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Mollisol profile

Mollisols are a soil order in USA soil taxonomy. Mollisols form in semi-arid to semi-humid areas, typically under a grassland cover. They are most commonly found latitudinally in a band of 50 degrees north of the equator, although there are some in South America, South-Eastern Australia (mainly South Australia) and South Africa. Their parent material is generally limestone, loess, or wind-blown sand. The main processes that lead to the formation of grassland Mollisols are melanisation, decomposition, humification and pedoturbation.

Mollisols have deep, high organic matter, nutrient-enriched surface soil (A horizon), typically between 60-80 cm thick. This fertile surface horizon, known as a mollic epipedon, is the defining feature of Mollisols. Mollic epipedons result from the long-term addition of organic materials derived from plant roots, and typically have soft, granular, soil structure. Mollisols occur in savannahs and mountain valleys (such as Central Asia, or the North American Great Plains). It was estimated that in 2003, only 14 to 26 percent of grassland ecosystems still remained in a relatively natural state (that is, they weren't used for agriculture due to the fertility of the A horizon). Globally, they represent ~7% of ice-free land area. Because of their productivity and abundance, the Mollisols represent one of the more economically important soil orders.

Mollisols are geologically by far the youngest soil order in USA Soil Taxonomy. Whereas all the other soil orders known today existed by the time of the Carboniferous Ice Age 280 million years ago, Mollisols are not known from the paleopedological record any earlier than the Eocene. Their development is very closely associated with the cooling and drying of the global climate that occurred during the Oligocene, Miocene and Pliocene

Contents

Suborders

Albolls — wet soils; aquic soil moisture regime with an eluvial horizon
Aquolls — wet soils; aquic soil moisture regime
Cryolls — cold climate; frigid or cryic soil temperature regime
Gelolls — very cold climate; mean annual soil temperature <0°C
Rendolls — lime parent material
Udolls — humid climate; udic moisture regime
Ustolls — subhumid climate; ustic moisture regime
Xerolls — Mediteranaean climate; xeric moisture regime

Soils which are in most ways similar to Mollisols but contain either continuous or discontinuous permafrost and are consequently affected by cryoturbation are common in the high mountain plateaux of Tibet and the Andean altiplano. Such soils are known as Molliturbels or Mollorthels and provide the best grazing land in such cold climates because they are not acidic like many other soils of very cold climates.

Other soils which have a mollic epipedon are classed as Vertisols because the presence of high shrink swell characteristics and relatively high clay contents takes precedence over the mollic epipedon. These are especially common is parts of South America in the Parana River basin that have abundant but erratic rainfall and extensive deposition of clay-rich minerals from the Andes. Mollic epipedons also occur in some Andisols but the andic properties take precedence.

See also

Pedogenesis
Pedology (soil study)
Soil classification
Soil science
Soil type

External links

References

Brady, N.C. and Weil, R.R. (1996). ‘The Nature and Properties of Soils.’ 11th edition. (Prentice Hall, New Jersey).
Buol, S.W., Southard, R.J., Graham, R.C., and McDaniel, P.A. (2003). ‘Soil Genesis and Classification.’ 5th edition. (Iowa State University Press - Blackwell, Ames.)

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