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Mollisols
Mollisols
Summary:
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Vegetation: prairie, grassland
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Climate: variety of soil temperature regimes (cryic to hypothermic)
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Soil moisture regime: variety of soil moisture regimes - aquic, udic, ustic, or xeric; average annual precipitation between 200 to 800 mm
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Major soil property: organic matter content, high base saturation,
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Diagnostic horizons: argillic, cambic (natric, calcic, petrocalcic, gypsic, albic, duripan)
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Epipedon: mollic
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Major processes: melanization, decomposition, humification, pedoturbation
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Characteristics: highly fertile soils
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Mollisols - Environmental Conditions
Climate: Mollisols occur in a variety of climatic zones, ranging from cryic (e.g. Mongolia, North Dakota), frigid (e.g. Iowa), mesic (e.g. Pakistan), or thermic (e.g. central Oklahoma) temperature regimes. The average annual precipitation amount ranges from 200 mm where short-grass steppe vegetation predominates to 800 mm where tall-grass vegetation grows. For example, climate in the Great Plains favor the development of Mollisols: severe, dry winters with much wind and relatively slight accumulation of snow; relatively moist springs and droughty summers with some thunderstorms and/or tornadoes (e.g. typical climate of the Great Plains). Mollisols occur under several soil moisture regimes: udic, ustic, xerix, and aquic.
Vegetation: Most of the Mollisols have formed under prairie or grassland vegetation. There are different types of prairie: In tall-grass prairie grasses stand 1 to 3-m at maturity, whereas in short-grass prairie grasses stand 13 to 30-cm in height. The prairie or grassland vegetation add plentiful raw organic matter to the soil, mostly by in situ root death. Legumes in the prairie or grassland community contribute considerable nitrogen to the soil. Prairies develop under relatively moist condistions, whereas grass steppe develop under drier climate. Prairie extension was largest approximately 5000 to 2000 B.P. Common species of prairie vegetation are bluestem (Andropogon gerardi), buffalo grass (Buchloe dactyloides), or western wheat grass (Agropyron smithii). Nowadays, most of the prairie in the U.S. is replaced by farmland. Mollisols are fertile soils and in the U.S. approximately 25 % of the land area are covered by Mollisols which produce much of the wheat, soybean, and alfalfa yield. A few Mollisols have formed under forest, under special conditions of poor drainage and/or calcareous or high base status parent material.
Relief: Mollisols cover a wide range of land forms (e.g. flat or gently rolling plains, undulating plains, mountain areas). Extensions of prairies by fire have formed preferentially on topography over which fire moves easily (e.g. ridge tops, windward slopes).
Parent Material: Mollisols occur on deposits and landscapes with a wide range of ages. Many Mollisols are formed on deposits associated with glaciations (unconsolidated Quaternary materials), where calcareous rich aolian deposits supported the formation of Mollisols. However, in other areas they develop in residuum weathered from sedimentary rocks.
Time: The age for development of Mollisols is indifferent and closely associates to the other environmental factors.
Mollisols - Processes
Melanization is defined as a process of darkening of the soil by addition of organic matter and it is the dominant process in Mollisols. Thus, the melanization that occurs in Mollisols is driven by the incorporation of organic matter directly into the mineral soil.
The prairie and grassland vegetation accumulate relatively large amounts of organic matter (accumulation of OM). Microbial decomposition of organic materials in the soil produces relatively stable, dark compounds (humification). Residue from plants partially decomposes on the soil surface and enriches the upper part of the A horizon through incorporation by soil fauna. Earthworms, ants, cicada nymphs, and rodents (e.g. gophers) are considered to be important agents in promoting the incorporation and breakdown of litter into the soil. The biological activity in Mollisols is greater than in forest soils, particularly the earthworm activity is considerable in Mollisols. Intensive pedoturbation obliterates the differentiation of horizons. In Mollisols several kinds of pedoturbation are recognized: (i) Faunal pedoturbation: soil mixing by animals such as ants, earthworms, moles, and rodents, (ii) Human induced pedoturbation: tillage operations, (iii) Congelli pedoturbation (cryoturbation): mixing by freeze-thaw cycles as in tundra and alpine landscapes, and (iv) Argilli pedoturbation: mixing of materials in the solum by shrink and swell movements of expansible clays as they wet and dry in the water cycles within the soil.
In some Mollisols there is also evidence of eluviation and illuviation of organic and some mineral colloids (clays, iron and manganese oxides) along voids between peds and the surfaces of which become coated with dark cutans (organo-argillans). For example, an eluviated horizon is present in the Albolls and an argillic horizon is found in Argiudolls. Percolation of water is influenced by systems of cracks, krotovinas, and macropores made by roots and soil fauna. In many medium-textured, well-drained Mollisols the presence of A and B horizons with nearly equal clay content can be explained by the following processes: (i) in climates where evapotranspiration exceeds precipitation clay might be translocated upwards from the B to the A horizon, (ii) rapid clay formation in the A horizon under well-drained soil moisture conditions and grassland vegetation, (iii) very slow eluviation in grassland soils, due to the complexing of mineral and organic colloids and the rapid adsorption of water by plant roots, or (iv) pedoturbation by prairie ants (Formica cinerea), which builds mounds where clay, organic material, phosphorus, and potassium is accumulated.
Deposition of loess material (dust) and blown out dry organic matter support the development of Mollisols (wind erosion). The deposited material is rich in calcium and other nutrients, which supports microbial activity. In many Mollisols the calcareous loess was leached of carbonates and varying degrees of acidity have developed. After leaching of carbonates, clay formation reaches its maximum and clay movement might occur when precipitation exceeds evapotransiration.
Water erosion can cause cumulization and the thickening of the mollic epipedon. These soils usually are at the base of slopes or on flood plains. They are defined by the denotion 'cumulic'. In intensively cultivated areas, as in the Midwest, many of the soils have lost a significant thickness of the surface horizon due to erosion.
Mollisols - Properties
A major characteristic of Mollisols is the high accumulation and decomposition of soil organic matter (SOM). SOM includes a variety of materials ranging form newly added material to the thoroughly decomposed and polymerized residual matter (humus). The grassland or prairie vegetation produce high amount of SOM, where as much as 80 % of the total biomass is in the roots. For example, the above-ground production of tall-grass prairie ranges from 1700 to 3500 kg/ha, whereas the dry weight of roots is about 3 times higher. Under prairie vegetation more than 50 % of the biomass is added to the soil annually, almost all the above ground parts and at least 30 % of the underground parts. As a result, most of the OM is deposited within the profile itself, the highest amount within the mollic epipedon. Due to decomposition and humification stable humus is formed, which is composed of complex organic compounds synthesized by the soil organisms and resistant polymers of phenolic and aromatic functional groups. The average C:N ratio for grassland soils is nearly constant, ranging from 10:12. Mollisols exhibit a mollic epipedon , which is dark in color, humus-rich, relatively fertile, and show a thickness of about 40 to 75 cm. If earthworm activity is high wormholes or macropores are formed which are pathways for preferential flow. Additional factors that are associated with the accumulation of organic matter in Mollisols are a high base saturation (> 50 %), high cation exchange capacity , and a high water holding capacity.
Generally, the A horizon shows a granular structure, whereas the B horizon exhibits blocky and prismatic soil structure. Many clay minerals have been formed from pedogenesis. Inherited micas have been depleted of potassium and valence charges of the layers have been lowered by weathering producing a wide array of clay minerals in Mollisols. Coatings are found on ped surfaces, which are called organo-argillans composed of mineral and organic components. The eluviation and illuviation of clay might form an argillic or a cambic diagnostic horizon. Because the formation of the argillic horizon is relatively slow, its presence in Mollisols indicates soils formed on older, more stable geographic surfaces. Krotovinas (filled burrows) develop due to the intense activity of the fauna.
Mollisols - Classification
While it is true that all Mollisols have mollic epipedons, the presence of a mollic epipedon does not automatically qualify a soil as a Mollisol. Epipedons that are made to meet the mollic criteria by the common practice of agricultural liming are excluded from criteria when placing a soil in the Mollisol order.
The criteria to qualify for a Mollisol are:
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Mollic epipedon
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Base saturation of 50 % or more in all horizons to a depth of 180 cm or a lithic or paralithic contact if shallower
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There are 7 suborders in the Mollisol order:
 Albolls: Albolls are Mollisols with an albic horizon, aquic conditions for some time in most years, and redox concentrations within 100 cm of the mineral soil surface. Below the albic horizon there is an argillic or natric horizon. Processes which develop Albolls are eluviation/illuviation and reduction of iron and manganese oxides due to wet soil moisture conditions. They occur on nearly level interfluve ridgetops or closed depressions.
Aquolls: They develop under aquic conditions thus they show soil properties associated with wetness: (i) redoximorphic features, (ii) accumulation of organic matter, (iii) a histic epipedon overlying the mollic epipedon, (iv) accumulation of calcium carbonate or exchangeable sodium near the soil surface.
Rendolls: They are formed in humid regions under forest, formed from calcareous parent materials (e.g. limestone, calcareous glacial till, chalk, shell deposits). The mollic epipedon must be less than 50-cm thick and may be rather weakly expressed due to the dilution effect of the light-colored, calcium-rich material from which it has formed. Rendolls do not have argillic or calcic horizons. This suborder is not subdivided into great groups, but a number of subgroups are identified on the basis of a shallow lithic contact, cryic soil temperature regime, vertic character, and presence or absence of a cambic horizon. They were classified as Rendzina in the previous U.S. classification.
Xerolls: Xerolls are Mollisols that have a xeric soil moisture regime. They ordinarily have a thick mollic epipedon, or cambic or argillic horizon and an accumulation of carbonates in the lower solum. They occur in the U.S. in Washington, Idaho, and Oregon.
Cryolls: This is the most extensive Mollisol suborder worldwide. Borolls form under a frigid and cryic soil temperature regime. They occur in Eastern Europe and Asia (the northern Russian steppes), and the northern Great Plains and in mountainous areas of the western United States.
Ustolls: That are the freely drained Mollisols of semiarid to subhumid climates with ustic soil moisture regime. Erratic rainfall occurs mostly during the growing season, and summer drought is a frequent, but erratic occurrence. They are the most extensive Mollisols in the U.S. found in the southern Great Plains, New Mexico, Texas, and Oklahoma. Most Ustolls show an accumulation of calcium carbonate in the soil profile (calcic horizon).
Udolls: Udolls are formed under udic soil moisture regime in continental climates of the temperate and tropical regions. They were formed on late-Pleistocene or Holocene glacial or other deposits, under tall-grass prairie. Their well-developed mollic epipedons usually are underlain by either argillic or cambic horizons. They occur in the western Corn Belt of the U.S. and in the humid parts of the South American Pampas.
Several soil moisture regimes are considered at subgroup level ranging from dry to wet conditions: Xeric (e.g. Xeric Argialbolls), aridic (e.g. Aridic Calcixerolls), udic (e.g. Udic Paleustolls), ustic (e.g. Ustic Argicryolls), and aquic (e.g. Aquic Natrustolls).
Great groups and subgroups are differentiated by subsurface diagnostic horizons: (i) argillic - e.g. Argialbolls, Argic Duraquolls, (ii) natric - e.g. Natraquolls, Natric Duraquolls, (iii) calcic - e.g. Calciaquolls, Calcic Haplocryolls, (iv) petrocalcic - e.g. Petrocalcic Palexerolls, (v) gypsic - e.g. Clcixerolls, (vi) albic - e.g. Albic Cryoborolls,or (vii) duripan - e.g. Duricryolls, Duric Natrixerolls (viii) cambic - e.g. Eutropeptic Rendolls.
Soils formed in volcanic parent material with low bulk densities (< 1.0 g/cm3) and more than 35 % fragments coarser 2.0 mm are denoted by 'andic', 'aquandic', or 'vitrandic' (e.g. Andic Cryoborolls, Aquandic Argialbolls, Vitrandic Durixerolls).
The term 'vertic' is used when Mollisols show characteristics such as cracking, wedge-shaped aggregates, slickensides, and high content of expandable clays (e.g. Vertic Cryaquolls, Vertic Haprendolls, Vertic Palexerolls).
Mollisols with a glossic horizon, i.e., interfingering of albic material into the subsurface horizon is called 'glossic' (e.g.Glossic Natriborolls, Glossic Natrustolls).
Some Mollisols are differentiated by soil texture. Mollisols that have a sandy or sandy-skeletal particle-size class throughout a layer extending from the mineral soil surface to the top of an argillic horizon at a depth of 50 cm to 100 cm are denoted as 'arenic' (e.g. Arenic Argiaquolls , Arenic Argiborolls). Other Mollisols that have a mollic epipedon 50 cm or more thick with a texture finer than loamy fine sand are called 'pachic' (e.g. Pachic Haplustolls, Pachic Argiustolls). Mollisols that have a sandy particle-size class throughout the upper 75 cm of the argillic horizon, or throughout the entire argillic horizon if it is less than 75 cm thick are classified as 'psammentic' (e.g. Psammentic Argiudolls ).
The thickness of the mollic epipedon differentiates Mollisols using the denotion 'entic'. For example, the mollic epipedon has to be less than 50 cm thick in Entic Vermustolls and less than 75 cm thick in Entic Vermudolls. Shallow Mollisols are classified as 'lithic' (e.g. Lithic Endoaquolls, Lithic Rendolls, Lithic Argicryolls). Thicker mollic epipedons are classified as 'cumulic', where the epipedon has to be > 50 cm thick (e.g. Cumulic Cryaquolls).
Mollisols with a high amount of wormholes, worm casts, or filled animal burrows are classified as 'vermic' (e.g. Vermudolls).
There are Mollisols with a histic epipedon (e.g. Histic Cryaquolls) and soils with a mollic epipedon which are actual buried Histosols that has its upper boundary within 100 cm of the mineral soil surface (e.g. Thapto-Histic Cryaquolls).
Wet soil moisture conditions form aquic or even oxyaquic Mollisols, where redoximorphic features are present (e.g. Aquic Natrixerolls, Oxyaquic Argiborolls).
Mollisols where the argillic horizon has its upper boundary 60 cm or more below the mineral soil surface are classified by the term 'pale' (e.g. Paleborolls). In those soils the argillic horizon is the result of an earlier weathering regime no longer present.
Mollisols - Distinguishing Characteristics
In Mollisols the significant characteristic is the presence of a mollic epipedon. There are similar soils which show a dark, humus-rich surface horizon high in exchangeable calcium and magnesium. Differences in chemical composition (e.g. phosphorus content) differentiate Mollisols from other soils with similar morphology but different genetic histories. The mollic epipedon may occur in soils of other orders in addition to Mollisols. Mollic epipedons are present in many Vertisols, in which case the plastic, shrink-swell nature of the clay is a more significant soil property than the mollic epipedon. Also, mollic epipedons are found in the Inceptisol order with cambic horizons that more significantly influence the profile than does the mollic epipedon, which in some cases may have been formed by lime applications. A few Alfisols also have mollic epipedons where nutrient cycling has extensively removed bases from the subsoil and concentrated them in the epipedon.
Prairie soils such as Argiudolls will develop into Albaquolls as weathering, clay production, and horizon differentiation proceed. Failure to meet the thickness criteria for a mollic epipedon results in potential classification of many of these soils as Mollic Hapludalfs if they are well drained and have an argillic horizon. Without an argillic horizon but with a cambic horizon in the profile, they are classified as Inceptisols.
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