MOLLISOLS
Further Readings
Fenton, T. E. 1983. Mollisols. pp. 125-163. In: Wilding, L. P., N.E. Smeck, and G. F. Hall (eds.) Pedogenesis and Soil Taxonomy: II. The Soil Orders. Elsevier, New York.
Main Concepts
1. Mollisols have a thick, dark mineral A horizon containing high concentrations of organic matter.
2. Their base saturation is relatively high (> 50%).
3. Mollisols generally (though not always) formed under prairie or savanna vegetation.
Morphology and Horizonation
Epipedons
1. Mollic required.
2. Histic may overlay mollic, which sometimes occurs in seasonally wet, poorly drained soils of low topographic position.
3. Mollic epipedons may also occur in other orders without producing Mollisol classification:
a. Oxisols - the presence of the oxic subsurface horizon takes precedence over the mollic epipedon in the classification scheme.
b. Ultisols - the presence of a low base status argillic or kandic horizon takes precedence over the mollic epipedon. Rare.
c. Vertisols - the presence of high shrink swell characteristics and relatively high clay contents takes precedence over the mollic epipedon. Common.
d. Andisols - andic soil properties takes precedence.
Subsurface Horizons Allowed
These are usually expressed at the great group level, i.e., Argiudoll, Calciaquoll.
1. Cambic - significant but limited development, generally shows either mottling or gleying, a color change, or structure development.
2. Argillic - presence of an argillic horizon having base saturation 35%, otherwise an Ultisol
3. Calcic or petrocalcic - accumulations of secondary calcium carbonate, very common
4. Gypsic or petrogypsic - accumulations of secondary gypsum, common in drier areas (xeric, ustic, or aridic moisture regimes)
5. Albic - horizon of eluviation. Either formed above an argillic or a natric horizon. Often dissects the mollic epipedon.
6. Duripans - horizon cemented by silica, generally found in aquic, xeric, aridic, or occasionally ustic moisture regimes.
7. Natric - argillic horizon with an accumulation of exchangeable sodium; produces columnar soil structure and illuviated organic matter. Usually in ustic or drier moisture regimes.
8. None - it may be an A / C soil. This morphology is common in sandy sediments.
Subsurface Horizons Not Allowed:
1. Oxic - is then classified as an Oxisol
2. Argillic with low base saturation (< 35 %). Then classified as an Ultisol.
3. Andic or histic materials.
Genesis
1. Organisms - generally vegetated with prairie grasses (excluding the Rendolls).
2. Climate - Occur in all moisture regimes, including aridic. Found in all temperature regimes, from pergelic to hyperthermic. Most commonly found in xeric, ustic, udic, and aquic moisture regimes, and associated with cryic, frigid, and mesic temperature regimes.
3. Topography - in all landscape positions, though more commonly found on summits and foot and toeslopes. Why?
4. Time - most are relatively young to moderately old.
5. Parent Materials - virtually all kinds, although they are less likely to be associated with very acidic parent materials because of the base saturation requirements.
6. Processes
a. Additions - obviously, large additions of organic materials. Leaves and other plant parts are added at the soil's surface, but roots are added throughout the solum. Root turnover is rapid, generally taking only about 4 years for a complete turnover.
b. Losses - leaching of soluble salts and carbonates
c. Translocations - movement of carbonates, clays, some soluble salts, organic materials
d. Transformations - oxidation / reduction, decomposition and melanization of added organic materials and transformation to humus.
Classification
Sub Orders/Great Groups
1. Aquolls - aquic moisture regime
a. Cryaquolls - in cryic or pergelic temperature regimes
b. Duraquolls - have a duripan
c. Natraquolls - have a natric horizon
d. Calciaquolls - have a calcic or gypsic horizon and are calcareous throughout
e. Argiaquolls - have an argillic horizon
f. Epiaquolls - perched water table
g. Endoaquolls - groundwater table present
2. Albolls - have a well-defined albic horizon
a. Argialbolls - have an argillic horizon without a natric horizon
b. Natralbolls - have a natric horizon with or without an argillic
3. Rendolls - apparently formed under forest vegetation. Generally have very calcareous parent materials
no great groups defined
4. Xerolls - formed in a xeric moisture regime
a. Durixerolls - have a duripan
b. Natrixerolls - have a natric horizon
c. Palexerolls - formed on older surfaces. Commonly have a petrocalcic horizon or an especially well developed argillic horizon
d. Calcixerolls - have a calcic or gypsic horizon and are calcareous throughout
e. Argixerolls - have an argillic horizon
f. Haploxerolls - others, the central concept
5. Borolls - formed under either a frigid, cryic, or pergelic temperature regime
a. Paleborolls - have a well developed argillic horizon
b. Cryoborolls - cryic or pergelic temperature regime
c. Natriborolls - have a natric horizon
d. Argiborolls - have an argillic horizon
e. Vermiborolls - have > 50% of the volume of the mollic epipedon composed of wormholes, wormcasts, or filled animal burrows
f. Calciborolls - have a calcic or petrocalcic horizon and are calcareous throughout
g. Haploborolls - others, the central concept
6. Ustolls - formed in a ustic moisture regime
a. Durustolls - have a duripan
b. Natrustolls - have a natric horizon
c. Paleustolls - formed on older surfaces. Commonly have a petrocalcic horizon or an especially well developed argillic horizon
e. Calciustolls - have a calcic or gypsic horizon and are calcareous throughout
f. Argiustolls - have an argillic horizon
g. Vermustolls - have > 50% of the volume of the mollic epipedon composed of wormholes, wormcasts, or filled animal burrows
h. Haplustolls - others, the central concept
7. Udolls -
a. Paleudolls - well developed argillic horizon
b. Calciudolls - have a calcic horizon without an overlying argillic horizon
c. Argiudolls - less well developed argillic horizon
d. Vermudolls - have > 50% of the volume of the mollic epipedon composed of wormholes, wormcasts, or filled animal burrows
e. Hapludolls - others, the central concept
Distribution
1. Major areas occur in the central plains of the United States and Canada, as well as large areas in the Rockies and the Intermountain west. Large acreages of Xerolls may be found in Oregon, Washington, and some parts of California.
2. Large acreages of Mollisols may be found worldwide in the steppes and grasslands of Russia and China, and in the pampas regions of Venezuela.
Use
1. Mollisols are some of the finest agricultural soils in the world. The high organic matter content of these soils promotes soil properties that are desirable for most agricultural practices, including:
a. soil structure, which affects aeration, water infiltration and drainage, erodibility
b. tilth
c. high cation exchange capacity
e. contains organic nitrogen, phosphorous, and sulfur, all of which are released upon oxidation / degradation
f. high base status, which means less liming required and the addition of other amendments, better micronutrient balance
2. Because of the relatively low rainfall present in many of the areas where Ustolls and Xerolls occur, many of these soils are, or were formerly, used for grazing lands.
3. Where irrigation water is present, Mollisols are often the preferred soils for irrigated agriculture.
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ANDISOLS
Further Readings
Because this soil order was established fairly recently (July, 1989), very little information has been published about soils classified in this order. However, many of the Andisols were formerly classified as the Andept suborder of Inceptisols. Therefore, some information regarding some of the soils now classified in the Andisol order will be available under that heading, but the classification will, of course, be different.
See: Foss, J. E., F. R. Moormann, and S. Rieger. Inceptisols. pp. 355-378. In: Wilding, L. P., N.E. Smeck, and G.F. Hall (eds.) Pedogenesis and Soil Taxonomy: II. The Soil Orders. Elsevier, New York.
Main Concepts
1. These soils formed mainly in volcanic ash or cinders. They have andic soil properties; that is, their mineralogical properties are dominated by short-range-ordered materials (allophane, imogolite, ferrihydrite, proto-imogolite, others) or aluminum-humus complexes.
2. Occasionally, weathering of materials that are not of volcanic origin may still produce these short-range-ordered materials; in the current scheme such soils are also classified as Andisols.
3. Andic soil properties must be exhibited by a layer at least 35 cm thick which must occur in the top 60 cm of the soil.
Andic Properties
1. Andisols are defined by the presence of andic properties. These are a set of chemical criteria based on oxalate-extractable Al and Fe, soil bulk density, and phosphate retention, all modified by textural classes. The intent was to provide inarguable, unambiguous criteria to delineate those soils dominated by volcanic materials from others, but the reality is that the selected criteria don’t do so. Many soils formed in non-volcanic parent materials meet andic property criteria and thus are classified as Andisols, even though they have very low concentrations of the short-range-ordered materials listed above. This is particularly true for weakly developed Spodosols.
Morphology and Horizonation
Epipedons
1. Ochric
2. Umbric
3. Histic
4. Mollic - same as always, but cannot have all the requirements to be a melanic epipedon.
5. Anthropic - similar to the mollic except that it has > 250 mg kg-1 P2O5 soluble in citric acid. Formed in kitchen middens and old fields.
6. Melanic - a thick, black horizon occurring at or near the surface. Contains high concentrations of organic carbon usually associated with short-range-order minerals or aluminum-humus complexes. Rather like a mollic epipedon, but occurring in andic soil materials.
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