soil genesis and classification volume 143 (buol/soil genesis and classification) || vertisols:...

385

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 a month 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).

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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)


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 California’s 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.



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, shrink–swell 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

Vernon-Badlandcomplex


Vernon-Badland complex


Spur loamStamford

clay

Stamfordclay

Red marine claysAlluvium

Calcareousred beds

Olton clay loam



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

Ground level

1ft.

2ft.

3ft.

Dry season :Soils crack

Basalt Rock

Surface soilfalls into the cracks

Wet seasonSoil expands

pushing up soil surface

Angular Structure

StructureGranular

Figure 19.4. Sketch illustrating the “self-swallowing model” of Vertisols.



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 10–60° 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)



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 shrink–swell cycles developed and eventually “swallowed” the A horizon. The high content of fine clay (<0.2 mm) in some of these soils (Kunze and Templin 1956), and a high fine clay/coarse clay ratio, may have been the product of lessivage on a large scale.

The fate of a Vertisol may be to undergo alteration of the 2:1 clays to nonexpanding types of clay. The profile would then cease to churn as intensively, and eluviation/illuviation processes would dominate. This interpretation would suggest that Vertisols represent intermediate stages of soil development on the genetic pathway to another soil order. An alternative interpretation is that many Vertisols are stable and persistent, primarily due to dominantly low slope gradients and the slow permeability and cohesive nature of the smectitic clays. In this sense, the Vertisols may be considered to be in a steady-state, almost terminal, condition, barring a significant climatic change or major environmental disturbance. As is often the case, soils with similar properties can be produced by a number of pathways, complicating the conceptual relationships between theories of soil genesis and classification based on soil properties.

Uses of VertisolsIn general, the high content of expanding lattice clay is of primary concern in the management of these soils. Agronomic uses of Vertisols vary widely, depending on the climate and the suitability of associated soils for crop production (Probert et al. 1987). The high clay content and associated slow permeability of these soils when wet makes them desirable for cropping systems that require retention of surface water,



as in the case of paddy rice cultivation. Dryland grain production is common on Vertisols in semiarid climates. Research has shown that Vertisols are fairly resilient under a variety of crop production systems but that the soils are susceptible to loss of organic carbon and to erosion during long-term cultivation and if not managed properly (Chevalier et al. 2000; Nordt and Wilding 2009). Few, if any, commercial forests or orchards are found on Vertisols due to water management difficulties and the potential for shrink-swell–induced root shearing.

Worldwide, the largest acreages of Vertisols are used for pasture. During the dry season, cracks may be wide enough to present hazardous footing for animals. Runoff from initial rains after a dry period is almost entirely infiltrated through the wide cracks. After the soil has become saturated and the cracks closed, runoff may approach 100%, making the soils susceptible to erosion, particularly if over-grazed and compacted by animal traffic (Blokhuis 2006).

Many engineering problems are associated with these soils. Structural failures are common due to differential movement of the soil surface on wetting and drying. Highways, buildings, fences, pipelines, and utility lines are moved about and distorted by the shrinking and swelling of the soil. Septic systems often fail due to slow permeability and disruption of distribution lines by shrinking and swelling. Percolation tests can be misleading and indicate too high a percolation rate if conducted during the dry season when cracks are open. It is necessary to keep these soils wet for several days to saturate and expand the clays fully to determine valid percolation values.

Classification of VertisolsThe common features of Vertisols include evidence of soil movement in the form of intersecting slickensides or wedge-shaped aggregates in a layer 25-cm or more thick within 100 cm of the mineral soil surface; clay content of at least 30% to a depth of at least 50 cm, or to a densic, paralithic, or lithic contact, duripan, or petrocalcic horizon if shallower; and cracks that open and close periodically. Although soils with slickensides and wedge-shaped aggregates often have a gilgai microtopography, this is not always the case. Further, gilgai can be partially or totally erased by cultivation, and it is sometimes possible for other microrelief features to be confused with gilgai. Therefore, gilgai surface topography is not used as a differentiating characteristic for Vertisols.

These soils have been called Grumosols (Oakes and Thorp 1950; Templin et al. 1956), Tropical Black clays or Regur (Kossovich 1912), and Tirs (del Villar 1944), as well as several other names (Simonson 1954a). It is also probable that many of these soils have been referred to as Rendzinas. The classification of Vertisols was revised significantly in 1992 to include shrink–swell soils with aquic conditions and with cryic soil temperature regimes, and to place more emphasis on diagnostic horizons and less emphasis on soil color at high categorical levels.

Because emphasis is placed on the shrink–swell characteristics of these soils at the order level, a wide array of diagnostic horizons can be included in Vertisols. Most



Vertisols have mollic epipedons (Torrerts and acidic [Dystr] great groups are common exceptions) and cambic horizons, but some have other diagnostic subsurface horizons, including argillic or natric horizons. Vertisols can have any soil temperature regime, but do not have permafrost (i.e., Gelisols).

Six suborders are recognized in the Vertisol order (Figure 19.6). They are determined on the basis of aquic conditions, cryic soil temperature regime, and on the length of time the cracks are open to the surface. Note that although the formative elements for soil moisture regimes are used in naming Xererts, Torrerts, Usterts, and Uderts, the names do not necessarily mean that the soils have those soil moisture regimes. Soil moisture regimes are poorly defined in Vertisols due to difficulties in identifying the soil moisture control section.

Aquerts are identified by aquic conditions for some time in most years and the presence of redoximorphic features. These soils may, at first, seem to violate the idea of a seasonal wet-dry cycle, until one considers that the aquic conditions need persist only long enough for some reduction and mobilization of iron and manganese to occur. Manganese oxides may be partly responsible for the dark color typical of many Vertisols. The slow permeability of clayey, smectitic soil horizons may give rise to aquic conditions in Vertisols with almost any annual precipitation pattern, although these conditions occur most commonly where soils pond water on the surface during the winter. These soils are used extensively for rice production in the Sacramento Valley of California.

Cryerts have a cryic soil temperature regime. These Vertisols are most extensive in the grassland and forest-grassland transition zones of the Canadian Prairies (Mermut et al. 1990; Mills et al. 1990), and possibly at similar latitudes in Russia.

Xererts have a thermic, mesic, or frigid soil temperature regime and, unless irrigated, have cracks that are open at least 60 consecutive days during the summer, but are closed at least 60 consecutive days during the winter. Xererts are most extensive in the western United States, primarily in California, but also occur to a limited extent in southeastern Australia and the Mediterranean region.

Torrerts, unless irrigated, have cracks that are closed for less than 60 consecutive days when the soil temperature at 50 cm is above 8°C. These soils are not extensive in the United States, and occur mostly in west Texas, New Mexico, Arizona, and

Cold

DryWet

Aqu

erts

Cryerts

Ude

e e

e

rr r

r

t

t

t ts

s

s s

U X

Torrerts

Figure 19.6. Diagram showing some relationships among suborders of Vertisols.



Table 19.1. Suborders and great groups in the Vertisols order

Suborders Great Groups

Aquerts Sulfaquerts have a sulfuric horizon or sulfidic materials within 100 cm of the surface.Salaquerts have a salic horizon within 100 cm of the surface.Duraquerts have a duripan within 100 cm of the surface.Natraquerts have a natric horizon within 100 cm of the surface.Calciaquerts have a calcic horizon within 100 cm of the surface.Dystraquerts have electrical conductivity less than 4.0 dS m−1 and pH in 0.01 M CaCl

2

of 4.5 or less in horizons 25 cm or more thick within 50 cm of the surface.Epiaquerts have episaturation.Endoaquerts—other Aquerts.

Cryerts Humicryerts have 10 kg m−2 or more organic carbon between mineral surface and 50 cm depth.

Haplocryerts—other Cryerts.Xererts Durixererts have a duripan within 100 cm of the surface.

Calcixererts have a calcic or petrocalcic horizon within 100 cm of the surface.Haploxererts—other Xererts.

Torrerts Salitorrerts have a salic horizon within 100 cm of the surface.Gypsitorrerts have a gypsic horizon within 100 cm of the surface.Calcitorrerts have a calcic or petrocalcic horizon within 100 cm of the surface.Haplotorrerts—other Torrerts.

Usterts Dystrusterts have electrical conductivity less than 4.0 dS m−1 and pH in 0.01 M CaCl2

of 4.5 or less in horizons 25 cm or more thick within 50 cm of the surface.Salusterts have a salic horizon within 100 cm of the surface.Gypsiusterts have a gypsic horizon within 100 cm of the surface.Calciusterts have a calcic or petrocalcic horizon within 100 cm of the surface.Haplusterts—other Usterts.

Uderts

Dystruderts have electrical conductivity less than 4.0 dS m−1 and pH in 0.01 M CaCl2

of 4.5 or less in horizons 25 cm or more thick within 50 cm of the surface. Hapluderts—other Uderts.

South Dakota, but they are the most extensive suborder of Vertisols in Australia (Isbell 1990).

Usterts, unless irrigated, have cracks that are open for at least 90 cumulative days per year. Globally, Usterts are the most extensive suborder, encompassing the Vertisols of the tropics and monsoonal climates in Australia, India, and, Africa. In the United States, Usterts are widely distributed from Texas (Black Prairie) to Montana, and also occur in “iso-” soil temperature regimes in Puerto Rico, Hawaii, and California.

Uderts have cracks that are open less than 90 cumulative days per year and less than 60 consecutive days during the summer. These are the Vertisols of the Gulf Coastal Plain and the Black Belt in Mississippi and Alabama.

Great groups are differentiated by subsurface diagnostic horizons, sulfidic mate-rials, reaction, organic carbon content, and nature of aquic conditions, depending on the particular suborder (Table 19.1).

A number of Inceptisols, Mollisols, Alfisols, Ultisols, Aridisols, Entisols, and Gelisols intergrade to Vertisols at the subgroup level (Vertic subgroups). These soils



have significant cracking and slickensides or wedge-shaped aggregates, but not enough to be Vertisols, or have a potential linear extensibility (COLE × thickness of layer involved) of at least 6 cm in the upper 100 cm of the profile (DeMent and Bartelli 1969). At the family level, only fine (30–60% clay, for Vertisols only) and very-fine particle size classes are identified (contrasting particle size classes are also recognized). In the U.S., the vast majority of Vertisols are in the smectitic mineralogy class; a few have mixed mineralogy; three have magnesic mineralogy, and one (in Hawaii) has halloysitic mineralogy.

PerspectiveVertisols are found on every continent except Antarctica. Main areas of occurrence are in Australia, India, Sudan, Ethiopia, China, and the United States. The most extensive areas of occurrence in the United States are in Texas, South Dakota, and California. The most common environmental feature in the areas of occurrence is the seasonal drying of the soil profile. Vertisols can have any soil temperature regime but do not have permafrost. The main pedogenic process is movement and shearing of plastic soil material by the shrinking and swelling of the predominant 2:1 expanding lattice soil clays during the seasonal changes in soil moisture. In some cases, this movement causes significant mixing of soil horizons by argillipedoturbation.

Six suborders are identified based on soil temperature regime (Cryerts), aquic conditions (Aquerts), and on the duration and timing of the opening and closing of cracks (Xererts, Torrerts, Usterts, and Uderts). Great groups are identified primarily on the basis of diagnostic subsurface horizons.

Pasture is the most extensive agricultural land use. Crops grown on these soils vary widely, depending on the climate and technology for water management. Engineering problems are common due to the shrinking and swelling.


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