Mollisols
Encyclopedia
Mollisols are a soil order in USDA soil taxonomy
. Mollisols form in semi-arid to semi-humid areas, typically under a grassland
cover. They are most commonly found in the mid-latitudes, namely in North America, mostly east of the Rocky Mountains, in South America in Argentina (Pampas) and Brazil, and in Asia in Mongolia and the Russian Steppes. Their parent material
is typically base-rich and calcareous and include 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 in depth. This fertile surface horizon, known as a mollic epipedon, is the defining diagnostic 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
). These environments have historically been strongly influenced by fire and abundant pedoturbation from organisms such as ants and earth worms. 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. As the world's most agriculturally productive soil order, the Mollisols represent one of the more economically important soil orders.
Though most of 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
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 classified 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 Paraná 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.
Pedology (soil study)
Soil classification
Soil science
Soil type
USDA soil taxonomy
USDA Soil Taxonomy developed by United States Department of Agriculture and the National Cooperative Soil Survey provides an elaborate classification of soil types according to several parameters and in several levels: Order, Suborder, Great Group, Subgroup, Family, and Series.- Example of...
. Mollisols form in semi-arid to semi-humid areas, typically under a grassland
Grassland
Grasslands are areas where the vegetation is dominated by grasses and other herbaceous plants . However, sedge and rush families can also be found. Grasslands occur naturally on all continents except Antarctica...
cover. They are most commonly found in the mid-latitudes, namely in North America, mostly east of the Rocky Mountains, in South America in Argentina (Pampas) and Brazil, and in Asia in Mongolia and the Russian Steppes. Their parent material
Parent material
In soil science, parent material is the underlying geological material in which soil horizons form...
is typically base-rich and calcareous and include limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
, loess
Loess
Loess is an aeolian sediment formed by the accumulation of wind-blown silt, typically in the 20–50 micrometre size range, twenty percent or less clay and the balance equal parts sand and silt that are loosely cemented by calcium carbonate...
, or wind-blown sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
. The main processes that lead to the formation of grassland Mollisols are melanisation, decomposition
Decomposition
Decomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...
, humification and pedoturbation.
Mollisols have deep, high organic matter, nutrient-enriched surface soil (A horizon), typically between 60–80 cm in depth. This fertile surface horizon, known as a mollic epipedon, is the defining diagnostic 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
Great Plains
The Great Plains are a broad expanse of flat land, much of it covered in prairie, steppe and grassland, which lies west of the Mississippi River and east of the Rocky Mountains in the United States and Canada. This area covers parts of the U.S...
). These environments have historically been strongly influenced by fire and abundant pedoturbation from organisms such as ants and earth worms. 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
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...
due to the fertility of the A horizon). Globally, they represent ~7% of ice-free land area. As the world's most agriculturally productive soil order, the Mollisols represent one of the more economically important soil orders.
Though most of the other soil orders known today existed by the time of the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
Ice Age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
280 million years ago, Mollisols are not known from the paleopedological record
Paleopedological record
The paleopedological record is, essentially, the fossil record of soils. The paleopedological record consists chiefly of paleosols buried by flood sediments, or preserved at geological unconformities, especially plateau escarpments or sides of river valleys...
any earlier than the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
. Their development is very closely associated with the cooling and drying of the global climate that occurred during the Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
, Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
and Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
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 regimeUdic moisture regimeThe udic moisture regime is common to soils of humid climates which have well-distributed rainfall, or which have enough rain in summer so that the amount of stored moisture plus rainfall is approximately equal to, or exceeds, the amount of evapotranspiration...
- 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
Permafrost
In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...
and are consequently affected by cryoturbation
Cryoturbation
In gelisols , cryoturbation refers to the mixing of materials from various horizons of the soil right down to the bedrock due to freezing and thawing.Cryoturbation occurs to varying degrees in most gelisols...
are common in the high mountain plateaux of Tibet
Tibet
Tibet is a plateau region in Asia, north-east of the Himalayas. It is the traditional homeland of the Tibetan people as well as some other ethnic groups such as Monpas, Qiang, and Lhobas, and is now also inhabited by considerable numbers of Han and Hui people...
and the Andean altiplano
Altiplano
The Altiplano , in west-central South America, where the Andes are at their widest, is the most extensive area of high plateau on Earth outside of Tibet...
. Such soils are known as Molliturbels
Gelisols
Gelisols are an order in USDA soil taxonomy. They are soils of very cold climates which are defined as containing permafrost within two meters of the soil surface...
or Mollorthels
Gelisols
Gelisols are an order in USDA soil taxonomy. They are soils of very cold climates which are defined as containing permafrost within two meters of the soil surface...
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 classified 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 Paraná River
Paraná River
The Paraná River is a river in south Central South America, running through Brazil, Paraguay and Argentina for some . It is second in length only to the Amazon River among South American rivers. The name Paraná is an abbreviation of the phrase "para rehe onáva", which comes from the Tupi language...
basin that have abundant but erratic rainfall and extensive deposition of clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
-rich minerals from the Andes
Andes
The Andes is the world's longest continental mountain range. It is a continual range of highlands along the western coast of South America. This range is about long, about to wide , and of an average height of about .Along its length, the Andes is split into several ranges, which are separated...
. Mollic epipedons also occur in some Andisols
Andisols
In USDA soil taxonomy, Andisols are soils formed in volcanic ash and defined as soils containing high proportions of glass and amorphous colloidal materials, including allophane, imogolite and ferrihydrite...
but the andic properties take precedence.
See also
PedogenesisPedogenesis
Pedogenesis is the science and study of the processes that lead to the formation of soil ' and first explored by the Russian geologist Vasily Dokuchaev , the so called grandfather of soil science, who determined that soil formed over time as a consequence of...
Pedology (soil study)
Pedology (soil study)
Pedology is the study of soils in their natural environment. It is one of two main branches of soil science, the other being edaphology...
Soil classification
Soil classification
Soil classification deals with the systematic categorization of soils based on distinguishing characteristics as well as criteria that dictate choices in use.- Overview :...
Soil science
Soil science
Soil science is the study of soil as a natural resource on the surface of the earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to the use and management of soils.Sometimes terms which...
Soil type
Soil type
In terms of soil texture, soil type usually refers to the different sizes of mineral particles in a particular sample. Soil is made up in part of finely ground rock particles, grouped according to size as sand, silt and clay...