- Written by Dr. Edward A. Nater, Dept. of Soil, Water and Climate, U of MN

Reference

Soil Survey Staff, 1994. Keys to Soil Taxonomy. Sixth Edition, Pocahontas Press, Inc., Blacksburg, VA. This book is the prime reference for all information about the U.S. Soil Taxonomic system.

Note: There have been several recent changes in Soil Taxonomy; this page will be updated in the near future to reflect the addition of a new order, Gelisols, and several key changes to the current Soil Orders.

Further Readings

Rust, R. H. 1983. Alfisols. pp. 253-282. 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. Alfisols are mineral soils that have a pedogenic accumulation of clay in the B horizon and base saturation ³ 35% and that have sufficient water during the growing season to support the growth of plants or have a fragipan that has clay films ³ 1mm in thickness.

Morphology and Horizonation

Epipedons

1. Histic - an organic soil horizon formed under aquic moisture conditions. Must be too thin to make this an organic soil. Usually very thin.

2. Ochric - a thin A horizon showing some incorporation of organic matter. Most common.

3. Umbric - similar to the mollic epipedon except that it has base saturation < 50 %. Generally formed under wet conditions in the Alfisols.

4. None - may have an O horizon at the surface and no A horizon.

Subsurface Horizons

1. Argillic - an increase (must be pedogenic) in illuvial layer lattice (phyllosilicate) clays. This may be in the form of lamellae as long as the combined thickness of lamellae >1 cm thick in the uppermost 1 m is ³ 15 cm.

2. Kandic - an argillic horizon composed mainly of kaolinite and/or halloysite.

3. Natric - an argillic horizon having high sodium content.

4. Albic - a horizon of eluviation.

5. Fragipan - a brittle horizon that appears cemented when dry but brittle when wet; has a high bulk density. Generally, but not always, below the argillic horizon.

6. Duripan - a horizon cemented by silica.

7. Plinthite - an iron-rich, humus-poor horizon containing clay, quartz and other minerals. Commonly occurs as red mottles or reticulate or platy patterns. Repeated wetting and drying, particularly with exposure to the sun produces irreversible hardening to ironstone.

8. Calcic - horizon of accumulation of calcium carbonate.

9. Gypsic - horizon of accumulation of gypsum.

10. Agric - horizon containing illuvial silt, clay, and humus. Formed under cultivation.

Morphology

1. A typical morphology would include a thin O horizon over a relatively light colored A horizon (ochric A, usually < 15 cm thick) over a moderately thick (15 - 25 cm), lighter colored E horizon (may be albic) over a relatively thick (25 - 75 cm), darker colored (usually redder, browner, or yellower due to the presence of iron oxides) B horizon showing an increase in clay content and one or more evidences of a pedogenic origin for the clay (oriented clay on ped faces or around pores or higher fine clay : coarse clay ratios in the B than in the A horizon).

2. Generally, changes in soil structure can be observed in the different horizons. These include: A horizon usually has crumb or granular structure; the E horizon may be structureless, or may have platy structure; and the B horizon usually has blocky or subangular blocky structure. Columnar or prismatic structure is also common.

3. The E and B horizons are often intermingled, such that parts of each are present in the other. Additionally, tonguing is a particular sort of intermingling where small to moderate vertical columns (tongues or fingers) of the E material penetrate the B material. It may be initiated by root penetration and subsequent decay or by degradation of the B horizon along structural planes.

Genesis

1. Organisms - generally formed under deciduous or coniferous forest; may also form under shrubs.

2. Climate - Moisture: from ustic and xeric to perudic. Temperature: from frigid to hyperthermic

3. Parent Materials - virtually anything allowed except for volcanic ash or organic. Most commonly formed in glacial till, outwash, loess, or on bedrock. Extremely acidic parent materials are rare.

4. Topography - can be found on virtually any slope, although less common on steeper slopes. Why? Alfisols may be found in sites of very poor to excessive drainage, although most Alfisols occur in moderately well or poorer.

5. Time - May be relatively young (a thousand years or so) to quite old (hundreds of thousands of years).

6. Processes - the basic requirement is the formation of a clay-rich B horizon by pedogenic processes. This requires the following three fundamental steps:

Classification

Sub Orders/Great Groups

1. Aqualfs - showing evidence of an aquic moisture regime.

1. Worldwide. Large areas of Alfisols are found in North America and Eurasia. As mapping becomes more detailed, more and more soils originally identified as Oxisols or Ultisols in Africa, South and Central America, and Australia are being identified as Alfisols.

Uses

1. Commonly used for farmland and forestry. Extensive areas of Alfisols have been cultivated in North America and Europe due to their high base status, high water holding capacity, and generally suitable climate.

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Further Readings

Miller, B. J. 1983. Ultisols. pp. 283-323. 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

Ultisols are mineral soils that have an argillic or kandic horizon and base saturation < 35 % at a prescribed depth in the argillic horizon, which depth depends on color, depth to lithic contact, or the presence of a fragipan. Alternately, if it has a fragipan with clay coatings ? 1 mm thick and base saturation <35 %, it may also be classified as an Ultisol. Formerly required to have a mesic or warmer temperature regime. No longer a requirement.

Morphology and Horizonation

Epipedons

1. Histic - an organic soil horizon formed under aquic moisture conditions.

2. Ochric - a thin horizon showing minimal incorporation of organic materials.

3. Umbric - similar to the mollic epipedon except that it has a base saturation less than 50 %.

4. Mollic - a thick mineral soil horizon having an accumulation of humic matter. Must have a base saturation greater than or equal to 50 %. Rare, but possible.

5. None - an O without an underlying A horizon.

Subsurface Horizons

1. Argillic

2. Kandic

3. Albic

4. Sombric - contains illuvial humus. Apparently restricted to cool moist soils of high plateaus and mountains in tropical or subtropical regions. Base saturation is generally < 50 %.

5. Fragipan

6. Plinthite

Morphology

A typical morphology would include a relatively light colored A horizon (ochric A, usually < 15 cm thick) over a moderately thick (15 - 25 cm), lighter colored E horizon (may be albic) over a relatively thick (25 - 200 cm), darker colored (usually redder or more yellow than the argillic found in an Alfisol due to higher concentrations of iron oxides) Bt horizon showing an increase in clay content and one or more evidences of a pedogenic origin for the clay (oriented clay on ped faces or around pores, an increase in the fine clay to coarse clay ratio in the B over the A horizon).

Genesis

1. Organisms - generally formed under deciduous or coniferous forest; may also form under shrubs. May have seen a variety of vegetative sequences over a long period of time.

2. Climate - Moisture: from ustic and xeric to perudic. Temperature: no requirements, but commonly from mesic to hyperthermic

3. Parent Materials - virtually anything possible.

4. Topography - occur on virtually any slope, but much more common on low slopes. Ultisols may be found in sites of very poor to excessive drainage.

5. Time - Relatively old. Depending on parent material, it takes time to leach bases.

6. Processes - the basic requirement is the formation of a clay-rich B horizon by pedogenic processes similar to those found in Alfisols. Both mechanisms of argillic horizon formation discussed above occur, but neoformation of clays appears to play a larger part in these (usually) more highly weathered soils. The other main process is loss of bases by leaching.

7. Pedoturbation is not a major factor, due to the thickness of the solum.

Classification

Sub Orders

1. Aquults - Have an aquic moisture environment.

1. Common in the tropics and subtropics, and occasionally on older stable landforms in temperate zones. Extensive areas occur in the southeastern U.S. and California (many now classified as Andisols), in Central and South America, and in Asia, Africa, and Australia.

2. Commonly used for agriculture and forestry. Generally require more liming and nutrient additions than Alfisols, but often have higher water-holding capacities and better physical properties, such as better structural aggregation and higher infiltration rates.

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Further Reading

Van Wambeke, A., H. Eswaran, A. J. Herbillon, and J. Comerma 1983. Oxisols. pp. 325-354. 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. Highly weathered soils that have less than 10 % weatherable minerals. The mineral fraction is dominated by quartz, iron and aluminum (hydr)oxides, kaolinite, and titanium oxides.

Morphology and Horizonation

Epipedons

1. Oxic - horizon of extreme weathering. Has less than 10 % weatherable minerals in the fine sand fraction, and has a particle-size of sandy loam or finer.

2. Kandic

3. Sombric - contains illuvial humus. Apparently restricted to cool moist soils of high plateaus and mountains in tropical or subtropical regions. Base saturation is generally < 50 %.

4. Plinthite

5. Petroferric contact - indicates the presence of an iron cemented layer within the profile. Unlike the placic horizon, it does not have significant organic matter content. Common in tropical soils.

Morphology

1. Dominated by a thick, highly weathered oxic horizon which usually extends to the soil's surface. This horizon is usually uniform and featureless, and is generally red or yellow due to the presence of iron oxides. Sometimes grayish due to the presence of kaolinite and aluminum oxides and lack of iron.

2. The epipedon may contain substantial quantities of organic matter; however, the deep colors imparted by iron oxides often make it difficult to recognize by field morphology alone.

3. Oxisols are generally very well aggregated, and are often highly resistant to erosion.

Genesis

1. Organisms - generally formed under tropical or subtropical forest or savanna. Due to the great age of many of these soils, however, they may have undergone several changes in vegetation.

2. Climate - Moisture: from ustic and xeric to perudic. Temperature: from thermic to hyperthermic, with very occasionally mesic or xeric (almost always paleosols). Many formed under climates far different than those to which they are now exposed.

3. Parent Materials - anything allowed, given sufficient time. Even highly basic materials like limestone or basalt will weather to form oxic materials. Quaternary glacial materials would be rare due to the different worldwide distribution of Oxisols (tropics) and glacial materials.

4. Topography - usually associated with level surfaces or low slopes. Why? Oxisols may be found in sites of very poor to excessive drainage.

5. Time - Almost always very old. Some may be millions of years old, some > 40 million years.

6. Processes - The main process is intense weathering and leaching, occurring over a long period of time.

Classification

Sub Orders

1. Aquox - have an aquic moisture condition (often have histic epipedons)

1. Extensive areas occur in the subtropics and tropics, with most of the acreage in South America and Africa occurring below the equator. Extensive areas are also present in southern Asia. Oxisols are also found in Hawaii, Puerto Rico, and many other tropical islands. The only known occurrence of an Oxisol in the contiguous 48 states is the Ione paleosol in central California.

2. Commonly used for agriculture and forestry. Because of the high inputs required for sustained agricultural production, many of these soils are farmed by shifting cultivation practices.

3. Nutrient problems are often difficult to overcome in Oxisols due to the lack of weatherable minerals, the extensive leaching that has occurred, their low pH values and tremendous buffering capacities, common phosphorous deficiencies, and other nutrient problems related to the high content of iron and aluminum oxides.

4. The major portion of the nutrient budget in many of these soils is tied up in the standing biomass, and the turnover of organic matter is very rapid.

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McKeague, J. A., F. DeConinck, and D. P. Franzmeier. 1983. Spodosols. pp. 217 - 252. 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. Spodosols are relatively acidic mineral soils that show strong evidence of illuviation of humic materials (generally fulvic acids) in association with non-crystalline Al and Fe sesquioxides (oxides, hydrous oxides, and hydroxides). Proper term is hydrous oxides or (hydr)oxides.

2. They often have a well-developed, thin to thick horizon of eluviation that usually qualifies as an albic horizon.

3. They are generally associated either coniferous forests, heath, or mosses in the boreal regions, or deciduous forests and heaths in temperate to tropical regions.

Morphology and Horizonation

Epipedons

1. Histic - an organic soil horizon formed under wet conditions.

2. Ochric - a thin A horizon showing some incorporation of organic matter.

3. Umbric - similar to the mollic epipedon except it has a base saturation less than 50%.

4. None - no epipedon is required. Many Spodosols have an O over E morphology.

5. Plaggen epipedons are not allowed.

Subsurface Horizons

1. Spodic or placic required.

3. Argillic or kandic - commonly occurs below the spodic horizon; a second E horizon is usually present between the spodic and the argillic. This constitutes a bisequum.

4. Fragipan

5. Duripan - a horizon cemented by Si. Somewhat different than the duripans commonly found in ustic, xeric, or aridic soils, but meets the same criteria.

6. Ortstein - nodules or a horizon cemented by Al and Fe organic complexes (essentially a cemented spodic horizon). Common in well developed Spodosols, particularly those in aquic moisture regimes.

Morphology

1. Spodosols commonly have four horizons:

1. Organisms - coniferous forest, deciduous forest, shrubs and mosses. Commonly under boreal spruce forests, though other conifers are common. Large acreages in Europe occur under heath and mosses. Slash and longleaf pines are the dominant overstory vegetation for Spodosols in Florida, while significant areas of Spodosols occur under forest and heath in the tropics. Dominated by plants that produce high concentrations of fulvic acids.

2. Climate - the majority form in regions of cool, humid or perhumid climate. However, large areas occur in mild perhumid areas of the west coast of Canada and Alaska, and in the warm humid region of the Atlantic coastal plain and Florida. They may occur in very cold climates and the tropics as well.

3. Parent Materials - generally sandy to coarse loamy materials. These are usually acidic. Although they may have been calcareous at one time, carbonates were removed by leaching before spodic horizon development started.

4. Topography - may occur on virtually any slope; relatively level surfaces are more common. The depth to water table is often significant; although many form in well-drained soils, a large proportion form over fluctuating water tables.

5. Time - Only a few hundred years are required for the formation of a Spodosol under the best of conditions; commonly, however, several thousand years are required for full development.

6. Processes

Sub Orders

1. Aquods - Spodosols occurring in aquic moisture environments

1. The largest areas occur under boreal forests of the USSR, Canada, and the US.

2. Cool, humid coastal climates also favor the development of Spodosols. Large acreages are found in British Columbia and Alaska, as well as along the northern coasts of Europe.

3. Temperate and tropical regions also have their share of Spodosols. Large sections of Florida and the coastal plains of the Southeast are covered with Spodosols, and they are known to occur in the tropics and subtropics of Australia, South America, and elsewhere as well.

Uses

1. The most common use is for forest growth, especially in the boreal forests.

2. Most Spodosols make very poor agricultural soils because they are sandy or coarse loamy, have low pH, and are often in aquic conditions. The nutrient pool of these soils is commonly bound up in organic matter in the spodic horizon. Agricultural practices, such as liming and plowing, may cause rapid oxidation of the spodic horizon, leading to loss of CEC, nutrients, and water-holding capacity. Most suitable agricultural use is for potatoes or other acid-loving crops.

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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:

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 ³ 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

Sub Orders/Great Groups

1. Aquolls - aquic moisture regime

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:

3. Where irrigation water is present, Mollisols are often the preferred soils for irrigated agriculture.

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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.

Subsurface Horizons (a wide-ranging variety)

1. Cambic

2. Placic

3. Duripan

4. Spodic

5. Argillic

6. Kandic

7. Calcic and petrocalcic

8. Petroferric contact

9. Oxic

Morphology

1. Quite variable, depending on the climate, vegetation, etc. Not necessarily distinctive. Andic properties do not produce a characteristic morphology, though soils often have very low bulk densities.

2. Epipedons may vary from ochric to umbric, mollic, histic, or melanic.

3. Eluviation / illuviation processes are often not very important, so there may be very little horizon differentiation except for the A and C horizons. Given enough time, however, striking, and rather different horizonation, can develop.

Genesis

1. Organisms - not important in defining the character of these soils. Many have thick grassland vegetation, but forests, savanna, and chaparral are also common.

2. Climate - the whole range from pergelic to hyperthermic, from aridic to perudic.

3. Parent Materials - Most are formed in volcanic ash, glass, or cinders, which are usually silicic, acidic, and poorly crystalline. May form in other materials, however.

4. Topography - Any landscape position and slope are allowed

5. Time - may be very young to very old

6. Processes - the main process involved is the rapid weathering of the primary parent materials (volcanic glass, ash, and cinders) and the formation of short-range-ordered secondary materials and aluminum-humus complexes.

7. The overwhelming concept behind this order is that the properties of these soils will be dominated more by parent material properties than by any other factor; thus, they are classified on that basis, regardless of horizonation, climate, vegetation, etc.

Classification

Sub Orders

1. Aquands - have evidence of wetness, such as a histic epipedon, mottling, or gleying.

1. Estimated to cover more than 124 million hectares worldwide (» 0.8 % of the earth's land surface). The vast majority are associated with the Pacific rim: Japan, west coast of North and South America and the east coast of Asia, and with several islands, such as New Zealand, Iceland, Hawaii.

Uses

1. Andisols are often very fertile, and they commonly have physical properties that make them very suitable for agricultural use. In addition, the location of most Andisols occurs in climates that are very suitable for agriculture.

2. Some Andisols have severe phosphorous fertility problems due to phosphorous sorption by one or more of the short-range-order materials present in the clay fraction. This may be particularly true for highly-weathered Andisols. Additionally, some Andisols have exchangeable and soluble Al concentrations that are toxic to all but the most resistant species. Because of their high buffering capacities, pH adjustment by liming is very expensive. These soils are very poor agricultural soils, but may still produce excellent timber growth.

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Ahmed, N. 1983. Vertisols. p. 91-123. In: Wilding, L. P., N.E. Smeck, and G. F. Hall (ed.), Pedogenesis and Soil Taxonomy: II. The Soil Orders. Elsevier, New York.

Main Concepts

1. Vertisols are soils that have high shrink / swell activity. Wide, deep cracks form in these soils during the dry season.

2. They are characterized by high pedoturbation (argillipedoturbation, if you will) due to the shrink / swell activity. The soil material is thoroughly mixed due to a "self-swallowing" process.

3. Vertisols have high clay content (> 30 % by definition). The clay mineralogy is usually, but not always, dominated by smectites. The clay has a high COLE (coefficient of linear extensibility) value.

Morphology and Horizonation

Epipedons

1. Mollic

2. Ochric

Subsurface Horizons

1. Calcic

2. Gypsic

3. Argillic

4. Natric

5. Salic

6. Duripan

Morphology

1. Most Vertisols are A/C soils; that is, they have only an A and a C horizon.

2. The A horizon is commonly very thick, usually more than 0.5 m, is dark gray to black, and has a relatively high organic matter content throughout its depth. Mollic epipedons are common.

3. Slickensides are commonly observed on ped faces, particularly in the lower part of the A horizon. These form by pressure deformation of peds during the swelling phase.

4. There is often an abrupt boundary with the C horizon material. It is "more or less undifferentiated" parent material, showing little pedogenic change. Often occurs over bedrock or some other contact zone between parent materials.

Genesis

Processes

1. The main process involved is argillipedoturbation, this "self-swallowing" process.

1. Parent material - the parent material is usually some type of basic rock, often one containing high quantities of illite or smectite.

2. Climate - seasonal wetting and drying cycles must occur in order to produce the cracks and initiate argillipedoturbation. No temperature restrictions exist (frigid and cryic temperature regimes were previously excluded from this order). Especially common in areas subjected to irregular high intensity storms following extended periods of dryness.

3. Organisms - grassland vegetation is most common, sometimes under woodlands and shrublands.

4. Topography - most occur on low slope areas (ca. < 8 %): valley floors, level plains, plateaus.

5. Time - many Vertisols occur on the most stable parts of the landscape, and are quite old. Others are much younger, but still probably several thousand years old.

6. One anomaly - Vertisols occur in Hawaii that do not have a smectitic mineralogy - they have a kaolinitic mineralogy. Little is known about high shrink/swell behavior in kaolinitic materials, but it apparently exists, and thus must be accounted for at some time in the future.

Classification

1. Aquerts - have an aquic moisture regime

1. Commonly used for agriculture. Vertisols often have serious management problems, such as poor drainage, difficulty in wetting, poor tilth, and high sodium content. Large acreages are used for cotton, sugar beets, cereals, pasture.

2. One curious problem associated with Vertisols is that grazing animals may be injured by falling into cracks during the dry season.

3. Out of the 320 M ha of Vertisols in the world, 60 % occur in the tropics, 30 % occur in the subtropics, and the rest occur outside the tropical-subtropical belt. Large areas occur in India, Australia, the U.S., and northern Africa.

4. Vertisols present severe engineering problems, and have many land use restrictions. The high shrink / swell behavior causes a shifting of foundations, roadbeds, telephone poles, fence posts, buried pipes and wires, septic systems, and other structures.

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Further Reading

Grossman, R.B. 1983. Entisols. pp. 55 - 90. In: Wilding, L.P., N.E. Smeck, and G.F. Hall (eds.) Pedogenesis and Soil Taxonomy: II. The Soil Orders.

Main Concept

1. Soils that show little or slight development. "Youthful" soils.

3. The most obvious cause of a lack of development is that the chronological age of these soils is very young. This is not always the case, however.

4. Causes of delayed or slowed development

Surface Horizons

1. Only ochric, histic, or anthropic epipedons (surface horizons) are allowed.

Subsurface Horizons

1. Most have none.

2. May have an albic horizon.

3. May have a spodic horizon if it occurs deeper than 2 meters.

4. A calcic or gypsic horizon or duripan if > 1 meter.

5. Other horizons if buried.

Generally an A / C soil

Classification

Sub Orders/Great Groups

1. Aquents - affected by wetness - either gleyed or mottled. Not in pergelic temperature regime.

Further Readings

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.

The fairly recent establishment of the Andisol order has greatly changed the definition and description of the Inceptisol order, where most Andisols formerly resided. Therefore, please read this chapter critically. References to the Andept suborder now mainly, but not always, refer to soils of the Andisol order.

Main Concepts

1. Soils that show a moderate amount of development: they have more development than an Entisol, but not enough to qualify as an Alfisol, Mollisol, Vertisol, or other order.

2. Often have a cambic subhorizon, formed by removal of carbonates, formation of mottles, limited movement of silicate clays and iron and aluminum (hydr)oxides, or other processes.

Classification Criteria (no, this is not a joke)

1. Soils that have one or more of the following:

Morphology and Horizonation

Epipedons

1. Histic - an organic soil horizon. Generally formed under wet conditions.

2. Ochric - a thin horizon showing minimal incorporation of organic materials.

3. Umbric - similar to the mollic epipedon except that it has a base saturation less than 50 %.

4. Plaggen - a manmade surface layer 50 cm thick produced by long-continued manuring.

5. Mollic - under certain conditions where the base saturation of the entire soil does not meet the depth criteria required for a Mollisol.

Subsurface Horizons

1. Cambic - an altered horizon. Recognized by development of soil structure or absence of rock structure in at least half the volume, and evidence of alteration (removal of carbonates, redoximorphic features (gleying or mottling), color change compared to the lower horizon, and is not classified as a spodic, argillic, kandic, or oxic horizon and is not cemented. Most common type.

2. Albic

3. Calcic or petrocalcic

4. Gypsic or petrogypsic

5. Salic - a horizon > 15 cm thick containing a secondary enrichment of salts more soluble in cold water than gypsum (sodium carbonate, sodium chloride, potassium chloride, some bicarbonates).

6. Sulfuric - formed by the oxidation of sulfides. Has a pH < 3.5 and jarosite mottles.

7. Duripan

8. Fragipan

9. Agric - illuvial horizon formed under cultivation. Has significant amounts of illuvial silt, clay, and humus.

10. Sombric - contains illuvial humus. Apparently restricted to cool moist soils of high plateaus and mountains in tropical or subtropical regions. Base saturation is generally < 50 %.

11. Placic - a thin black or dark reddish pan cemented by iron, possibly in combination with manganese or organic matter. Generally 2 - 10 mm thick. Inceptisols having a placic horizon do not meet other criteria required for the Spodosol order.

Morphology

1. Generally have one of the following morphologies:

Genesis

1. Organisms - almost any vegetation may be involved

2. Climate - common in pergelic and cryic climates, but may occur in any temperature regime. Common in all moisture regimes except for aridic, where it is not allowed.

3. Parent Materials - may be any mineral material except for andic materials

4. Topography - any topographic position allowed. Commonly found in lower slopes or steep slopes, and often in low lying areas (Aquepts); sometimes common on slopes in areas of Mollisols or Alfisols.

5. Time - generally relatively young, though not always the case.

6. Processes - any processes that cause the development of a B horizon are allowed. Includes eluviation and illuviation, oxidation of iron oxides, structural aggregate formation, others.

Classification

Sub Orders

1. Aquepts - have an aquic moisture regime

3. Tropepts - have an isomesic or warmer iso temperature regime

1. Widespread except in arid regions. Very common in cryic and pergelic temperature regimes.

Uses

1. Commonly used for agriculture and forestry. In temperate climates, they often make some of the best agricultural soils because they have undergone very little leaching and loss of bases, etc.

Back to top

Nettleton, W. D., and F. F. Peterson. 1983. Aridisols. p. 165-215. In: Wilding, L. P., N.E. Smeck, and G. F. Hall (eds.) Pedogenesis and Soil Taxonomy: II. The Soil Orders.

Main Concepts

1. Mineral soils having aridic moisture regimes that have sufficient development to not be classified as Entisols (Inceptisols are disallowed in aridic moisture regimes).

Morphology and Horizonation

1. Epipedons

Genesis

1. Parent material - virtually anything allowed except for andic, oxic, or organic materials

2. Climate - aridic moisture regime; any temperature regime

3. Organisms - severely restricted by moisture regime; may have been different in the past.

4. Topography - any topographic position allowed.

5. Time - there are no time restrictions; however, sufficient time must have elapsed for the development of the soil beyond the Entisol order, and this may be a substantial time period in an aridic environment.

6. Processes -

1. Argids - have an argillic, natric, or (I'm presuming, though it's not specified) a kandic horizon

1. In the natural state, Aridisols have few uses other than as grazing lands and wild lands. If sufficient irrigation water is present, however, these soils may be some of the most productive soils in agriculture, due to great stores of nutrients resulting from a lack of leaching.

Back to top

Everett, K. R. 1983. Histosols. pp. 1-53. In: Wilding, L. P., N.E. Smeck, and G. F. Hall (eds.) Pedogenesis and Soil Taxonomy: II. The Soil Orders. Elsevier, New York.

Basic Concepts

1. Histosols are organic soils. They generally form in situations of poor drainage. In many respects, they are organic sediments that have accumulated over long periods of time.

Terminology and Description

1. Textures - based on fiber content (fibers are those particles retained by a 100 mesh sieve, i.e., its openings are 0.15 mm diameter)

Mode of Formation

1. Lake fill

2. Swamping or paludification

Morphology - see Greenwood series description

Classification

1. Folists - have a lithic contact within 1 m and have < 75 % Sphagnum fibers

1. Histosols are distributed throughout the world, even in fairly arid environments in areas where water accumulates. Large areas of Histosols are present in boreal and tundra regions of Asia and North America. However, significant areas are also present in the tropics, subtropics, and more moderate climatic regions of the world.

2. Organic soils are commonly used for agriculture when drained because of their (often) high native fertility, good physical characteristics, and the easy availability of water. Problems arise from low pH, the presence of sulfidic materials, and subsidence after drainage. Other uses include forestry, and mining of organic soil materials for fuel, agricultural and horticultural amendments, and bedding for animals.