter-spaces. Biological soil crusts often form a continuous photosynthetic layer on the soil surface in these openings. These crusts are biologically dormant during the prevailing dry conditions but are almost immediately activated by rainfall. Biological crusts stabilize the soil surface against wind and water erosion, trap windblown silt and clay particles, and fix nitrogen. Various studies have shown that these crusts can increase, decrease, or have no effect on infiltration. The effect of biological soil crusts on hydrologic behavior, such as infiltration and runoff, seems to vary with the dominant biotic component and local site characteristics (Evans and Johansen 1999).
Vesicular horizons (Figure 10.1. and Figure 10.6A) are another very common surface feature of arid zone soils worldwide. These horizons form in fine-textured eolian material at the surface or, commonly, immediately below desert pavement. They are commonly labeled Av horizons, but this is not an accepted designation in the USDA system (Springer 1958; Anderson etal. 2002). The more strongly developed vesicular horizons have a compound structure consisting of prismatic or columnar units, often on the order of 10 cm in diameter, which in turn have platy structure and vesicular pores (Figure 10.6b). Repeated wetting and drying cycles, often associated with intense summer thunderstorms, produce these features. As the soil wets, escaping gases form bubbles (vesicular pores) that are preserved when the soil dries (Springer 1958). With repeated wetting and drying, the bubbles enlarge, coalesce, and eventu-ally collapse. The planes of weakness established by the collapsed bubbles yield the characteristic platy structure (Miller 1971). The coarse prismatic structure results
Figure 10.5. Examples of (A) physical and (B) biological crusts on Aridisols. In each case the crust isabout 1-cm thick. In (A), rain splash and dispersion of sodium-saturated, smectite-rich silty clay soilmaterial destroyed its original structure and produced a platy crust (Carrizo Plain, California). In (B), a biological soil crust composed mainly of cyanolichen stabilizes the soil surface and fixes nitrogen (Clark Mountains piedmont, Mojave Desert, California). (Photo by Nicole Pietrasiak)
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from relatively slow desiccation of uniformly textured soil material (Chadwick and Graham 2000). When wet, the prismatic structural units swell, closing the cracks between them. This, along with the platy structure and lack of pore continuity, result in low permeability of water (Young etal. 2004), in contrast to the rapid infiltration that happens in uncrusted coppice sand dunes (Figure 10.1) and vegetated areas (Gile 1966a; Eckert etal. 1979; Reid etal. 1993). The low infiltration rates typical of desert pavement/vesicular horizon surface conditions cause the subsoil accumulation of high levels of eolian-derived salts, including nitrate, while adjacent soils under shrubs are well-leached and nonsaline (Graham etal. 2008).
The complexity of processes and morphology in strongly developed vesicular horizons is described by Anderson etal. (2002). The columnar peds of a 10-cm-thick vesicular horizon in a Typic Natrargid (Figure 10.6b) were found to have up to 40% clay and 12% pedogenic calcite in their interiors, whereas the material adhering to the ped sides contained less than 7% clay and 2% calcite. Desert dust enriched in clay and
Figure 10.6. Examples of desert pavement and associated vesicular horizons in the Mojave Desert, California: (A) a 3-cm-thick vesicular horizon (photo by Nathan Bailey), (B) desert pavement overlying an 8-cm-thick vesicular horizon with columnar structure, (C) closely interlocking rock fragments make up a desert pavement (knife is 30 cm long), and (D) water ponding on desert pavement during a rainstorm.
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calcite is trapped by the rough surface of the overlying desert pavement. It is eluviated down the cracks between the columns, and then is carried laterally by infiltrating water into ped interiors via cracks between platy structural units. Argillans and siltans line the platy surfaces in the lower part of the columns. Several lines of evidence suggest that these clay- and calcite-rich vesicular layers experienced much of their development within the last 5,000 years, when desiccation of local playas provided a source of abundant eolian materials.
Many Aridisols that developed in stony material have surface pebble layers (Figure 10.6b, 10.6c, Figure 10.7), variously called desert pavement, stone pavement, gibber, gobi, sai, hammada, and reg. Desert pavements form on stable geomorphic surfaces, such as alluvial fans and lava flows. They are composed of pebbles, cobbles, and stones derived from the underlying alluvium or bedrock. A number of mechanisms have been proposed for the formation of the pavement, including concentration of clasts at the surface by wind and water erosion and upward migration of clasts through the soil by shrink-swell heaving and other physical processes (Cooke 1970). Strong evidence has now accumulated showing that most pavements form on accreting landscapes as rock fragments on the original surface are rafted upward by infiltrat-ingeolian material (Wells etal. 1985; McFadden etal. 1987). The pavement itself serves as a dust trap (Yaalon and Ganor 1973; Gile 1975a; Peterson 1977). Fine particles lodge between clasts and are later eluviated down-profile during rains, becoming part of the underlying vesicular layer, as described above. Thus, desert pavement and the underlying vesicular layer function together as a unit to trap dust,
Figure 10.7. Photograph of a fine, smectitic, thermic Natric Petroaragid near Laguna Chapala, Baja California. Note the vesicular horizon (07 cm) under a desert pavement, the natric horizon (770 cm), and calcic horizon (1570 cm). Laminar petrocalcic material caps the boulders in the subsoil. For color detail, please see color plate section.
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incorporate it into the soil, and build the surface upward. Surficial weathering, including fracturing by heat stress and salt crystallization (Cooke 1970), splits vulnerable clasts, and the fragments become part of the pavement (McFadden etal. 2005). Over tens of thousands of years these processes lead to a closely interlocked mosaic of clasts that nearly completely covers the surface. The effectiveness of the pavement as a dust trap decreases as the surface fragments become truncated and more interlocked. At this point, the surface neither retains much new eolian material, nor loses much material to erosion, unless disturbed. Eventually, however, landscapes with highly developed desert pavement will self-destruct, as shown for pavement landscapes older than 700,000 years in the Mojave Desert of California (Wells etal. 1985). The tightly interlocked pavement and the underlying soil enriched in pedog-enic clay and calcite combine to yield very low infiltration rates so th...