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.
Vertisols: Shrinking and Swelling Dark Clay Soils
Dark, clayey soils that shrink and swell upon drying and wetting are found on every continent except Antarctica (Dudal 1963, 1965), between about 50 N and 45 S latitudes. Vertisols occupy about 320 million ha (Blokhuis 2006) or about 2.4% of the land area. Extensive areas are located in India (80 million ha), Australia (70 million ha) and Sudan (50 million ha), and the United States, China, and Ethiopia (12 to 15 million ha each). Vertisols are also locally extensive in a number of other locations including Ghana, Egypt, Chad, Cuba, Puerto Rico, Taiwan, and Uruguay (Coulombe et al. 2000; Hagenzieker 1964; Isbell 1990; Troeh 1969). In the United States, Vertisols are most extensive in Texas (6.5 million ha), South Dakota (1.5 million ha), California (1 million ha), and Montana (0.6 million ha) but are reported to occur in 25 states and territories (Coulombe et al. 2000).
SettingA common feature in the Vertisol environments is a seasonal drying of the soil profile. Rainfall patterns associated with Vertisols are varied. Although a dry season is a necessary feature, the duration of the dry season is highly variable. The modal situation for the Vertisols involves an annual wet-dry, monsoon type (ustic) climate. The more arid Vertisol areas (Torrerts) remain dry for most of the year, with only amonth or two of wetness. On the other end of the Vertisol range, soils are commonly wet (Aquerts), with moisture deficiency present for only a few weeks, often at irregular intervals, during the year.
A peculiar pedogenic landform occurs on at least 50% of the terrain occupied by Vertisols (Thorp 1957). The entire landscape may be crumpled into a complex microtopography of microknolls and microbasins (Figure 19.1). This microtopography is most commonly called gilgai, but is also referred to as crabhole, Bay of Biscay, hushabye, or polygonal topography. The magnitude of the microrelief appears to be greatest in the udic and ustic soil moisture regimes, and more subdued or absent in the xeric and torric regimes. Gilgai topography may take on a variety of forms at the landscape scale and has been referred to as normal, lattice, wavy, tank, stony, and melon-hole (Hallsworth et al. 1955; Hallsworth and Beckmann 1969), or lattice, dendritic, or wavy (Hagenzieker 1963).
Buol_c19.indd 385Buol_c19.indd 385 7/1/2011 1:21:20 PM7/1/2011 1:21:20 PM
386 Soil Genesis and Classification
Diapir, also called mukkara in Australia, is another term used to describe intrusions of subsoil materials through the upper layers, can be identified by a contrast in color and/or texture (Figure 19.2), and often coincides with a microknoll.
Microclimates and small-scale hydrology differ between the microrelief features of knolls and basins in a Vertisol landscape, causing differences in vegetative species, biomass production, and redox environment. The gilgai landscape also often records a complex history of past and present climates, superimposed on the local microclimates, that affected the distribution of C
3 versus C
3 plants and the dissolution, movement, and
precipitation of carbonates (Kovda et al. 2003, 2006). The basins have higher humidity due to moisture release from the cracks and water ponding during wet periods, denser vegetation, and higher organic carbon contents and may be more saline than the microknolls. Mobilization of reduced Mn during the wet season may result in a significant accumulation of exchangeable Mn (Gehring et al. 1997) or of Mn-oxide nodules in the microbasins (Weitkamp et al. 1996). The knolls are drier, have higher temperatures and greater calcium carbonate contents, and are in an erosional position (Newman 1986; Wilding et al. 1990). The microrelief and distribution of soil properties are often repeated at a regular interval across the landscape. The horizontal distance from one microknoll to the next often ranges from about 3 to 10 m.
Vertisols form from a wide variety of parent material, but a common feature is a neutral to alkaline reaction. The most common parent materials include calcareous sedimentary rocks, mafic igneous rocks, volcanic ash, and alluvium from these
Figure 19.1. Gilgai landscape of microknolls and microbasins filled with water during the wet season in Texas. (From Eswaran et al. 1999)
Buol_c19.indd 386Buol_c19.indd 386 7/1/2011 1:21:21 PM7/1/2011 1:21:21 PM
19 / Vertisols: Shrinking and Swelling Dark Clay Soils 387
materials. Most Vertisols occur on nearly level to gently sloping landscapes. For example, Simonson (1954a) reported that Vertisols in India were mostly confined to landscapes with slopes from 1 to 8%. Although some Vertisols were present on steeper slopes, they were much less common on rolling landscapes, and were largely absent in hilly areas. In the Coast Ranges and foothills surrounding Californias Central Valley, Vertisols formed from marine sedimentary rocks and from coarse-grained mafic igneous rocks are mapped on slopes as great as 50% (Andrews 1972; Huntington 1971).
Grasses and forbs dominate the vegetative cover on most Vertisols. Some Vertisol landscapes have shrub or woodland vegetative communities, but most large woody species are not well adapted to the shrink/swell soil properties, possibly due to root shearing and compression during drying and wetting cycles (Ahmad 1983).
Pedogenic ProcessesAlthough there are several processes active in the formation of Vertisols, the major process seems to be shearing of wet, plastic soil materials, which may result in argillipedoturbation. To consider fully the development of the Vertisol profiles, one must first account for the high content of clay (>30% by definition) and the predominance of 2:1 expanding clay (Dixon and Nash 1968). It is not difficult to explain the presence of the necessary clay where the soils are developed from
Figure 19.2. A Hapludert in Texas. The dark-colored microbasin is on the left; the lighter-colored diapir on the right creates a microknoll. The scale on the left is in decimeters and feet. (From Eswaran et al. 1999) For color detail, please see color plate section.
Buol_c19.indd 387Buol_c19.indd 387 7/1/2011 1:21:21 PM7/1/2011 1:21:21 PM
388 Soil Genesis and Classification
argillaceous limestones, marine clays (Figure 19.3), shales, or clayey, smectitic alluvium. It would appear that those Vertisols developed on basalt, however, require a fairly extensive weathering period, especially in arid to semiarid regions, unless the solum is developed from dust, volcanic ash, or colluvium deposited over the basalt.
The weathering environment of the profile must be such that the 2:1 expandable clays are not completely weathered to 1:1 clays or interlayered to the extent their expanding properties are destroyed. These environmental conditions are generally met when leaching is limited by some combination of the following: an arid or semiarid climate, a horizon or layer of slow permeability at a shallow depth below the soil (for example, a lithic contact, petrocalcic horizon, or duripan), or simply the slow permeability of the smectitic soil material itself. Under these circumstances, soil solution silica and basic cation concentrations are maintained at levels high enough for smectite to be stable, and aluminum concentrations are low, so interlayering cannot occur. Once the required content of clay and dominant 2:1 expanding clay have been achieved, shrinkswell processes begin to operate. The slow permeability and cohesiveness of the clayey, smectitic soil material (self-preserving properties) contribute to the preservation of paleo-Vertisols in the sedimentary rock record. The fossil Vertisols provide some of the best means for reconstructing and modeling paleo-environments (Driese 2009). Vertisols with mixed, even kaolinitic, mineralogy
Figure 19.3. Soilscape pattern of Vertisols and associated soils in Runnels County, Texas. Soils identified are Stamford (fine, smectitic, thermic Chromic Haplusterts), Weymouth (fine-loamy, mixed, superactive, thermic Typic Haplustepts), Vernon (fine, mixed, active, thermic Typic Haplustepts), Olton (fine, mixed, superactive, thermic Aridic Paleustolls), Spur (fine-loamy, mixed, superactive thermic Fluventic Haplustolls). Badland identifies nonsoil land that is steep to very steep, barren land, dissected by many intermittent drainage channels. (After Wiedenfeld et al. 1970)
Weymouth clay loam
Weymouth clay loam Vernon-Badlandcomplex
Red marine claysAlluvium
Olton clay loam
Buol_c19.indd 388Buol_c19.indd 388 7/1/2011 1:21:22 PM7/1/2011 1:21:22 PM
19 / Vertisols: Shrinking and Swelling Dark Clay Soils 389
have been reported (Ahmad 1983; Coulombe et al. 2000), but expansible smectite, even as a subdominant component, must be largely responsible for the shrink/swell behavior.
Two major models have been proposed to account for the soil properties observed in Vertisols. The classic model, that represents processes by which the soil inverts itself (hence, Vertisol) is called the self-swallowing model, which operates in the following manner (Figure 19.4). During the dry season, the soil cracks to the surface, due to the shrinkage of the 2:1 expanding clays. The cracks often extend to a depth of 1 m or more, but cracking depth is variable and seems to be related to the depth of wetting of the profile during the wet season and the severity of drying subsequently. While the cracks are open, surface soil material falls into them. The surface material can be dislodged by several mechanisms such as animal activity, wind, or at the onset of the rainy season by water. The clays hydrate and expand on rewetting. As expansion takes place, the cracks close, but because of the extra material now present in the lower parts of the profile, a greater volume is required, and the expanding material presses and slides the aggregates against each other, developing a lentil, an angular blocky structure (wedge-shaped aggregate) with slickenside features on the ped faces (Krishna and Perumal 1948). This expansion buckles the landscape, forming the gilgai microrelief. The higher organic carbon content of the microbasins may be due in part to admixtures of subsurface material into the microknoll area and slight erosion of organic-rich fines from the knolls to the basins (Templin et al. 1956). The apparently homogeneous properties, particularly soil color and clay content, of the upper parts of many Vertisols lend support to the self-swallowing model and often lead to the conclusion that the dark color is associated with organic matter derived from
Dry season :Soils crack
Surface soilfalls into the cracks
Wet seasonSoil expands
pushing up soil surface
Figure 19.4. Sketch illustrating the self-swallowing model of Vertisols.
Buol_c19.indd 389Buol_c19.indd 389 7/1/2011 1:21:22 PM7/1/2011 1:21:22 PM
390 Soil Genesis and Classification
incorporation of surface soil materials. Whereas the sloughing of surface materials into cracks no doubt occurs to some degree in all Vertisols, this model does not fully account for many properties of a large number of Vertisols, namely decreasing organic carbon content and increasing mean residence time of organic matter with depth and the clear differentiation of subsurface horizons with accumulations of soluble salts, gypsum, carbonates, and in some cases, even clay (Dasog et al. 1987; Wilding et al. 1990; Southard and Graham 1992).
A second model, the soil mechanics model (Wilding and Tessier 1988; Nordt et al. 2004), or shear failure model (Coulombe et al. 2000) has been proposed to explain the profile distributions of these properties (Figure 19.5). This model is based on the failure along shear planes (slickensides) of plastic soil materials when swelling pressures generated by hydration of clays exceed the shear strength of the soil material. Stress is relieved by an upward movement that is somewhat constrained by the weight of the overlying soil material, resulting in a failure shear plane that is usually inclined at 1060 above the horizontal. This model does not require that surface material fall into cracks. Instead, subsurface material is transported upward along the slickensides to produce the microknolls of the gilgai relief, thereby exposing it to weathering and leaching processes. The slickensides, in turn, intercept percolating water and focus flow to the microbasins, where accumulation of salts, gypsum, carbonates, and Mn-oxides occurs. The presence of
Figure 19.5. Schematic illustration of the soil mechanics of Vertisol development. The Bkss horizons are sometimes referred to as vertic horizons rather than cambic horizons. (After Coulombe et al. 1996)
Buol_c19.indd 390Buol_c19.indd 390 7/1/2011 1:21:22 PM7/1/2011 1:21:22 PM
19 / Vertisols: Shrinking and Swelling Dark Clay Soils 391
Mn-oxides, produced by cycles of mild reduction, followed by oxidization and precipitation of the Mn, may be largely responsible for the dark soil colors typical of many Vertisols. Once microrelief is established, soil processes are driven largely by small-scale variations in hydrology and microclimate, and less so by pedoturbation.
It is difficult to assign all Vertisols to a similar place in a genetic scheme of soil classification. In many cases, Vertisols form from fine-textured, smectitic alluvium (Andrews 1972) and develop the characteristic cracks, slickensides, and wedge-shaped aggregate soil structure very quickly. Graham and Southard (1983) advanced another possible mode of genesis of Vertisols occurring in association with Mollisols in Utah. They concluded that some of the Vertisols there were formed when erosion removed the A horizons of Mollisols, exposing their cracking, clayey argillic horizons at the surface, leading to development of characteristics of Vertisols. Erosion of the former Mollisols is postulated as being a result of the loss of the Gambel oak trees (Quercus gambelii) now on the present Mollisol sites (which strongly retain surface soils against erosion) and the invasion of the present Vertisol areas by wyethia (Wyethia amplexicaulis). The wyethia has a long single taproot, which allows it to survive by penetrating the Vertisol clays, but the plant lacks surficial roots to hold topsoil in place. In some cases, Vertisols may be the end product of a developmental sequence involving soils whose B horizons became so clayey (for example, an argillic horizon) that shrinkswell cycles developed and eventually swallowed the Ahorizon. The high content of fine clay (
392 Soil Genesis and Classi...