In the FAO and Soil Taxonomy Soil Classifications, a vertisol
/ b> is that soil, generally black, where there is a high content of expansive clay known as montmorillonite that forms deep cracks in the dry seasons, or in years. Alternative expansions and contractions cause auto mulching , where the soil material mixes consistently with each other, causing vertisols with an extremely deep horizon A with no B horizon. (A soil with no B horizon is called < i> soil A / C soil) . This also results in the rise of internal material to the surface by creating microreliefs known as gilgai.
Vertisols are typically formed from highly basic rocks such as basalt in seasonally moist or subject climates drought and flood. Depending on parental material and climate, they may range from gray or reddish to the more familiar black (black earth in Argentina and Australia).
Vertisols are in the 50 N at 45 ° S of the terrestrial equator. The largest areas where vertisols dominate are Eastern Australia (especially Queensland, New South Wales), the Decan Plateau in India, Southern Sudan, Ethiopia and Chad (the Gezira), and province of Entre Ríos between the Paraná River and the Uruguay River and its adjacent western area of Uruguay in South America. Other areas where vertisols dominate include southern Texas and northern Mexico, northern Nigeria, Thrace, and parts of eastern China.
In their natural climate, vertisols are covered with grasses or forests pasture. Its heavy and unstable texture hamper forest growth.
When irrigation is done, crops such as cotton, wheat, sorghum, and rice grow well. Vertisols are especially good for growing rice because of their impermeability when they are saturated. Rainfed agriculture is very difficult because they can only be worked in a very narrow range of humidity: they are extremely hard dry, and too wet plastics. In Australia, vertisols are highly prized because they are the only soils without phosphorus deficiencies available. Some call them crusted vertisols , by a hard and fine dry "fragipan" layer, which can persist for 2 to 3 years before loosening enough to allow sowing.
Vertisols produce peculiar forms of the landscape, such as gilgais, small and ephemeral circular shaped lakes formed in a depression on the surface of a soil very rich in clay or vertisol. These soils expand and contract easily resulting in elevations and subsidence of the terrain that take on regular geometric forms. Also the term is applied to the surface microrelieves in the same type of lands, consisting of mounds and depressions.