Vertisol is a soil order in the USDA soil taxonomy and a Reference Soil Group in the World Reference Base for Soil Resources (WRB). It is also defined in many other soil classification systems, including the Australian Soil Classification, where it is called vertosol.
Characteristics
Vertisols are characterized by a high content of expansive clay minerals, many of which belong to the montmorillonite group. These clays undergo pronounced volume changes with changes in moisture content: they swell when wet and shrink when dry, forming deep cracks (at least 1 cm wide) that extend downward from the land surface. This process, known as argillipedoturbation or self-ploughing, causes the soil material to mix itself consistently, often resulting in an extremely deep A horizon with no B horizon (an A/C soil). The heaving of underlying material to the surface frequently creates a microrelief known as gilgai.
Vertisols have a clay-size-particle content of 30 percent or more by mass in all horizons of the upper half-metre of the soil profile. They typically exhibit slickensides (polished and grooved ped surfaces) or wedge-shaped structural aggregates within 100 cm of the mineral surface.
Formation and Occurrence
Vertisols typically form from highly basic rocks, such as basalt, in climates that are seasonally humid or subject to erratic droughts and floods, or where drainage is impeded. The natural vegetation is grassland, savanna, or grassy woodland, as the heavy texture and unstable behavior make it difficult for many tree species to grow.
Vertisols are found between 50°N and 45°S of the equator. Major areas include:
- Eastern Australia (especially inland Queensland and New South Wales)
- The Deccan Plateau of India
- Parts of Sudan, Ethiopia, Kenya, Chad (the Gezira), and South Africa
- The lower Paraná River in South America
- Southern Texas and adjacent Mexico
- Northeast Nigeria, Thrace, New Caledonia, and parts of eastern China
Depending on parent material and climate, they can range from grey or red to deep black. Common local names include "black earths" (Australia), "black gumbo" (East Texas), "black cotton soils" (East Africa), and "turf soil" or "vlei soils" (South Africa).
Utilization
When irrigation is available, crops such as cotton, wheat, sorghum, and rice can be grown. Vertisols are especially suitable for rice because they are nearly impermeable when saturated. Rainfed farming is difficult because vertisols can be worked only under a narrow range of moisture conditions: they are very hard when dry and very sticky when wet. In Australia, they are highly regarded as among the few soils not acutely deficient in available phosphorus.
The shrinking and swelling of vertisols can damage buildings and roads, leading to extensive subsidence. They are generally used for grazing of cattle or sheep.
Classification (USDA Soil Taxonomy)
Vertisols are subdivided into six suborders:
- Aquerts: Show aquic conditions and redoximorphic features; permeability is slowed by high clay content.
- Cryerts: Have a cryic soil temperature regime; found in the Canadian Prairies and similar latitudes in Russia.
- Xererts: Have cracks open for at least 60 consecutive days in summer but closed for at least 60 consecutive days in winter; found in the eastern Mediterranean and parts of California.
- Torrerts: Have cracks closed for less than 60 consecutive days when soil temperature at 50 cm is above 8 °C; found in west Texas, New Mexico, Arizona, and Australia.
- Usterts: Have cracks open for at least 90 cumulative days per year; the most extensive suborder globally, encompassing vertisols of the tropics and monsoonal climates.
- Uderts: Have cracks open less than 90 cumulative days per year; of small extent globally, found in Uruguay, eastern Argentina, parts of Queensland, and the "Black Belt" of Mississippi and Alabama.
In the WRB classification, the diagnostic feature is a vertic horizon — a clayey subsurface horizon with slickensides or wedge-shaped structural aggregates within 100 cm of the soil surface.
