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Vertisols

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Environmental Conditions
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Processes
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Pedogenic Models for the Formation of Vertisols
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Properties
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Classification
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Distinguishing Characteristics

Vertisols
Summary:
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Vegetation: grassland, deep rooting tree species
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Climate: seasonal variations in precipitation and temperature; any soil temperature regime except pergelic
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Soil moisture regime: erratic soil moisture regime
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Major soil property: high clay content (predominance of 2:1 type expanding clay -> montmorillonite, smectite), high CEC, low permeability, slickensides, gilgai micro-relief, dark color of low chroma, medium to low organic matter content (0.5 - 3 %)
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Diagnostic horizons: cambic (argillic, natric)
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Epipedon: mollic
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Major processes: shrinking and swelling, pedoturbation
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Characteristics: stage of weathering relatively unadvanced or minimal, lack in horizon differentiation
 
 
Vertisols - Environmental Conditions
Climate: Vertisols occur in almost every major climatic zone. Australia with over 80 million ha has the largest occurence of Vertisols, where they are formed mainly in areas with aridic moisture regime, with less in ustic and xeric zones. Generally, the seasonal variations in precipitation and temperature, which favor the formation of smectitic clays as well as provide many of the physical attributes of these soils, would be considered as prerequisites for the formation of Vertisols. The variation in climatic conditions result in weathering of primary and secondary minerals during wet season, but encourage the accumulation of basic cations in the dry season. Areas where Vertisols are found are characterized by a period when the potential evapotranspiration exceeds precipitation (fry period). During periods with sufficient moisture deficit cracking occurs, although the intensity in cold temperature regions, such as Canada, is much lower than in the warmer regions. Generally, higher rainfall results in higher intensity of cracking, increased organic matter contents, and increased leaching of carbonates and salts.
 
Vegetation: Since it is known that Vertisols occur in a wide variety of climatic types, the natural vegetation associated with this soil order is equally variable. The natural vegetation types are, to a certain extent, limited by the soil properties such as the high clay content, shring-swell characteristics, and soil structure. Both climate and soil properties limit the vegetation types to grasses and slow-growing, deep-rooting tree species (e.g. Acacia). The main features of the natural vegetation in these soils are tolerance to drought, as well as development of deep roots to overcome root damage by a consequence of the annual cracking. Most Vertisols have has grassland or savanna vegetation as the native vegetation, but some had formed under forest. Present use of Vertisols comprise wheat, rice, cotton, and sorghum, pastureland in the the south of the United States. Vertisols in India are used for grain legume, oil seed crops, and cotton cultivated in a ridge and furrow system. In Australia most Vertisols are used for grazing by sheep and cattle or dry land agriculture. Large areas of Vertisols in Africa are largely un-utilized except for extensive grazing.
 
Relief: Two different scales should be examined (i) macro, and (ii) micro scale.
(i) Macro relief: Vertisols generally occur in areas with slopes not exceeding 5 % gradient, since at higher gradients soil erosion would occur. Often the level areas lack an integrated drainage network, hence infiltration is slow in these soils, which results in surface ponding.
(ii) Micro relief: Gilgai-relief. The development of gilgai-relief is due to shrinking and swelling of Vertisols and soil movement, i.e., the soil mass cannot re-occupy the original volume since surficial material has fallen into the cracks during dry season. As such, part of the soil mass is forced upwards forming the mounds (or knolls). The formation of a mound provides a locally preferred site for the further release of pressure, thereby perpetuating the formation of other mounds and depressions in an area.
 
Parent Material: Vertisols develop on a wide range of parent material including alluvial, colluvial and lacustrine deposits, marl and other calcareaous rocks, limestone, shales, igneous, metamorphic and volcanic rocks of basic nature. The materials that form Vertisols can be either allochtonous or autochthonous in origin. In most cases the materials are recently deposited and soil formation is still in the early stages. Vertisols may develop in situ from the parent materials. The smectites (clay minerals) in these soils could be derived from the original rock or form as a result of neogenesis or transformations from primary minerals. A high pH and high potentials of Si as well as Mg smectites develop, a process which is also favored by poor drainage conditions. Calcareous parent material or unconsolidated sediments which are dominantely basic in character and low in quartz favor the formation of Vertisols. Vertisols occupy approximately 1 percent of the land mass of the U.S. and are dominant mainly in Texas, Alabama and Mississippi. Those soils have developed on gentle slopes either in calcareous clay or in residuum weathered from soft, calcareous sedimentary rocks. Vertisols are known to develop on basalts in Australia, gneisses and sandstones in India, or glacio-lacustrine in Saskatchewan. The parent material although variable in origin, are rich in feldspars and ferro-magnesian minerals and yield clay residues on weathering. Where parent materials are not basic, alkaline earth elements can be added by seepage or by flood water.
 
Time: Most Vertisols develop on young landscapes but they may occur on old geomorphic surfaces. It is believed that the stage of weathering in Vertisols is relatively unadvanced or minimal.
 
 
Vertisols - Processes
Vertisols form under multiple genetic pathways which are complex. In general, soil forming processes that lead to the formation of Vertisols are those which control the formation and stability of smectites in the soil. However, subsidiary processes, such as fluctuations in the moisture status, accumulation of organic matter, carbonates, gypsum or soluble salts and acidification processes through leaching, result in the differences within the Vertisols.
 
The development of Vertisols requires conditions that ensure the formation and preservation of smectites. These clay minerals may form either in situ through the weathering and development of a solum (autochtonous Vertisols) or from a sediment which is composed of materials that can produce vertic properties (allochtonous Vertisols). The latter is geographically more extensive and occupies the lower parts of the landscape. The development of smectitic clays is favored by a high pH with sufficient Ca 2+ and Mg2+ in the soil system. The presence of a relatively impermeable layer at some depth within the soil prevents the leaching of the various components needed to form smectites.
 
Shrinking and swelling cause shearing and consequently the formation of slickensides. This process is attributed to smectitic clays and alternations in dry and wet seasons. As a result of this process, Vertisols develop deep and wide cracks in a polygonal pattern.
 
Pedoturbation (churning) is a process which homogenizes the soil profile due to the infilling of the cracks by surficial material during dry season. The process in Vertisols is also called self-mulching or self-swallowing.
During the drying cycles, cracks develop, whereas on moistening, shear stresses form which result in the formation of slickensides and/or smoothened surface of sphenoids. Both features require the material to be in a plastic state. The lateral pressures developed in these soils are much greater than the vertical swelling pressures. Within the soil, the vertical component of the swelling pressures includes the weight of the overlying material. The moisture conditions above and below a point within the soil determines the net pressure and angle of shear. As such, the near surface horizon develop cracks and may have only a few slickensides since both the horizontal and vertical pressures are small (the net pressure being much lower than the sheat strength of the material). In deeper horizons, typically from 50 to about 125 cm below the surface, slickensides development is maximum. In these deeper layers, the net pressure is much greater than the shear strength of the material and soil movement occurs with swelling. Sphenoids develop as a result of the existence of much lower vertical and horizontal pressures in comparison to that needed for the development of slickensides. In the typical case, sphenoids would be found in between the surface horizon with cracks and deeper horizons with slickensides. Their development has been related to lower clay contents, as well as smaller proportions of smectitic clays in the colloidal fractions.
 
Clay translocation is not phenomenal in Vertisols, nevertheless, the presence of smectitic clays has all the conditions necessary for dispersion, translocation, and accumulation in subsurface horizons in Vertisols. In some Vertisols there is some evidence of illuviated clays in the lower soil profile, which is subjected to the least amount of pedoturbation. This process tends to obliterate all evidence of the illuviation process and it is unlikely that well-defined clay skins will be preserved, instead any translocated clay is probably engulfed in the matrix and/or slickensides as a result of shrink-swell processes.
 
 
Pedogenic Models for the Formation of Vertisols
(I) Pedoturbation Model (Self-Swallowing Model)
Prerequisite for the formation of Vertisols is the presence of expanding clays (smectites). After clay formation shrink-swell processes begin to operate. During the dry season the soil cracks. While the cracks are open, surface soil material falls into them due to wind, animal acitivity, or water erosion. On rewetting the clays hydrate and expand. As expansion takes place, the cracks close, but because of the 'additional' 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 slickensides.
 
(II) Soil Mechanistic Model
This model is based on the failure along shear planes (slickensides) of plastic soil material 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 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 falls into cracks. Instead, surface material is transported upward along the slickensides to produce the microknolls of the gilgai-relief. Once microrelief is established, soil processes are driven largely by small-scale variations in hydrology and microclimate, and less so by pedoturbation.
 
 
Vertisols - Properties
Five zones or horizons with distinct structural attributes may be recognized in Vertisols, although not all may be present in a particular profile and may not occur in a strictly ordered sequence.
 
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Zone 1: This zone is from the surface to approximately 25 cm or the plow layer if present. It is characterized by large prisms, up to 30 cm in width, that result from cracking. The material is hard or very hard when dry and the prismatic elements may part to coarse angular blocky elements.
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Zone 2: This zone is typically 10 to 30 cm thick and characterized by coarse angular blocky elements that may occur aggregated into discernable prisms. If overlain by a plow zone, it may represent a root restricting layer for agricultural crops.
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Zone 3: This zone may vary in thickness from 10 to over 100cm. Soil Taxonomy refers to the structural elements in this zone as 'wedge-shaped natural structural aggregates that have their long axis tilted 10 to 60 degrees from the horizontal'. These structural aggregates have an orthorhombic form, are generally 5 to 10 cm long along their long axis; and smooth or striated ped faces. Their mode of formation is related to the slickensides, which are characteristic of zone 4.
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Zone 4:This is the zone of slickenside formation and ranges from 25 to 100 cm in thickness. The term slickenside refers to a surface that has a polished and shiny appearence that also may be grooved or striated. The term does not refer to a soil structural element, which is a 3-D entity. In this zone the slickensides occupy areas ranging from 600 to 2000 cm 2. Their surface topography is not flat, but curved or slightly undulating. The net result of the inclined arrangement is to produce a set of intersecting slickensides arranged in a synclinal form. The deepest part of the syncline is between 50 and 125 cm below the surface, while the shallower arms may reach within 25 cm of the surface. The amplitude of the two arms represents the amplitude of the gilgai and may vary from about 3 m to more than 25 m. The thickness and expression of zones 2 &3 are a function of the depth at which the arms of the slickensides approach the surface.
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Zone 5: This zone underlies zone 4 or is directly below zone 3. It is subject to only slight moisture variation and is massive and may show accumulations of gypsum, carbonates and other translocated soluble salts.
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Variations from the model profile is the rule rather than the exception. One or more of zones 2, 3 or 4 may be absent, but 'conceptually zone 3, 4 or both must be present for recognition as a Vertisol. The expression of zones 2, 3 and 4 will show considerable variation as a function of soil moisture content and variation in intrinsic soil attributes (variation in clay type and content), however their relative positions are usually sequential.
 
Generally, the clay content is very high in Vertisols and the dominant clay minerals are 2:1 type minerals (smectites, montmorillonites). These clay minerals have the outstanding feature to expand (swell) when wet and shrink when dry. Therefore, pronounced changes in volumes with changes in soil moisture result in deep cracks in the dry season and very plastic and sticky soil consistency when wet. Due to the high clay content of expanding character the cation exchange capacity of the whole soil is high. A high clay content is also associated with slow permeability but the water adsorption is high.
 
Slickensides are a pronounced characteristic of Vertisols. They are defined as polished and grooved surfaces produced by one soil mass sliding past another. The formation of slickenside features is related to swelling pressures which exceed the shear strength of the soil under overburden-pressure confinement. The shear strength of a soil is a function of cohesion plus the angle of internal friction, which is low in clay soils. The cohesion is a function of bulk density, clay content, clay mineralogy, and is inversely related to moisture content. Generally, lateral swelling pressures in soils are much larger than vertical swelling pressures, as the latter is substantially reduced by the overburden pressure. Maximum slickensides are between 50 and 125 cm depth, however, fewer slickensides are found at depths between 25 and 50 cm. At such depths both vertical and horizontal pressures are small. As the moisture changes become limited at 125 cm depth, slickensides become scarce below this depth. Shearing occurs at an angle of 30 to 50 degrees from the horizontal and it is dependent on moisture and the swelling pressures, which vary vertically, horizontally, and temporally. Slickensides can only form when the material is plastic. The slickensides (stress cutans) differ markedly from clay skins (argillans) which occur on the ped surfaces resulting from clay translocation. The latter have sharp outer and inner boundaries with distinct extinction patterns and are often finely layered (laminar fabric). The relatively small slickensides developed by pedogenesis must not be confuse with large slickensides of the substratum which in alluvial and lacustrine sediments is a feature of the parent material.
 
The organic matter content is generally low (0.5 - 3 %) in spite of the usual dark soil color. Complexation or chelation of organic colloids to clay minerals of the smectite group probably darkens the mineral. Some of the dark color may also be related to presence of manganese oxides. The dark black color may be also due to the parent material (e.g. Vertisols derived from basalt). The Chrom great groups of Vertisols are brownish in color and typically have small amounts of montmorillonites. These great groups typically have large amounts of iron oxyhydroxides and are well-drained.
 
Kankars (carbonate glaebules or nodules) are basically lime concretions that are found in Vertisols. Many Vertisols are formed in calcareous parent material and have kankars throughout the profile. In deeper horizons, it is also common to find calcic horizons. Drying, in the presence of Fe and Mn, results in the formation of hard concretions.
The structure of Vertisols is almost a temporal characteristic. The size, shape, grade, and consistence of the structural elements are all related to the moisture conditions at the time the soil is inspected. The depth at which the different structural elements are expressed may also be a function of moisture conditions in different parts of the profile. Ideally, structural assessments should be made under different moisture conditions. Often Vertisols show an angular blocky structure (wedge-shaped aggegates).
 
Typical microrelief features are knolls (mounds) and basins (depressions) in a Vertisol landscape. The basins are wetter than the knolls due to moisture release from the cracks and water ponding during wet periods. They exhibit higher organic matter contents and are often more saline than the microknolls. The knolls are drier, have a higher calcium carbonate content and are in the erosional positions. Minibasins and microknolls show a repetitive but irregular pattern within a Vertisol landscape with distances of about 3 to 10 m between the knolls. The topography related to Vertisols is called ' Gilgai micro-topography '. Various forms of gilgai have been reported: round, mushroom, tank, wavy, lattice, stony, and depressional. The form is related to landscape shape, clay content and type, and soil moisture regime. A fine, angular blocky structure, described by some as 'nutty' may develop in surfaces that have a very high montmorillonitic clay content. In the dry season they show a very hard consistence and appear as loose gravel strewn on the surface. In previous classification systems these soils were called Grumosols.
 
Most Vertisols have a mollic epipedon and a cambic diagnostic horizon, but some have other diagnostic subsurface horizons, including argillic or natric. 
 
 
Vertisols - Classification
The requirements to qualify for a Vertisol are the following:
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Clay content of at least 30 % to a depth of at least 50 cm, or a lithic or paralithic contact, duripan, or a petrocalcic horizon if shallower
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Cracks that open and close periodically
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Evidence of soil movement (e.g. slickensides, wedge-shaped aggregates)
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Any soil temperature regime, except pergelic (i.e., Gelisols)
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Soil moisture regime must be erratic to allow for cracking in dry season and swelling in wet season
 
Gilgai surface topography is not considered as a requirement to meet a Vertisol. Cultivation practice may erase gilgai microtopography.
 
Six suborders are recognized in the Vertisol order. They are differentiated by aquic conditions, soil moisture regime, and on the cracking characteristics of the soil. 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.
 
Aquerts: Vertisols which are subdued aquic conditions for some time in most years and show redoximorphic features are grouped as Aquerts. Because of the high clay content the permeability is slowed down and aquic conditions are likely to occur. In general, when precipitation exceeds evapotranspiration ponding may occur. Under wet soil moisture conditions iron and manganese is mobilized and reduced. The manganese may be partly responsible for the dark color of the soil profile.
 
Cryerts: They have a cryic soil temperature regime. Cryerts are most extensive in the grassland and forest-grassland transitions zones of the Canadian Prairies and at similar latitudes in the Soviet Union.
 
Xererts: They have a thermic, mesic, or frigid soil temperature regime. They show cracks that are open at least 60 consecutive days during the summer, but are closed at least 60 consecutive days during winter. Xererts are most extensive in the western United States, primarily in California.
 
Torrerts: They 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 U.S., and occur mostly in west Texas, New Mexico, Arizona, and South Dakota, but are the most extensive suborder of Vertisols in Australia.
 
Usterts: They have cracks that are open for at least 90 cumulative days per year. Globally, this suborder is the most extensive of the Vertisols order, encompassing the Vertisols of the tropics and monsoonal climates in Australia, India, and Africa. In the U.S. the Usterts are common in Texas, Montana, Hawaii, and California.
 
Uderts: They 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 (e.g. salic, calcic, natric, gypsic horizons), the presence of a duripan (e.g. Duraquerts, Durixererts), organic carbon content (e.g. Humicryerts), or reaction (electrical conductivity is less than 4 dS/m and pH in 1:1 water of 5 or less in 25 cm or more within top 50 cm - e.g. Dystrusters, Dystraquerts).
 
Several soil moisture regimes are considered at sugroup level ranging from dry to wet conditons: Xeric (e.g. Xeric Epiaquerts), aridic (e.g. Aridic Epiaquerts), udic (e.g Udic Haplusterts), ustic (e.g. Ustic Dystraquerts), and aquic (e.g. Aquic Dystrusterts, Aquic Salitorrerts).
 
Soil color is used to differentiate the 'aeric' subgroup of Vertisols. Soils that have in one or more horizons between either an Ap horizon or a depth of 25 cm from the mineral soil surface, whichever is deeper, 50 percent or more colors as follows: (i) a hue of 2.5 of redder and either (ii) a color value, moist, of 6 or more and a chroma of 3 or more; or (iii) a color value, moist, of 5 or less and a chroma of 2 or more; or (iv) a hue of 5Y and a chroma of 3 or more; or (v) a chroma of 2 or more, and no redox concentrations (e.g. Aeric Endoaquerts). Soil color is used also to differentiate the 'chromic' subgroup of Vertisols. The chromic characteristic encompass soils that have, in one or more horizons within 30 cm of the mineral soil surface, 50 percent or more colors as follows: (i) a color value, moist, of 4 or more; or (ii) a color value, dry, of 6 or more; or (iii) a chroma of 3 or more (e.g. Chromic Epiaquerts).
 
Shallow Vertisols are classified using the designation 'leptic' (soil with a densic, lithic, or paralithic contact within 100 cm of the mineral soil surface) or 'lithic' (e.g. Leptic Salaquerts or Lithic Haploxererts).
 
Vertisols which are low in clay content are differentiated as 'entic'. To meet the 'entic' designation the Vertisol must have a layer 25 cm or more thick that contains less than 27 percent clay in its fine-earth fraction and has its upper boundary within 100 cm of the mineral soil surface (e.g. Entic Salaquerts, Entic Haplotorrerts).
 
Soils are defined by the designation 'halic' if their salt content is high. They must meet the following criterion : throughout a layer 15 cm or thicker the electrical conductivity must be at least 15 dS/m or more (1:1 soil:water) for 6 or more months per year in 6 or more out of 10 years (e.g. Halic Durixererts). Vertisols with a high sodium content are classified as 'sodic' (e.g. Sodic Durixererts). They must have an exchangeable sodium percentage of 15 or more (or a sodium adsorption ratio of 13 or more) for 6 or more months per year in 6 or more out of 10 years.
 
 
Vertisols - Distinguishing Characteristics
Soils at higher elevations and on steeper slopes formed in the same parent material as Vertisols are classified as Inceptisols and Alfisols and they may have vertic properties. In a catenary association Alfisols may occur on the top of the slopes - Entisols, Inceptisols, and Alfisols with vertic properties on the erosional hillslope positions - and Vertisols on the lower slopes and in the depressions. The main associated soils formed in calcareous parent material are Ustolls, Aqualfs in the less calcareous clays, and soils in vertic subgroups of Ustolls and Aquolls on nearly level slopes. With advancement of leaching and the formation of an argillic horizon, the soil would evolve into Alfisols (e.g. Vertic Hapludalfs). Leaching also promotes the destruction of smectites, i.e., the vertic properties of the soils are destroyed and Alfisols are formed. A number of Inceptisols, Entisols, Alfisols, Mollisols, Ultisols, and Aridisols intergrade to Vertisols at the subgroup level. These soils have vertic characteristics such as cracking, slickensides or wedge-shaped aggregates, but not enough to be Vertisols.
 
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