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In both the FAO and USA soil taxonomy, a vertisol is a soil in which there is a high content of expanding and shrinking clay known as montmorillonite that forms deep cracks in certain seasons or years. This shrinking and swelling causes self-mulching, where the soil material consistently mixes itself, causing vertisols to have an extremely deep A horizon and no B horizon. (A soil with no B horizon is called an A/C soil). This heaving of the underlying material to the surface often creates microrelief known as gilgai. Vertisols typically form from highly basic rocks such as basalt in climates that are seasonally humid or subject to erratic droughts and floods. Depending on the parent material and the climate, they can range from grey or red to the more familiar deep black (known as black earths in Australia). Vertisols are found between 50° N and 45° S of the equator. Major areas where vertisols are dominant are eastern Australia (especially inland Queensland and New South Wales), the Deccan Plateau of India, southern Sudan and adjacent Chad (the Gezira), and the lower Parana River in South America. Other areas where vertisols are dominant include southern Texas and adjacent Mexico, northeast Nigeria, Thrace, and parts of eastern China. In their natural state, vertisols are covered with grassland or grassy woodland. The heavy texture and unstable behaviour prevents forest from growing. The shrinking and swelling of vertisols can damage buildings and roads, leading to extensive subsidence. Vertisols are generally used for grazing of cattle or sheep. It is not unknown for livestock to be injured through falling into cracks in dry periods. However, the shrink-swell activity allows rapid recovery from compaction. When irrigation is available, crops such as cotton, wheat, sorghum and rice can be grown. Vertisols are especially suitable for rice because they are almost impermeable when saturated. Rainfed farming is very difficult because vertisols can be worked only under a very narrow range of moisture conditions: they are very hard when dry and very sticky when wet. However, in Australia, vertisols are highly regarded, because they are the only soils that are not always acutely deficient in available phosphorus. Some, known as crusty vertisols, have a thin, hard crust when dry that can persist for 2 to 3 years before they have crumbled enough to permit seeding. In the USA soil taxonomy, vertisols are subdivided into:
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