soilorders

12) Soil Orders

12.13) Summaries / Orders

xxxback to: xxxMain Pagexxx Table of Contentsxxx 11.) U.S. Soil Taxonomy

12) Soil Orders

In chapter 12 the eleven soil orders of Soil Taxonomy are described. Because the Soil Taxonomy is a hierarchical system we will also use a hierarchical structure starting each chapter with a brief summary and adding more and more detail. The requirements, diagnostic horizons, and properties to meet each soil order are given. Additionally, attention is given on the links between processes and properties for each soil order. More information about soil orders, suborders, and the great groups can be looked up in the Keys to Soil Taxonomy (Soil Survey Staff) USDA-NRCS.

Further Reading

Soil Survey Staff. 1994. Keys to Soil Taxonomy USDA - Soil Conservation Service. 6th ed., Washington D.C.

Soil Survey Staff. 1997. Keys to Soil Taxonomy USDA - Soil Conservation Service. 7th ed., Washington D.C.

Wilding L.P., Smeck N.E., and Hall G.F., 1983. Pedogenesis and Soil Taxonomy I. Concepts and Interactions - II. The Soil Orders. Elsevier Sci. Publ., New York.

Buol S.W., Hole F.D., McCracken R.J., and Southard R.J., 1997. Soil Genesis and Classification. Iowa State University Press, Ames, Iowa.

12.1) Alfisols

Summary:

Vegetation: deciduous forest (prairie, grassland)

Climate: thermic or warmer, mesic or cooler

Soil moisture regime: erratic soil moisture regime

Major soil property: medium to high base saturation

Diagnostic horizons: albic, argillic (natric, kandic)

Epipedon: ochric (mollic, umbric)

Major processes: weathering, eluviation/illuviation

12.1.1) Environmental Conditions

Climate: The climatic conditions under which Alfisols form are thermic or warmer and mesic or cooler. Therefore, most Alfisols are in temperate regions, but these soils are also extensive in tropical and subtropical zones. Alfisols can occur generally in zones with a temperature range from below 0^oC to above 22^oC. Important for the development of Alfisols is the change between periods of high moisture content and high soil temperature, to break down the primary mineral components and to leach the weathered products, and low moisture content and low soil temperatures, which permit the precipitation or accumulation of the weathered products. Most Alfisols have an udic, ustic, or xerix moisture regime, and many have aquic conditions, but they are not known to have a perudic moisture regime. The suborder of Aqualfs requires higher soil moisture conditions compared to the development of other suborders of the Alfisols.

Vegetation: Most Alfisols are formed under broadleaf decidious forest, but they occur also under grassland and prairie vegetation. In forested ecosystems, the trees deliver the bulk of their annual production of organic matter aboveground, which is different from grassland soils. In those ecosystems the organic matter is enriched by the huge rootsystem of the grass or prairie cover. While present vegetation may be deciduous forest, earlier vegetation may have been grass or conifers.

Relief: In most Alfisols the drainage is not restricted with the water table occuring below the solum during major portions of the nonfrozen period. For instance, the suborder of Aqualfs is often functionally related to landscape position. Alfisols develop under several drainage conditions ranging from excessive on hill crest and steep slopes (e.g Lithic Hapludalfs) to poorly drained footslopes and level plains (e.g. Albaqualfs). Alfisols do not develop on very steep slopes, alluvial floodplains, and very poorly drained depressions. High elevations combined with limited rainfall favor Alfisol formation in the tropics.

Parent Material: The parent material has a major impact on the formation of clay minerals within soil. The resistance to weathering and the composition of primary minerals determine in combination with the other soil forming factors which clay minerals are formed. Generally, a wide variety of clay minerals ranging from kaolinites, hydrous micas, montmorillonites to vermiculites can occur. It should be stressed that several clay minerals do have a potential to adsorb exchangeable bases (high cation exchange capacity), which is a critera that should be met to qualify for an Alfisol. Most Alfisols are present on relative old landscapes (beginning Holocene or older) whereever the supply of primary minerals is plentiful.

Time: Most Alfisols need a longer period of time for development. Several studies postulated that the time to develop Alfisols is at least 200 y, where an argillic horizon is approached, to 1000 y for a clear expression of an Alfisol profile, and even longer periods, depending on the other soil forming factors.

12.1.2) Processes

The weathering of primary mineral components is a prerequisite for further processes to form Alfisols. Water is the master ingredient to accelerate physical and chemical weathering, particularly for hydration, hydrolysis, and oxidation. If the primary minerals are weathering in an alkaline environment then carbonates often initially dominate the weathering products. The release of H⁺ ions for Ca²⁺, Mg²⁺, and a variety of other cations, from the roots of vegetation fosters also the process of weathering. At the same time, under forest vegetation, most profiles show Ca²⁺ and Mg²⁺ higher in amount in the surface horizon than in horizons below. This may be attributed to recycling through leaf fall and decay. On the other hand, lower Ca²⁺ and Mg²⁺ values in the lower horizons of Alfisol solum can be an indication of more intense weathering.

The litter is decomposed to form an A horizon (decomposition, humification, mineralization). Under deciduous forest often an O and A horizon is found. There is relatively little accumulation of organic matter in the mineral horizons due to cycling of nutrients in the upper horizons. Biocycling of nutrients from B horizons to A and O horizons is an important process in most forested Alfisols. This explains the high content of bases (Ca, Mg, and K) in the ochric epipedon.

Eluviation of clay (in organic and inorganic form) from the A and E horizons in the initial material, of clay formed by mineral weathering, and of clay progressively added in eolian material is a dominant process in the formation of Alfisols. The eluviated material is illuviated in the underlying B horizon (illuviation), i.e., an argillic horizon is formed. Therefore, particularly the E horizon, is depleteted in organic colloids, clay minerals, and / or oxides and hydroxides,i.e., an albic diagnostic horizon is formed. The process of clay translocation is also called lessivage. An erratic moisture regime favors the formation of an argillic horizon, because the processes of weathering and translocation are supported by percolation water and the precipitation of the translocated material by dry moisture conditions.

The details of eluviation and illuviation can highlight the complexity of a variety of subprocesses involved in the development of Alfisols. Leaching of carbonates from the toplayers appear to be a prerequisite before clay can migrate. The presence of exchangeable calcium (from calcium carbonate) flocculates clay particles, creating particles that are too large to be transported in suspension. Removal of the calcium leaves the solum in a condition favorable for the dispersion of clay particles. When the clay particles are dispersed in an aqueous suspension translocation from the A and E horizons into the B horizon occurs with or without aid of complexing organic compounds, and possibly by migration of Si, Fe and Al under the influence of percolating water. Fine clays move more readily than coarse clay, therefore, the fine clay to total clay ratios are typically higher in the B horizon (0.6 - 0.8) than in the A and E horizons (0.3 - 0.6). Freshly formed clays tend to move more readily than older clays. The influence of organic matter on the transport phenomena of clay colloids in soils has been stressed by many authors. Organic matter is known to act as an electron donor for the reduction and solubilization of iron oxides which are leached. Sesquioxides do act as a cohesion agent. Furthermore, the presence of organic acids tends to destabilize the soil micro-aggregates and produce dispersible clays which are subsequently leached.

Argillans (clay coatings) are formed in the B horizon, which are often fewer in the upper B compared to the lower B horizon(s). This can be explained by shrink-swell cycles (freezing-thawing, wetting-drying), soil creep, and biologic mixing, which are more intense in the upper horizon. The precipitation of clays, often with sequioxides and organic matter, in the argillic horizon may be brought about by (i) depletion of percolating waters through sorption by peds, (ii) swelling shuts of voids and consequent slowing of percolating water, (iii) sieve action by clogging of fine pores, (iv) flocculation of the negatively charged clay by positively charged iron oxides in the Bt horizon or by calcium in the higher-base saturation lower solum, and (v) low pH which favors flocculation. The accumulation of clay may be masked by other processes such as pedoturbation.

Additionally, there might be in situ formation of clay minerals in the B horizon by weathering of primary minerals such as feldspars, micas, and ferromagnesian minerals, or by neosynthesis from illuvial weathering products. In young Alfisols the illuviation is the dominant process for the formation of an argillic horizon, whereas through time the in situ formation of clays within the argillic horizon becomes more dominant.

If the accumulation of clay materials in the Bt horizon is high it results in a decrease of percolation and subsequent waterlogging (reducing, anerobic environmental conditions). The slower permeability also favors the in situ weathering of primary minerals to clays. For example, Palexeralfs form on earlier-Pleistocene deposits when clay accumulation and slow permeability is sufficient to cause perching of a seasonal water table in the winter. Under such conditions iron oxide concretions form in horizons affected by a perched water table above dense B horizons.

In most Alfisols there is also a removal of Fe and Al from the E horizon to the B horizon. This can be attributed to the cheluviation of metal ions and organic colloids that form metal-organic complexes which are translocated.

12.1.3) Properties

A typical Alfisol profile looks like:

On uncultivated sites: A very thin O horizon is common; on cultivated sites: no O horizon

Thin A (less than 15 cm), weakly expressed crumb or granular structure

Moderate thin E horizon (15 - 25 cm), platy structure, light-colored

B horizon, usually with several subdivisions, which is normally between 25 - 75 cm thick, moderate to strong angular or subangular blocky structure, a lower case 't' is used to denote for an accumulation of silicate clay

Characteristics of the albic diagnostic horizon:

High in silt-size and larger particles

High amount of stable minerals such as quartz, tourmaline and rutile

Absence of organic matter

Particles are not aggregated

Higher pH compared to the argillic horizon (pH 6.5 - 7.0)

Higher Eh compared to the argillic horizon

Low cation exchange capacity

Platy structure

Characteristics of the argillic diagnostic horizon:

Accumulation of clay (organic and mineral colloids). The illuviated materials are deposited on structural aggregates, along root channels and on the surfaces of coarser particles (e.g. argillans)

Accumulation of iron and aluminium oxides (partly adsorbed to clay minerals)

The colloidal organic matter is mostly in the form of organo-clay complexes

Lower pH compared to the albic horizon (pH 4.5 - 6.0)

Lower Eh compared to the albic horizon

High cation exchange capacity

Blocky structure

12.1.4) Classification

The requirements to qualify for an Alfisol are the following:

High base status: > 35 % base saturation at a depth of 125 cm below the upper boundary of the argillic, natric, or kandic horizon

An argillic horizon that is not under a spodic or oxic horizon

Any soil temperature regime is allowed, except pergelic

The suborders of Alfisols are distinguished by soil temperature and soil moisture (Figure 12.1.4). The suborders, great groups, and subgroups of Alfisols are described in the Keys to Soil Taxonomy.

Figure 12.1.4. Diagram showing some relationships between suborders of Alfisols.

Aqualfs: They have aquic conditions for some time in most years within 50 cm of the mineral horizon and redoximorphic features in the upper 12.5 cm of the argillic, natric, or kandic horizon. Their appearance is normally controlled by gray redox depletions and higher-chroma redox concentrations. In some, ground water is near the surface during a considerable part of the year but drops to depths below the argillic (or natric, kandic horizon) in another part of the year. In others, the ground water may be deep most of the year but horizons that have low hydraulic conductivity restrict the downward movement of water and extend the period of saturation. Aqualfs occur in many parts of the world, mostly in small areas in deposits of late-Pleistocene age, where they occupy depressional areas or low-gradient landscapes subject to seasonal high water tables. Nearly all Aqualfs are believed to have supported forests at some time in the past. Most Aqualfs, except those that have a frigid or cryic temperature regime, have some artificial drainage or other water control and are cultivated. Rice is a common crop on Aqualfs that have a thermic or warmer temperature regime.

Boralfs: Boralfs are the more or less freely drained Alfisols of cold regions. They have a cryic temperature regime and an udic moisture regime is considered normal. Boralfs are not extensive. They form in North America, eastern Europe, and Asia above 49^o north latitude and in some high mountains south of that latitude. In the mountains, they tend to form below the Spodosols or Inceptisols. Most of them are or have been under a coniferous forest. Characteristically, Boralfs have an O horizon, an albic horizon, and an argillic horizon. A thin A horizon is present in some. In regions of the least rainfall, they are neutral or slightly acid in all horizons and a Bk horizon may underlie the argillic horizon. In many of the more humid areas of their occurence, the lower part of the albic horizon and the upper part of the argillic horizon are strongly or very strongly acid. Boralfs in the U.S. generally developed in Pleistocene deposits, mostly Wisconsinan age and under forest.

Udalfs: Udalfs are the more or less frequently drained Alfisols that have udic moisture regime and a frigid, mesic, isomesic, or warmer temperature regime. They are principally but not entirely on late-Pleistocene deposits and erosion surfaces of about the same age. Some of the Udalfs that are on older surfaces are underlain by limestone or other calcareous sediments. Udalfs are very extensive in the United States and in western Europe. All of them are believed to have had forest vegetation at some time during development. Most Udalfs with a mesic or warmer temperature regime have or had a deciduous forest vegetation and many of the frigid temperature regime have or had mixed coniferous and deciduous trees. Many Udalfs have been cleared of forests and intensively farmed, and as a result of erosion many now have only an argillic or a kandic horizon below the Ap horizon that is mostly material part of the argillic or kandic horizon. Others are on stable surfaces and retain most of their eluvial horizon above the argillic or kandic horizon. Normally, the undisturbed soil has a thin A horizon darkened by humus. A few Udalfs have a natric horizon. Others have a fragipan in or below the argillic or kandic horizon.

Ustalfs: They have an ustic moisture regime and a frigid, mesic, isomesic, or warmer temperature regime. They do not have, near the soil surface, both redoximorphic features with low chroma and aquic moisture regime for some time in normal years or artificial drainage. Moisture moves through most of these soils to deeper layers only in occasional years. If there are carbonates in the parent material or in the dust that settles on the surface, they tend to have a Bk or a calcic horizon below or in the argillic or kandic horizon. The dry season or seasons are pronounced enough that trees are either deciduous or xerophytic. Many of these soils have or have had a savanna vegetation and some were grasslands. Most of these soils are used for cropland of for grazingland. Ustalfs are the Alfisols of subhumid to semiarid regions. Sorghum, wheat, and cotton are common crops. Droughts are common. They occur in the United States mostly on the southern Great Plains. They are common in Africa, India, South America, Austalia, and southeastern Asia. The Ustalfs may be on erosion surfaces or deposits of late Wisconsian age, but many occur on old surfaces. In those soils the minerals have been strongly weathered, possibly in an environment more humid than the present one. At least, the clays in many of these older soils are kaolinitic. The base saturation in them at present probably reflects additions of bases in dust and rain.

Xeralfs: They have xeric moisture regime common of regions that have Mediterranean climate. They are dry for extended periods in summer, but in winter, moisture moves through the soil to deeper layers in at least occasional years, if not in normal years. Small grains, and other annuals are common crops where there is no irrigation. Grapes and olives are also common crops where the climate is thermic. With irrigation, a wide variety of crops can be grown. The Xeralfs formed in South Africa, Chile, Western Australia, Southern Australia and the Western United States. Most border the Mediterranean Sea or lie to the east of an ocean in midlatitudes. In the world as a whole, the Xeralfs are not extensive soils, but in the regions where they occur, they are extensive. The vegetation, before the soils were farmed, was a mixture of annual grasses, forbs, and woody shrubs on the warmest and driest Xeralfs and coniferous forest on the coolest and most moist Xeralfs. Xeralfs formed on surfaces that are different ages. Some formed on erosion surfaces or in deposits of late-Wisconsinan age, and some, as in Australia, are on old surfaces and have characteristics that probably reflect an environment greatly different from the present one. It is common in the oldest Xeralfs that the boudary between the A and B horizons is very abrupt. The epipedon of some Xeralfs is hard and massive when dry.

Great groups and subgroups are classified according to following features:

Alfisol may have (i) a fragipan (e.g. Fragixeralfs, Fragiaquic Paleudalfs), (ii) a duripan (e.g. Durixeralfs, Durudands, Durustands ), (iii) a kandic horizon (e.g. Kandiaqualfs), (iv) a natric horizon (e.g. Natraqualfs), (v) a salic horizon (e.g. Salidic Natrustalfs), (vi) a calcic horizon (e.g. Calcic Rhodoxeralfs), (vii) a petrocalcic horizon (e.g. Petrocalcic Natrustalfs), (viii) or plinthite horizon (e.g. Plinthustalfs, Plinthic Paleustalfs).

(i) Fragipans are found in some great groups of Alfisols. It is postulated that the majority of fragipans have developed nearly concurrent with the argillic horizon, sometimes as a part of it, in other cases immediately below. The dense, brittle character of the fragipan is attributed to various cementing agents such as silicate clays, oxides of iron, manganese and aluminum, and colloid silica. These are weathering products of the upper horizons, which are translocated and accumulated in lower horizons. The phenomena of large polygonal cracking commonly observed in the fragipan zone suggests a time of desiccation, probably on a recurring basis, with accumulated in-filling.

Recent research on the formation of fragipans suggest that the 'hydroconsolidation process', i.e., a structure collapse when loaded and wetted may contributed to fragipan formation (Bryant, 1989; Assallay et al., 1998). The classic occurences of hydroconsolidation are in loess soils with a clay content of 5 to 30 %. Fragipans occur more or less at a constant depth of about 40 to 80 cm below the soil surface.

(ii) In some Alfisols there is a duripan, i.e., a horizon of silica cementation. For example, the processes to form duripans are the slow weathering of feldspars and ferromagnesian minerals in older landscapes or rapid weathering of volcanic glass.

(iii) A kandic horizon is a subsoil diagnostic horizon having a clay increase relative to overlying horizons and low activity clays, with <= 16 cmol/kg clay CEC.

(iv) In soils with high Na content the sodium ion is important in the dispersion and mobilization of clay. Under such environmental conditions natric horizons can form, where pH may be as high as 10 or 11. Sodium is a cation which is weakly absorbed and is leached easily. Soil layers high in sodium are dispersed when wet, and show a low permeability and low aeration. Natric horizons are expressed in the great groups of Alfisols, for example, in Natrixeralfs, Natrudalfs, or Natrustalfs. (v) Salic horizons are enriched in secondary soluble salt such that the electrical conductivity exceeds 30 dS/m more than 90 days each year.

(vi) A calcic horizon is a mineral soil horizon of secondary carbonate enrichment that is more than 15 cm thick, has a CaCO₃equivalent of > 150 g/kg. (vii) If a horizon of indurated carbonates occur the formed diagnostic horizon is called petrocalcic. In general, a shift to a drier regime with periods of evaporation would contribute to carbonate accumulation.

(viii) Plinthite is a weakly-cemented iron-rich, humus poor mixture of clay with other diluents that commonly occurs as dark red redox concentrations that form platy, polygonal, or reticulate patterns. Plinthite changes irreversibly to ironstone hardpans or irregular aggregates on exposure to repeated wetting and drying.

A tonguing of the A horizon into the B horizon is also found in some Alfisols. The matrix of the tongues is similar to that of the eluvial horizon. It may be initiated by tree-root penetration and decay. These soils are classified in the great groups of Alfisols, such as, Glossaqualfs, Glossocryalfs, Glossudalfs or in the subgroups of Alfisols, such as Glossaquic Paleudalfs, Glossaquic Natrudalfs, or Glossic Natraqualfs.

Alfisols with vertic soil characteristics, i.e., cracks that are 5 mm or more wide through a thickness of 30 cm or more for some time in most years, and slickensides or wedge-shaped aggregates in a layer 15 cm or more thick that has its upper boundary within 125 cm of the mineral soil surface; or a linear extensibility of 6.0 cm or more between the mineral soil surface and either a depth of 100 cm or a densic, lithic, or paralithic contact, whichever is shallower (e.g. Vertic Natraqualfs).

'Albic' materials, i.e., soil materials with a color white to gray mainly due to the color of primary sand and silt particles and from which clay and/or free iron oxides have been removed, is used to define Alfisols at the subgroup level (e.g. Albic Natraqualfs).

Alfisols with recognizable bioturbation such as filled animal burrows, wormholes, or casts are named 'Vermic' (e.g. Vermic Natraqualfs, Vermic Fragiaqualfs).

Soil color is used to define 'Aeric' - chroma of 2 or more and no redox depletions (e.g. Aeric Kandiaqualfs), 'Udollic' - color value moist of 3 or less (e.g. Udollic Albaqualfs), and 'Rhodic' - a hue of 2.5YR or redder and a value (moist) of 3 or less (e.g Rhodic Kandiustalfs) characteristics of Alfisols.

Epipedons are also used to distinguish Alfisols at the subgroup level: 'Mollic' (e.g. Mollic Natraqualfs), 'Umbric' (e.g. Umbric Fragiaqualfs), or 'Histic' epipedon (e.g. Histic Glossaqualfs). 'Humic' is used for Alfisols with high organic matter content (e.g. Humic Fragiaqualfs).

Soil texture is used to classify Alfisols at the subgroup level: 'Arenic' or 'Grossarenic' show a sandy or sandy-skeletal particle-size class (e.g. Arenic Kandiaqualfs, Grossarenic Kandiaqualfs), 'Psammentic' subgroups show a sandy particle-size class throughout the argillic horizon (e.g. Psammentic Cryoboralfs).

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 Palexeralfs, Aquandic Albaqualfs, Vitrandic Fragiudalfs).

Alfisols that have episaturation, i.e., when a soil is saturated with water in one or more layers within 200 cm of the mineral soil surface and also has one or more unsaturated layers with an upper boundary above 200 cm, below the saturated layers(s) the prefix 'Epi' (e.g. Epiaqualfs) is used. Alfisols with wet soil moisture conditions and redox depletions with a chroma of 2 or less (e.g. Aquic Paleboralfs) are named 'Aquic' or 'Oxyaquic' when soils are saturated with water, in one or more layers within 100 cm of the mineral soil surface, for 1 month or more per year in 6 or more out of 10 years (e.g. Oxyaquic Paleboralfs).

Alfisols which are shallow are classified as 'Lithic' (e.g. Lithic Cryoboralfs) and soils which show less pronounced characteristics of an Alfisol are classified as 'Inceptic' (e.g. Inceptic Fragixeralfs).

Alfisols with high base saturation are named 'Eutr' (e.g. Eutroboralfs, Eutric Glossocryalfs). If the base saturation (by sum of cations) is less than 75 % throughout the argillic horizon the prefix 'Ultic' is used for classification (e.g. Ultic Paleustalfs).

A special feature is the presence of lamellae, which are subhorizons (two or more), each with an overlying eluvial horizon. The lamellae layers are of pedogenic origin. Alfisols with these features are classified as 'Lamellic' (e.g. Lamellic Eutroboralfs). Alfisols which are relatively old soils showing pronounced characteristics to qualify for this order are denoted by 'Pale' (e.g. Paleustalfs).

Soil temperature regimes are used to classify Alfisols at the great group and subgroup level: 'Cryic' (e.g. ), 'Xeric' (e.g. Xeric Palecryalfs), 'Ustic' (e.g. ), 'Aridic' (e.g. Aridic Kandiustalfs), 'Udic' (Udic Paleustalfs), 'Torrertic' (e.g. Torrertic Natrustalfs).

12.1.5) Distinguishing Characteristics

If the Entisols are considered of soils in the stage of minimum organization the Alfisols show a higher degree of organization. Weathering and eluviation / illuviation altered Entisols or Inceptisols to form Alfisols. Transitions between areas of Alfisols and Spodosols lie in ecotones between mixed deciduous forest and coniferous forest. The Ustalfs tend to form a belt between the Aridisols of arid regions and the Udalfs, Ultisols, Oxisols, and Inceptisols of humid regions. A lower content of organic matter in the surface horizon distinguishs the Alfisols from the Mollisols, which develop under grassland or prairie. The soil moisture is not high enough to accumulate organic matter to form Histosols. A pergelic soil temperature regime would develop Gelisols. Other soil orders with argillic horizons are Ultisols, Mollisols, and Aridisols.

References

Assallay, A.M., I. Jefferson, C.D.F. Rogers, and I.J. Smalley. 1998. Fragipan formation in loess soils: development of the Bryant hydroconsolidation hypothesis. Geoderma 83: 1-16.

Bryant, R.B., 1989. Physical processes of fragipan formation. In: Smeck, N.E., Ciolkosz I. (Eds.). Fragipans: Their occurence, classification and genesis. Soil Sci. Soc. Am. Apec. Publ. 24: 141-150.

xxxback to: xxxMain Pagexxx Table of Contentsxxx 11.) U.S. Soil Taxonomy

12.2) Andisols

Summary:

Vegetation: variety of vegetation types

Climate: all soil temperature regimes, except pergelic

Soil moisture regime: all soil moisture regimes

Major soil property: andic soil properties (low bulk density, oxalate extractable aluminum and iron, short-range-order minerals compounds - amorphous material, high phosphate sorption capacity) related to volcanic origin of materials.

Diagnostic horizons: cambic

Epipedon: histic, melanic

Major processes: weathering, humification, melanization, leaching, P-fixation

12.2.1) Environmental Conditions

Climate: Andisols form in all soil moisture and all soil temperature regimes, except pergelic. Formation of Andisols in arid regions is limited because of slow weathering of volcanic parent materials.

Vegetation: Andisols develop under a variety of vegetation types ranging from coniferous and deciduous forest, tundra, to shrubs.

Relief: Andisols are found on any topography, however, often they occur on steep slopes formed by volcanic activity.

Parent Material: The vast majority of Andisols formed from pyroclastic deposits (volcanic ejecta) such as ash, pumice, cinders, and lava. Volcanic terrains have a greater variety of rock-types than other surface environment on earth. These terrains include lavas, pyroclastic deposits (from explosions), and deposits from a wide range of sedimentary processes that occur in volcanic terrains. The nature of volcanic material ejected from a volcano varies greatly in time and space and determines the size of particles, composition of materials, and depth of volcanic material deposited. Rapid cooling of the molten materials upon ejection prevents crystallization of minerals with long range atomic order, and the resulting product is vitric material or volcanic glass, which are dominated by amorphous, short-range-order minerals.

Time: Because volcanoclastic material is more weatherable than crystalline materials Andisols do not need very long time periods to form.

12.2.2) Processes

Volcanic ash is chemically/mineralogically distinct from most other soil parent materials. It is composed largely of vitric or glassy materials containing varying amounts of Al and Si. Volcanic glass lacks a well-defined crystal structure (i.e., amorphous) and is quite soluble. Environmental conditions, notably vegetation and soil moisture regime together with chemical composition (Al:Si ratio, base status, pH etc.) strongly influence weathering pathways of volcanic glass.

Allophane and imogolite are common early-stage residual weathering products of volcanic glass and both have poorly-ordered structures. Allophane forms inside glass fragments where Si concentration and pH are high and has a characteristic spherule shape. Imogolite tends to form on the exterior of glass fragments under conditions of lower pH and Si concentration, and has a characteristic thread-like morphology. Both allophane and imogolite may complex with organic matter. In some instances, where organic matter is rapidly accumulating, neither allophane or imogolite form in large amounts. Instead, opaline silica and Al-humus complexes are formed, which appear to inhibit allophane and imogolite formation.

Allophane, imogolite and humus complexes are generally transformed under leaching conditions. In Si-rich environments, halloysite formation is favored, under more basic conditions gibbsite is favored. In non-allophanic ashes 2:1 clays occur although their pathways of formation are not well-defined. Soil moisture regimes influence transformation rates - crystalline clay formation is favored under regimes that include dry seasons (e.g., ustic and drier) and moist regimes (udic) favor persistence of amorphous complexes.

The weathering products such as Al, Fe, and non-crystalline aluminosilicates stabilize humic substances and render them recalcitrant to decomposition, i.e., humic acids are accumulated (humification). Al, Fe-humus complexes are only sparingly soluble and therefore they accumulate at the surface, forming dark thick surface horizon especially under grass vegetation and humid climate (histic or melanic epipedons). The formation of Al, Fe-humus complexes is associated with a change in soil color (black color -organic matter), which is called melanization.

Leaching of base cations is associated with the free drainage of many Andisols, i.e., percolating water leaches the cations out of the soil.

A characteristic of Andisols is their tendency to fix phosphate in a plant-unavailable form. The highest P fixation is found in those Andisols that are fine textured and have relatively high Al/Si ratios. The phosphate is apparently bound by the aluminum via an anion exchange for hydroxyl.

12.2.3) Properties

Andisols are dominated by short-range-order compounds (e.g. allophane, imogolite), including organo-metallic complexes, ferrihydrite, and aluminosilicates, that formed largely in situ.

A typical soil profile show a thick, dark-colored, greasy mineral horizon (e.g. melanic epipedon), a weakly developed cambic subsurface horizon (Bw), and relatively unaltered volcanic or volcanoclastic parent material (C). Histic or melanic epipedons are common in Andisols. A melanic epipedon has to be 30-cm or thicker with a black color and a histic epipedon requires more than 12 % to 18 % organic carbon, depending on clay content. Typically, Fe-Al-humus complexes are found in the A horizon, whereas short-range-order minerals are found in the Bw horizon.

In general, the pH-functional cation exchange capacity (CEC) is high, due to a high surface area of the mineral and organic compounds in Andisols. The %-base saturation is often low because of high percolation and leaching of cations in many Andisols.

Physical soil properties of Andisols comprise a low bulk density, high macroporosity with rapid drainage at low soil moisture tensions, and weak mechanical strength. When they are dry Andisols are highly susceptible to wind erosion.

12.2.4) Classification

To qualify for an Andisol a soil have to have andic soil properties in 60 % or more of the thickness of soil material within 60 cm of the mineral soil surface, or on the top of an organic layer with andic properties. Andic soil materials contain less than 25 % organic carbon (by weight) and, in the fine-earth fraction (> 2 mm), meet one or both of the following:

Al plus 1/2 Fe extractable % (by ammonium oxalate - amorphous phases) totals 2% or more

A bulk density, measured at 33 kPa water of 0.9 g/cm³ or less,

Phosphate retention of 85% or more.

In cases where the particle size is composed of 30% or more particles in the 0.02 to 2.00 mm fraction, the limits listed above are modified to account for less of an active amorphous component in the soil and thus lower limits on P-adsorption and amounts of amorphous Al/Fe.

There are 7 different suborders in the Andisol order distinguished by soil moisture regime, water holding capacity, or organic matter content:

Aquands: Aquands are Andisols that have a histic epipedon or have aquic conditions which result in redoximorphic features. Aquands occur locally in depressions and along floodplains where water tables are at or near the soil surface for at least part of the year.

Cryands: They are defined as Andisols with cryic soil temperature regimes. These soils are the Andisols of high latitude (e.g. Alaska, Kamchatka) and high altitude (e.g. Sierra Nevada in the U.S.).

Torrands: They are defined as Andisols with aridic soil moisture regimes. Vegetation is mostly desert shrubs.

Xerands: They are defined as Andisols with xeric soil moisture regimes.

Vitrands: They are Andisols that have a low water-holding capacity. Vitrands are restricted to ustic and udic soil moisture regimes.

Ustands: They are defined as Andisols with ustic soil moisture regimes. These are the Andisols of the intertropical regions that experience seasonal precipitation distribution.

Udants: They are defined as Andisols with udic soil moisture regimes (most extensive Andisols).

Shallow Andisols that have a lithic contact within 50 cm either of the mineral soil surface, or of the top of an organic layer with andic soil properties, whichever is shallower are denoted 'Lithic' (e.g. Lithic Cryaquands, Lithic Haploxerands).

Andisols with very low base status (that have extractable bases plus KCl-extractable Al3+ totaling less than 2.0 cmol(+)/kg in the fine-earth fraction) are named 'Acrudoxic' (e.g. Acrudoxic Placudands), low base status soils that have more than 2.0 cmol(+)/kg Al3+ (by KCl) in the fine-earth fraction are named 'Alic' (e.g. Alic Epiaquands), and Andisols that have extractable bases plus KCl-extractable Al3+ totaling less than 15.0 cmol(+)/kg are labeled 'Dystric' (e.g. Dystric Haplustand), whereas Andisols with high base status (that have a sum of extractable bases of more than 25.0 cmol(+)/kg in the fine-earth fraction) are named 'Eutric' (e.g. Eutric Placudands).

Soil moisture regime is used to distinguish Andisols at the great group and subgroup level: xeric (e.g. Xeric Vitricryands), ustic (e.g. Ustivitrands), udic (e.g. Udivitrands), aquic (e.g. Aquic Ustivitrands), and 'oxyaquic', i.e., soils that are saturated with water, in one or more layers within 100 cm of the mineral soil surface, for 1 month or more per year in 6 or more out of 10 years (e.g. Oxyaquic Vitricryands). Andisols with episaturation, i.e., the soil is saturated with water in one or more layers within 200 cm of the mineral soil surface and also has one or more unsaturated layers with an upper boundary above 200 cm depth, below the saturated layer(s) (a perched water table) are denoted by 'Epi' (e.g. Epiaquands).

Epipedons are used to classify 'Melanic' and 'Histic' Andisols (e.g. Melanaquands, Histic Cryaquands). Andisols, which show a layer 10 cm or more thick with characteristics of a mollic epipedon and more than 3 % organic carbon are named 'Thaptic' (e.g. Thaptic Cryaquands). Andisols, which have more than 6.0 percent organic carbon and colors of a mollic epipedon throughout a layer 50 cm or more thick within 60 cm either of the mineral soil surface, or of the top of an organic layer with andic soil properties, whichever is shallower are named 'Pachic' (e.g. Pachic Melanoxerands). Generally, Pachic is term to identify a thickened mollic epipedon.

Water retention characteristics are used to classify Andisols at the great group and subgroup level. Andisols that have a 1500-kPa water retention of less than 15 % on air-dried samples and of less than 30 % on undried samples dominant in the upper 60 cm are named 'Vitric' (e.g. Vitraquands, Vitric Haplocryands). Andisols that have, undried, a 1500-kPa water retention of 70 % or more throughout a layer 35 cm or more thick within 100 cm either of the mineral soil surface, or of the top of an organic layer with andic soil properties, whichever is shallower are named 'Hydric' (e.g. Hydrocryands, Hydric Melanaquands).

Diagnostic horizons are used to classify 'Petrocalcic', i.e., an indurated calcic horizon (e.g. Petrocalcic Vitritorrands), 'Calcic', i.e., a horizon with secondary accumulation of carbonates (e.g. Calcic Vitritorrands), 'Alfic', i.e., the presence of an argillic or kandic horizon (e.g. Alfic Vitrixerands), 'Ultic', i.e., the presence of an argillic or kandic horizon plus a base saturation (by sum of cations) of less than 35 percent throughout its upper 50 cm (e.g. Ultic Haploxerands), 'Oxic', i.e., an horizon with sandy loam or finer and a high content of low-charge 1:1 clays (e.g. Oxic Haplustands), 'Placic', i.e., a 2 to 10-mm thick dark reddish brown to black iron or manganese pan (e.g. Placaquands), or presence of a duripan, i.e., a horizon cemented by illuvial silica (e.g. Duric Placaquands).

12.25) Distinguishing Characteristics

The geographic distribution of Andisols is closely related to volcanoes that are active or have been active during the Holocene. Soils formed on older volcanic deposits are dominated by crystalline aluminosilicates or the material is mixed with other parent material, therefore, the criteria to qualify for Andisols are not given. Andisols are limited to soils formed on volcanic materials that have weathered enough to produce short-range-order organo-metallic and aluminosilicate compounds, but that have not weathered to the point where crystalline materials predominate or where significant transformations has occured.

Soils from a variety of soil orders may be found on volcanic terrains, but Andisols are almost exclusively confined to the pyroclastic materials. Soils developed in pyroclastic and other fragmental volcanic materials occupy only about 0.8% of the earth's surface. However, because of their very distinct characteristics, they are recognized as a separate soil order in soil taxonomy.

Most Andisols are formed from specific parent material (volcanic ejecta). Few soil orders, except Histosols, have such a specific range of parent materials and depositional environments.

The separation between Spodosols and Andisols is difficult, because short-range order aluminosilicates and organo-metallic complexes occur in the B horizons of soils of both orders. A distinguishing characteristic is the transformations in situ and lack of intensive illuviation of these compounds in Andisols.

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12.3.1) Aridisols

Summary:

Vegetation: Present vegetation - species adapted to arid climate; former vegetation - not specified

Climate: Arid regions (cold and warm deserts); cryid or frigid - thermic or hypothermic soil temperature regime

Soil moisture regime: aridic, torric

Major soil property: crusts, desert pavement, accumulation of material such as clay, CaCO₃, or salts

Diagnostic horizons: cambic, argillic, calcic, petrocalcic, natric, gypsic, petrogypsic, salic

Epipedon: ochric, anthropic

Major processes: weathering, silication, calcification, hardening, salinization, solodization, deflation

12.3.1) Environmental Conditions

Climate: Arid regions including cold polar, cool temperate and warm deserts, which occupy about 36% of the land surface based on climate and about 35% based on vegetation. Aridisols may also occur in semi-arid areas outside of zones broadly classified as arid - e.g. where local conditions impose aridity - steep, south-facing slopes in N-hemisphere, physical properties that limit water infiltration. Aridisols are classified on the basis of their soil moisture regime (more specifically referenced to the soil moisture control section), which is dry in all parts >50% of the time in most years, and not moist for as much as 90 consecutive days when the soil is warm enough (>80C) for plant growth. In an aridic (&torric) soil moisture regime, potential evapotranspiration greatly exceeds precipitation during most of the year. In most years, little or no water percolates through the soil. This hydrologic regime has a distinctive influence on the development of such soils. During Quarternary time, most deserts have changed back and forth from cooler-moister, to warmer-more-arid climates, therefore, change in climatic conditions have to be considered when talking about Aridisols.

Vegetation: Present vegetation comprises species adapted to dry climate such as cactus (Cactaceae), mesquite (Prosopis), creosotebush (Larrea), Yucca (Yucca), sagebrush (Artemisia), or shadscale (Atirplex). Species have to live in an environment with sparse organic matter, low microbial population, and lack of nutrients such as nitrogen and phosphorous. Use of Aridisols is limited because of lack of water, low biotic activity and low nutrient status. Irrigation can be used to improve crop growth on Aridisols, but issues of internal permeability, salinization and alkalization arising from the irrigation water should be addressed.

Relief: They form on plain terraces and on steep slopes

Parent Material: They occur on land surfaces of Pleistocene or greater age, therefore, they occur on parent material such as crystalline rocks. Aridisols do develop on fluvial and eolian materials, extensively in large deserts such as the Gobi, Namib, or Kalahari desert. Aridisols occur on gypsiferous material formed from marine sedimentary rocks, on unconsolidated sediments, or limestone.

Time: Most Arisisols are found on landscapes that are relatively old and stable (up to more than million years).

12.3.2) Processes

In arid regions chemical and physical reactions operate in the same way as in humid regions, although with less intensity and at shallower depths. Physical weathering such as weathering due to the crystallization of salts or thermal expansion and contraction of the constituents minerals is favored in arid regions. Chemical weathering is retarded because of lack in water although the importance of chemical weathering has been proved in many pedological research studies.

Because of sparse vegetation and low humification rates little humus accumulates in the typic Aridisol, i.e., in many Soils ochric epipedons are found.

Evidence of leaching below the average depth of water storage is commonly observed in Aridisols and is explained by: (i) more humid paleoclimates, and/or (ii) the influence of occasional, exceptionally large precipitation events.

Examination of soil forming processes in arid zones invariably requires consideration of possible paleoclimatic influences (i.e. some features in the soil may have formed under conditions quite different from those operating at present), the periodic occurrence of large precipitation events that can punctuate the otherwise dry environment of these regions and local variation in factors that prescribe soil genesis. It seems to be contradictory that horizons accumulated with clay, sodium, salts, gypsum, or silica occurs in Aridisols which is associated with illuviation of those materials. A prerequisite for leaching or eluviation/illuviation is rainfall. Aridisols occur on landscapes that are more than one million years old, a time scale that has allowed for development of accumulations of clay, carbonates, and silica.

A predominant influence on soil formation in arid zones is that potential evapotranspiration greatly exceeds precipitation during most of the year. Thus, drainage of water through the soil is limited. The occurrence of horizons enriched in secondary minerals is strongly controlled by the distinct hydrology of arid regions which favors limited leaching from the solum. The source of secondary enrichment may be atmospheric, from groundwater and weathering of soil minerals. Thus, in evaluating the occurrence and significance of enrichment it is important to evaluate mineral source(s), hydrology and relative age of the soil-landscape. Relative age is important because many of the processes of enrichment are necessarily time dependent. The processes associated with the accumulation of materials in Aridisols are: (i) lessivage or eluviation/illuviation of clays - argillic horizons, (ii) silication, i.e., the accumulation of silica - duripans, (iii) calcification, i.e., the accumulation of CaCO₃ - calcic or petrocalcic horizons. The hardening of soil material may lead to a decrease in volume of voids by infilling with salts and silica. This process is responsible for the formation of petrocalcic, petrogypsic horizons or duripans.

The composition of the initial material in which some Aridisols are forming that contain argillic, natric and calcic horizons does not readily explain their internal enrichment in phyllosilicate clays or carbonates. Thus, it has been suggested that aeolian inputs may explain this enrichment. However, in some settings subsurface water enriched with clays and especially carbonates may also account for formation of Bt, Btn and Bk(m) horizons.

Soluble salt accumulation (salinization) is usually associated with depressional landscape positions, such as playas, and a source of saline ground water. Saline accumulations such as sulfates and chlorites of Ca, Mg, K, and Na are also associated with some irrigated agricultural areas. The accumulation of Na salts is called solodization. The accumulation of salts is often associated with a natural or artificially high water table (irrigation) feeding capillary water to, or near to the soil surface where salt accumulates upon evaporation. Salinization of irrigated agricultural areas in semi-arid and arid areas is a problem that has plagued the human race since the dawn of 'civilization'.

Rubification, i.e, the reddening of the soil due to oxidation of Fe-bearing minerals is often observed in Aridisols. Soil moisture conditions in arid regions favors oxidation over redoxidation.

The processes deflation and deposition are responsible for the development of 'desert pavement' (surface pebble layers). Deflation is the sorting out, lifting, and removal of loose, dry, fine grained soil particles by the turbulent action of the wind. It is assumed that vertical sorting of stones, i.e., the gradual upward migration of pebbles that have been heaved up by swelling clay, with local supplement action by frost, growth of salt crystalls, and expansion of entrapped air, with preferential collapse of fines into voids too small to accept pebbles during subsequent desiccation support developing a surface pebble layer. The pavement serves as a dust trap but inhibits loss of soil particles by wind erosion.

12.3.3) Properties

Pedogenic processes produced numerous soil features associated with dry climate: (i) crusts, (ii) desert pavement, (iii) cambic horizons, (iv) argillic horizons, (v) natric horizons, (vi) carbonate accumulations (calcic and petrocalcic horizons), (vii) duripans, (viii) salic and gypsic horizons.

(i) Crusts are surficial layers generally less than 10- to 20-cm thick. They are dominated by fine material composed of compound polygonally prismatic and platy fragments that are coherent when dry. When silt particles dominate they may exhibit vesicular porosity. The distinctive morphology of crusts probably results from repeated wetting and drying, entrapped air during wetting likely accounts for vesicle formation. The impact of soil crusts to infiltration is high, because crusts slow the permeability to water in contrast to rapid infiltration that happens in uncrusted soils.

(ii) Desert pavement is a surface pebble layer. Several pathways, which probably operate over tens of thousands of years may account for the same end product, these include: (a) removal of fine particles from surface by wind/water, leaving a 'lag' of coarser fragments, (b) vertical sorting of coarse fragments towards surface via wet/dry, freeze/thaw, and uplift by swelling clay, salt growth, air entrapment below, concomitant downward movement of fines, and (c) over time, pavement becomes 'flat' and covered with a thin veneer of 'varnish', composed of Fe, Mn and silicate clays, microbiological processes may contribute to its formation in some settings.

(iii) Cambic horizons (Bw) have a texture of loamy very fine sand or finer and contain some weatherable minerals. They are characterized by the alteration or removal of mineral material as indicated by mottling or gray colors, stronger chromas or redder hues than in underlying horizons. Carbonates are leached out in low-carbonate parent material, whereas in highly calcareous parent materials, evidence of carbonate removal may take the form of carbonate coatings on undersides of pebbles in the cambic horizon.

(iv) Argillic horizons (horizons enriched in clay - Bt) may form due to in situ weathering or illuviation of clay in the Bt horizon. Carbonates have to be leached before illuvial clay can accumulate in argillic horizons because clay flocculates in the presence of carbonates.

(v) Natric horizons (n in combination with any master horizon) satisfy the requirements of an argillic horizon, but also has prismatic, columnar, or blocky structure, and > 15 % saturation with exchangeable Na⁺. Sodium has characteristic effects of soil physical properties. In the presence of Na clay and humus disperse into individual hydrated particles instead of remaining flocculated. Sodic soils readily lose their structure, deflocculation occurs, the soil structure is destroyed, and pores clog at the surface, therefore the permeability at the surface is reduced.

(vi) Calcic and petrocalcic horizons (Bk and Bkm or Ck and Ckm) show an accumulation of carbonate and they commonly lie below argillic and cambic horizons in Aridisols. Generally, carbonates are leached out before clay are translocated to form an argillic horizon. Calcic horizons develop over time into petrocalcic horizons, which are indurated calcic horizons cemented by calcium carbonate and in some places with magnesium carbonate. Petrocalcic horizons cannot be penetrated with a spade or auger when dry and the cemented layer is impenetrable to roots.

(vii) Duripans (Bqm or Cqm) are subsurface soil horizons cemented by illuvial silica, usually opal or microcrystalline forms, to the degree that less than 50 % of the volume of air-dry fragments will slake in water or HCl. Often the duripans in Aridisols have a considerable content of calcium carbonate and can be distinguished only by the test described above.

(viii) Salic horizons (Bz or Cz) are enriched with secondary salts more soluble than gypsum. A salic horizon is 15 cm or more in thickness and contains at least 20 g/kg salt. A gypsic horizon (By or Cy) is enriched of secondary CaSO₄, is > 15 cm thick, and has at least 50 g/kg more gypsum than the C horizon. High pH values (> 9) are associated with nutrient deficiencies or toxicities induced by high pH. Calcium is immobilized because high pH promotes the formation of carbonate from CO₂, and carbonated precipitates with Ca, as CaCO₃. A high pH also affects the sorption behavior of these cations in the soil.

Most Aridisols show a low permeability because of the presence of accumulated or cemented layers. The nutrient status of often low, however, supplies of micronutrients are usually abundant, although they may not be available because of the high pH.

12.3.4) Classification

An criterion of salinity is the electrical conductivity (EC) of the saturation extract. Soils are considered saline if their EC exceeds 4 dS/m. Usefuls measures of sodicity are the exchangeable sodium percentage (ESP) and the sodium adsorption ratio (SAR). The ESP is the exchangeable Na expressed as a percentage of the total exchangeable cations. The SAR is a modified ratio of Na to other major cations (Ca and Mg) in the saturation extract.

ESP = 100 (exch. Na) / (exch. Na + exch. Ca + exch. Mg)

(the cation amounts are expressed in mols of charge (gram equivalents).

SAR = (Na) / Wurzel (Ca * Mg) /2

(the cations are expressed in mols of charge (gram equivalents) per liter.

Three classes of salt-affected soils are recognized and defined in terms of electrical conductivity and exchangeable sodium percentage:

Saline: Has a saturation extract conductivity of 4 mmhos/cm or greater and has a low exchangeable sodium percentage.

Sodic: Has an exchangeable sodium percentage of 15% or greater but has a low salt content.

Saline-sodic: Has both the salt concentration to qualify as saline and sufficient exchangeable sodium to qualify as sodic.

The requirements to classify for an Aridisol are:

an aridic soil moisture regime

an ochric or anthropic epipedon, and

one or more of the following subsurface horizons within 100 cm of the soil surface: argillic, cambic, natric, salic, gypsic, petrogypsic, calcic, petrocalcic, or duripan.

The Aridisols are composed of 7 suborders distinguished by (i) soil temperature regime, and (c) occurrence of particular diagnostic horizons:

Cryids: Cryic soil temperature regime, MAT higher than 0^oC but less than 8^oC.

Salids: Salic horizon that has its upper boundary within 100 cm of the surface.

Durids: Duripan that has its upper boundary within 100 cm of the surface.

Gypsids: Gypsic of petrogypsic horizon that has its upper boundary within 100 cm of the surface and lacks an overlying petrocalcic horizon.

Argids: Argillic or natric horizon that has its upper boundary and does not have petrocalcic horizon within 100 cm of the surface.

Calcids: Calcic or petrocalcic horizon that has its upper boundary within 100 cm of the surface.

Cambids: Other Aridisols

12.3.5) Distinguishing Characteristics

Soils with a dominance of attributes not specifically associated with arid-zone soil forming processes are assigned to other pertinent Orders even though their hydrologic regime is the same as that used for Aridisols. In such instances, the prefix 'Torr' or 'Torri' is used to identify these soils. This prefix refers to the Torric soil moisture regime which is identical to the Aridic soil moisture regime and is defined as:

Dry in all parts more than half the time that the soil temperature at a depth of 50 cm is above 5^oC

Never moist in some or all parts for as long as 90 consecutive days when the soil temperature at a depth of 50 cm is at or above 8^oC

Other soil orders such as the Entisols and Mollisols use the prefixes 'Torric', 'Ustic', and 'Xeric' to classify soils developed in regions with dry climate. Many Aridisols are closely associated with the occurence of Entisols.

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12.4.1) Entisols

Summary:

Vegetation: not specified, bare soil

Climate: pergelic to hypothermic

Soil moisture regime: dry to aquic

Major soil property: featureless soil bodies

Diagnostic horizons: typically absent, albic

Epipedon: ochric

Characteristic: little or no evidence of soil development

12.4.1) Environmental Conditions

Climate: Entisols may form in a variety of climates. For example, an arid or pergelic climate may limit the amount of soil development to inhibit the formation of other soil orders. A pronounced saturation of the soil profile or even submergence for long enough periods inhibit soil development and soils persist in the Entisol order.

Vegetation: Harsh environments may limit root and plant growth due to consolidated highly resistant bedrock, infertility or toxicity of initial material, submergence, or high erosion rates. When adequately fertilized and their water supply is controlled some Entisols can be used in agriculture (rangeland, grazing land). However, restrictions on their depth, clay content, or water balance limit intensive use of large areas of these soils. Some Entisols are intensively farmed, for example, river alluvium Entisols.

Relief: Entisols may be present on very steep slopes on hard bedrock where soil formation is inhibited. Mass movement may remove material from such an area as fast or faster than most pedogenic horizons form. Other Entisols form on level to gently sloping relief in deposited material such as alluvium or colluvium.

Parent Material: Entisols are on land surfaces that are very young (alluvium, colluvium, mudflows), extremely hard rocks (e.g. Orthents), or disturbed material (e.g. mined land, highly compacted soils, toxic material). They also occur on deep bodies of water and glaciers which are transitions between 'soils' and 'not soils'. Psamments are Entisols formed in sandy material and are found in Alabama and Georgia and used mostly for grazing. They are also typical of the shifting sands of the Sahara Desert and Saudi Arabia. In serpentine barrens, Entisols may be associated with bedrock outcrops. Entisols may be also associated with salt flats.

Time: Shortness of time since exposure of initial materials to the active factors of soil formation limits soil development. Fresh lava flows, marine or lacustrine deposits newly exposed by uplift of land or by lake drainage, provide sites for very young soils. Human activity may force the formation of Entisols. Deforestation may induce soil erosion where highly eroded, shallow Entisols remain. For example, wide areas are formed due to deforestation and erosion in southern Europe and in Southern America.

12.4.2) Processes

The characteristic of Entisols is that there is little or no evidence of soil development. They form a transition between the other soil orders of Soil Taxonomy and non-soil material such as bare rock, deep water or ice at the surface of the earth. In chapter 15.1.1. the environmental conditions for Entisols were described which often inhibit soil formation, i.e., some factors slow down soil forming processes. For example, submerged or waterlogged soils exclude oxidation and retard weathering. Sparse vegetation results in low litter amounts which retards the accumulation of organic matter in the topsoil. The high compactness of rock may inhibit the penetration of roots and therefore inhibit plant growth.

The impact of most soil forming processes is not great enough to produce soil features recognized as diagnostic for other soil orders. Entisols may be 'climax soils' which are in equilibrium with the environment, they may form by soil degradation (e.g. soil erosion) from other soil orders, or they may develop from 'non-soil areas'.

12.4.3) Properties

Entisols are soils without properties that are diagnostic of the other orders. Besides an ochric epipedon and an albic diagnostic horizon they may have some fragments of diagnostic horizons that are not arranged in any discernible order.

12.4.4) Classification

In the Entisols order there are 5 suborders:

Aquents: Entisols which are permanently or seasonally wet (saturated) are mapped as Aquents. They show pronounced redoximorphic features.

Arents: They are better drained than Aquents (lacking their redoximorphic features) and exhibit fragments of diagnostic horizons below the Ap horizon. Arents are deeply disturbed by farming, mining, or construction.

Psamments: The soil texture of Psamments is loamy fine sand or coarser. They are subject to movement by wind if dry.

Fluvents: The soil texture of Fluvents is loamy and clayey (finer in texture than loamy fine sand). They are found on stratified alluvial material.

Orthents: The soil texture of Orthents is loamy and clayey. They are better drained than Aquents with a regular decrease in content of organic matter with depth.

The suborders are subdivided into great groups on the basis of several factors: mean annual soil temperature and range of soil temperature, content of sand and quartz, stratification, presence of sulfidic material, and low-bearing capacity.

Hydraquents are formed in sediments that have accumulated under water and remained continously submerged. To qualify for a Hydraquent the n-value must be > 0.7. The n-values is used to define the grams of water associated with 1 gram of clay and obtained from the relationship:

A = nL + nbH + pR

where

A: water content per 100 g of dry soil

L: clay percentage

H: organic matter percentage

R: non-clay content

b: ratio of water retention by organic matter to clay (commonly taken as 3)

p: water associated with the non-clay (commonly 0.2)

In many subgroups of the Entisol order soils with aquic conditions some time in most years, redox depletions with a chroma of 2 or less are considered (e.g. Aquic Cryopsamments). In other Entisols that are saturated with water, in one or more layers within 100 cm of the mineral soil surface, for 1 month or more per year in 6 or more out of 10 years the term 'oxyaquic' is used (e.g. Oxyaquic Cryopsamments). Influence of soil temperature is considered on great group and subgroup level using designations such as ustic, xeric, torri, or udic.

Accumulation of iron sulfides (FeS₂) are found in lagoonal soils or distrubed soils in coal mine spoil. They are classified on great group level (e.g. Sulfquents) and subgroup level (e.g. Sulfic Hydraquents, Sulfic Fluvaquents). Some Entisols show mollic characteristics (e.g Mollic Cryofluvents, Mollic Ustifluvents).

Shallow soils with a lithic contact within 50 cm of the soil surface are common in the Entisol order (e.g. Lithic Cryopsamments, Lithic Quarzipsamments, Lithic Xerorthents).

Soils which show a fine-earth fraction containing 30 percent or more particles 0.02 to 2.0 mm in diameter of which 5 percent or more is volcanic glass, and [(Al plus 1/2 Fe, percent extracted by ammonium oxalate) times 60] plus the volcanic glass (percent) is 30 or more throughout one or more horizons with a total thickness of 18 cm or more within 75 cm of the mineral soil surface, are grouped as 'vitrandic' (e.g. Vitrandic Xerofluvents, Vitrandic Cryorthents). Entisols that have, throughout one or more horizons with a total thickness of 18 cm or more within 75 cm of the mineral soil surface, a fine-earth fraction with both a bulk density of 1.0 g/cm³ or less, measured at 33 kPa water retention, and aluminum plus 1/2 iron percentages (by ammonium oxalate) totaling more than 1.0 are grouped as 'andic' (e.g. Andic Cryofluvents).

12.4.5) Distinguishing Characteristics

Entisols are transitions between the other soil orders and non-soils. Non-soils are very unstable areas either because of water erosion (e.g. badlands, beaches, riverwash), wind erosion (e.g. dunes), areas impenetrable to roots (e.g. rock outcrops), areas that restrict plant growth (e.g. salt flats, slickens, toxic areas), or areas that are too cold to support plant growth. Many young soils are excluded from Entisols because presence of a mollic epipedon. Because cambic horizons cannot occur in soil materials coarser than very fine sand weathered sandy soils are grouped in the Entisol order.

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12.5.1) Gelisols

Summary:

Vegetation: lichens, moss, liverwort, sedges, grass

Climate: pergelic

Soil moisture regime: variety of soil moisture regimes

Major soil property: accumulation of organic matter, special features formed by cryoturbation

Diagnostic horizons: -

Epipedon: histic

Major processes: cryoturbation

Characteristics: soils that contain within 200 cm of the ground surface permafrost (permanently frozen ground)

12.5.1) Environmental Conditions

Climate: Gelisols develop in climatic regions where temperatures continuously are at or below 0° C (e.g. alpine, polar regions) - pergelic temperature regime. They occur in arid regions and areas with effective precipitation. Permafrost (i.e., permanently frozen soil) is a characteristic environmental factor for the development of Gelisols. The distribution of permafrost comprise two zones: (i) continuous permafrost: the zone at the highest latitudes and elevations where permafrost is ubiquitous; the southern boundary corresponds to the -7 °C isotherm. (ii) discontinuous (sporadic) permafrost: the zone in which permafrost occurs only in some materials; the southern boundary corresponds to the 0 to -2 °C isotherm.

Vegetation: Cold climate inhibits the growth of many species; only species adapted to harsh cold environmental conditions can survive, for example, lichens, sphagnum moss, liverwort, sedges, grass, Picea Betula, Salix.

Relief: There is no limitation in relief for the formation of Gelisols.

Parent Material: Gelisols can form in any parent material. Often they form in glacial drift material.

Time: At very low temperatures (< 0 to -70° C) pedogenic processes are slowed down, i.e., soil development is very slow. Many soils in cold regions are very old, e.g. in Antarctica millions of years.

12.5.2) Processes

Definition of Permafrost (permanently frozen soil): A condition existing below the ground surface, irrespective of texture, water content, or geologic character, in which the temperature of the material remained below 0 °C continuously for two or more years. The soil above the permafrost that thaws in the summer is referred to as the 'active layer'.

Cryopedogenesis is the sum of all subprocesses occuring in cryogenic soils, including compaction (desiccation), displacement (alignment, rotation, sorting, inclusions), and pore formation.

Cryoturbation (frost churning), which is mixing of soil due to freezing and thawing, results in the disruption of horizons, displacemetn of soil material, the incorporation of organic matter into lower horizons, and the orientation of stones in the soil profile. Cryoturbation in the soil profile is manifested by irregular and broken horizons and textural bands, involutions, organic matter accumulation on the permafrost table, oriented stones, silt caps and accumulations, and deformed soil material associated with movements due to ice- and sand-wedge growth.

At 0 °C, the increase in volume with the conversion from water to ice is 9 percent. When the moisture contained in rocks freezes, and the accompanying internal pressures are sufficiently great to exceed the strength of the rock, the rock ruptures (thermal cracking). Freezing, associated with an expansion of soil water, and thawing, associated with a contraction or collapse of soil layers, result in a new terrain called hilly thermokarst.

Frost cracking of ground into polygons results from shrinkage of the ground during cold dry winters. Water from the active layer in summer seeps into cracks and freezes, starting the growth of vertical ice wedges. With the approach of winter, refreezing of the moist soil may be by simultaneous, slow upward extension of cementing ice above the permafrost table and downward extension of surface freezing ground. Subsoil between these two approaching freezing fronts develops a massive condition from centuries of this seasonal compaction.

Patterned ground formation is a process which results in special features such as circles of stones, nets, polygons, steps, or stripes.

During summer periods the upper few centimeters or several decimeters of a pedon thaws. On slopes (> 1 % gradient) the upper soil layer, which is highly saturated with meltwater flows above the upper surface of the permafrost, called permafrost table. This process is called solifluction. Within solifluction layers stones are transported downhills to depression areas.

At low temperatures, particles of snow are as hard as grains of bedrock and can 'sandblast' ventifacts (wind erosion).

Pedogenic processes in very cold environments such as weathering, transformations and translocations of mineral and organic materials are slow. Because decomposition is retarded in cold climate and organic matter is accumulated and histic epipedons are formed.

12.5.3) Properties

The dark black epipedon in many Gelisols is classified as histic, which is formed by low decomposition rates.

Freezing and thawing in the zone above the permafrost table forms features such as unsorted and sorted circles of stones, nets, polygons, steps, stripes, mounds, pingos, peat rings, and beaded drainage patterns. Freezing and thawing forms platy and vesicular structures in surface mineral horizons, and blocky, prismatic, and massive structures in subsoil. Ice lenses may form close to the permafrost table.

Because pedogenic processes are retarded in cold regions the soil landscapes with Gelisols are fragile. It takes very long time periods to wipe out the impact of disturbances, for example, produced by human activity such as extracting geologic materials or digging of soil pits.

12.5.4) Classification

Gelisols occur in arctic regions such as Antarctica, Russia, Canada, Alaska. An estimated 13.4 % (18 million km²) of soils of the planet are occupied by permafrost. The basic requirement to form Gelisols is:

the presence of permafrost within 100 cm of the soil surface; or

gelic materials within 100 cm of the soil surface and permafrost within 200 cm of the soil surface.

Gelic materials are mineral or organic materials that have evidence of cryoturbation and/or ice segregation in the active layer (seasonal thaw layer) and/or upper part of the permafrost.

New soil horizon symbols are:

jj: cryoturbation

ff: dry permafrost

Wfm: glacic horizon (> 75 % ground ice in a layer >= 30 cm thick)

There are three different suborders of Gelisols:

Histels: Histels are organic soils similar to Histosols exept that they have permafrost within 2 meters below ground surface. They have 80 % or more organic materials from the soil surface to a depth of 50 cm or to a glacic layer or densic, lithic, or paralithic contact, whichever is shallowest. These soils occur predominantely in Subarctic and Low Arctic regions of continuous or widespread permafrost. Less than one-third of the active layer (the soil between the ground surface and a permafrost table) or an ice layer which is at least 30-cm thick has been cryoturbated.

Turbels: Turbels are soils that show marked influence of cryoturbation (more than one-third of the active-layer portion of the pedon) such as irregular, broken, or distorted horizon boundaries and involutions and areas with patterned ground. They commonly contain tongues of mineral and organic horizons, organic and mineral intrusions and oriented rock fragments. Organic matter is accumulated on top of the permafrost and ice wedges are a common features in Turbels. These soils occur primarily in the zone of continuous permafrost.

Orthels: Orthels are soils that show little or no cryoturbation (less than one-third of the pedon). Patterned ground (except for polygons) generally is lacking. These soils occur primarily within the zone of discontinuous permafrost, in alpine areas where precipitation is greater than 1400 mm per year.

The decomposition stage of organic material (fiber in the OM) distinguishs Gelisols on the great group and subgroup level. 'Fibric' (e.g. Fibristels), 'Hemic' (e.g. Hemistels), and 'Sapric' (e.g. Sapristels) organic material is considered to distinguis Gelisols at the great group level. 'Sphagnic' indicates the presence of sphagnum moss which influences soil development. In 'Humic' Gelisols a mollic, umbric, or histic epipedon is present (e.g. Humiturbels), in 'Umbric' Gelisols there is an umbric epipedon (e.g. Umbriorthels), and in 'Mollic' Gelisols show a mollic epipedon (e.g. Molliorthels).

'Glacic' (> 75 % ground ice in a layer >= 30 cm thick) is used at great group and subgroup level to classify Gelisols (e.g. Glacic Folistels, Glacic Aquaturbels).

The presence of calcium sulfate defines 'Gypsic' Gelisols (e.g. Gypsic Anhyturbels), soluble salts define 'Salic' Gelisols (e.g. Salic Anhyturbels), carbonates defines 'Calcic' Gelisols (e.g. Calcic Anhyturbels).

Gelisols with an argillic diagnostic horizon (e.g. Argiorthels) or a spodic horizon (Spodic Psammiorthels) are also considered in the classification of Gelisols.

Gelisols developed in sandy parent material are designated by the term 'Psammentic' (e.g. Psammentic Aquorthels, Psammiturbels).

'Sulfuric' Gelisols show a mineral or organic horizon that has a pH < 3.5, inhibits growth of plant roots, and has yellow mottles of jarosite (e.g. Sulfuric Aquaturbels).

There are two general types of permafrost: (i) dry permafrost, which contains insufficient interstitial water to cement the soil matrix, and (ii) wet frozen or ice-cemented permafrost, which contains sufficient moisture to cement the soil matrix. Gelisols formed in dry permafrost regions are classified as 'Anhy' (e.g. Anhyturbels, Anhyorthels). The denotion 'Foli' is used to classify Gelisols which are saturated with water only a few days each year (e.g. Folistels). Gelisols, which are saturated seasonally are classified as 'Aquic' (e.g. Aquistatels, Aquic Umbriorthels).

Gelisols formed in volcanic material (e.g. volcanic glass) which does not meet the criteria of the Andisol order are considered by the formative element 'vitrandic' (e.g. Vitrandic Molliorthels) or 'andic' if the fine-earth fraction exhibits a bulk density of 1.0 g/cm³ or less (e.g. Andic Andic Molliorthels).

Soil depth distinguishs between 'Lithic' - shallow (e.g. Lithic Anhyorthels) and 'Cumulic' - accumulated (e.g. Cumulic Umbriorthels) Gelisols. Gelisols, which have a mineral layer 30 cm or more thick that has its upper boundary within the control section below the surface tier are classified as 'Terric' (e.g. Terric Sapristels).

12.5.5) Distinguishing Characteristics

In the past, Soil Taxonomy has identified Gelisols as pergelic subgroups of Entisols, Inceptisols, Histosols, Mollisols, and Spodosols.

Gelisols and Histosols show high organic matter contents, whereas Gelisols are limited to cold climate.

Further Reading

Bockheim, J.G., and C. Tarnacai. 1998. Recognition of cryoturbation for classifying permafrost-affected soils. Geoderma, 81(3-4): 281-293.

Campbell, I.B., and G.G.C. Claridge. 1987. Antarctica: Soils, weathering processes and environment. Develop. in Soil Sci. 16, Elsevier, N.Y. 368 pp.

Gilichinsky, D.A. (ed.) 1992. Cryosols: the effects of cryogenesis on the processes and pecularities of soil formation. Proc. 1st Internat. Conf. on Cryopedology, Nov. 10-14, 1992, Russian Acad. Sci., Pushchino.

Rieger, S. 1983. The genesis and classification of cold soils. Academic Press, N.Y 230 pp.

Tedrow, J.C.F. 1977. Soils of the polar landscapes. Rutgers Univ. Press, New Brunswick, N.Y. 638 pp.

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12.6) Histosols

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12.7) Inceptisols

Summary:

Vegetation: not specified

Climate: variety of climates excluding arid

Soil moisture regime: variety of soil moisture regimes except aridic

Major soil property: few diagnostic features

Diagnostic horizons: cambic but no spodic, argillic, kandic, natric, and oxic horizon

Epipedon: ochric, umbric, histic, or plaggen (mollic)

Major processes: mass movement, soil erosion, deposition

Characteristics: Environmental conditions inhibit soil-forming processes

12.7.1) Environmental Conditions

Climate: Inceptisols form under a variety of climates except aridic conditions. Soil moisture regimes can be variable ranging from poorly drained soils to well-drained soils on steep slopes. By definition, Inceptisols cannot have an aridic soil moisture regime. Climate which inhibits soil development such as low temperatures or low precipitation favors the development of Inceptisols. The suborder of Aquepts requires higher soil moisture conditions compared to the other suborders of Inceptisols.

Vegetation: Inceptisols occur under forested ecosystems, grassland or agricultural land. Although Inceptisols are not limited to forest environments, most of the soils classified into this order occur under forest ecosystems. Some Inceptisols (Umbrepts) were probably developed under prairie vegetation. Present use may be restricted by the shallowness of the solum (e.g. on steep slopes) or by poor drainage (e.g. in depression areas). Those Inceptisols are suited only to forestry and/or wildlife habitat.

Relief: Most Inceptisols develop on steep slopes where soil erosion removes parts of the topsoil continuously. Other Inceptisols are formed on convex toeslope areas where slope is level to gently rolling. These Inceptisols develop in deep colluvium where sediment has been / is deposited.

Parent Material: Inceptisols are extensive in areas of glacial deposits or on recent deposits in valleys or deltas. Where they occupy upland positions on young geomorphic surfaces, both primary and secondary minerals are present. Most Inceptisols are present on geologically young sediments (e.g. alluvium, colluvium, loess). Parent materials which is highly calcareous or resistant to weathering inhibit soil development but favor the development of Inceptisols.

Time: Most Inceptisols are formed on young landscapes (< Holocene), where time limited the development of soil diagnostic features. There are Inceptisols where the solum is permanently altered by loss of soil particles due to erosion or by the deposition of soil particles. These processes might be acting smooth but continuously or sporadically in space and time. In tropical zones the speed of development of Inceptisols into other soil orders is greater than in temperate or cold zones, it may be slowed down by retarded weathering of resistant rocks.

12.7.2) Processes

Virtually many pedogenic processes are active to some extent in Inceptisol profiles but none predominates. The genesis of Inceptisols includes multiple pathways depending on the processes occuring on a given landscape and geographic area. Environmental factors can slow down weathering (e.g. low temperatures, low precipitation, or resistant parent material) and soil development to form other soil orders is retarded or even inhibited.

Soil erosion on steep slopes can alter the topsoil extremely. When erosion has leveled the slope erosion rates become lower and more distinct pedogenic features like argillic horizons are formed. Usually Inceptisols are formed in underlying volumes of parent material as erosion lowers the landscape by removing the volume of material that was soil. Long time periods and high erosion rates are necessary to develop an Inceptisol on a steep slope (shallow soil, AC horizons) to a further developed soil (deep soil profile, ABC horizons).

Inceptisols form also in colluvium at the base of steep slopes. Processes to form colluvium are mass movement, soil creep (slow mass movement), and deposition. Due to the hillslope processes and weathering morphological features are being formed and destroyed continuously.

Inceptisols may be also found on alluvial deposits where temporary flooding alters the soil profile due to the deposition of soil particles on the soil surface and the soil profile becomes saturated. For example, Inceptisols in the southern Mississippi River Valey are developed on alluvial deposits. A high water table favors the reduction of iron and aluminium oxides.

In depression areas or valley bottoms poorly drained Inceptisols are found where gleization produces redoximorphic features. In those areas leaching may be more extensive than in other landscape positions, but the process of lessivage and thus argillic horizon formation is somewhat retarded, probably because the soils do not undergo frequent desiccation. In areas of acid rocks, soils formed in landscape depressions tend to be more leached and somewhat lower in base content than soils in surrounding areas. In landscapes of high base status soils, the associated poorly drained Inceptisols in depression areas usually have higher base status than the surrounding soils. This can be attributed to the enrichment of the low-lying parts of the landscape by lateral processes such as transport of bases attached to soil particles, in surface runoff, or lateral subsurface flow. In some materials saturated with brackish water sulfides may accumulate and sulfuric horizons may be formed. When oxidized, usually by artificial drainage, sulfuric acid is formed. These unique Inceptisols are commonly known as 'cat-clays'.

Decomposition, humification, and mineralization result in the accumulation of organic matter. The soil organic matter is higher in the suborders Umbrepts and Aquepts compared to the suborder Ochrepts.

12.7.3) Properties

The cambic subsurface diagnostic horizon of Inceptisols is composed of very fine sand, loamy fine sand or finer texture, with some weak indication of either an argillic or spodic horizon, but not enough to qualify as either. Typically, these soils have an ochric or umbric epipedon over a cambic horizon. The ochric epipedon is generally a light-colored, low organic matter horizon. The umbric epipedon is similar to the mollic epipedon except for having a base saturation less than 50 %. Some poorly drained Inceptisols have a histic epipedon where organic matter content is high. Soils with mollic epipedons are Inceptisols when base saturation at pH 7 is less than 50 % in some horizon between the mollic epipedon and a depth of 180 cm or a lithic or paralithic contact if shallower.

Shallow Inceptisols show only few horizons, for example AC, AR or ABC. Due to erosion the development of soil structure is weak.

12.7.4) Classification

The requirements to qualify for an Inceptisol are the following:

Usually a cambic diagnostic horizon is present but no spodic, argillic, kandic, natric, or oxic horizon

Soils that lack subsoil development but have umbric, histic, or plaggen epipedons

Soil texture: loamy or finer textured mineral soils

They exhibit profile development sufficient to exclude them from Entisols but lack features though to represent mature soil formation

No andic soil properties are permitted in any layer thicker than 35 cm within the top 60 cm

No aridic soil moisture regime is allowed

The suborders of Inceptisols are distinguished by soil moisture, epipedon properties, and soil temperature regime (Figure 18.1.4.1).

Figure 18.1.4.1. Diagram showing some relationships between suborders of the Inceptisols

Aquepts: They show redoximorphic features and are saturated with water at some period in the year. Aquepts usually have cambic horizons and commonly in the US, they have fragipans. Aquepts are found in the Flood Plains of the Mississippi River Valley, the lacustrine regions in the Midwest, and the lower Coastal Plain along the Atlantic and Gulf Coast.

Plaggepts: They have dark brown or black plaggen epipedons. Plaggepts were formed by anthropic activity mainly in Europe and are of small extent.

Tropepts: They are formed in isomesic or a warmer iso soil temperature regime.

Ochrepts: They have an ochric epipedon or if their soil temperature regime is mesic or warmer they have thin (< 25 cm) mollic or umbric epipedons. Their soil organic matter content is low.

Umbrepts: They have umbric, mollic, or anthropic epipedons. They are freely drained Inceptisols that are acid, dark reddish or brownish, and high in organic matter.

Soil properties, soil temperature and moisture regimes distinguish the great groups and subgroups of Inceptisols.

Cryic or pergelic soil temperature (e.g. Cryaquepts), ustic moisture regime (e.g. Ustochrepts, Ustic Humitropepts), and aridic moisture regime (e.g. Aridic Ustochrepts) is considered in the Inceptisol Order.

A sulfuric horizon is considered on the great group (e.g. Sulfaquepts) and subgroup level (e.g. Sulfic Cryaquepts). The presence of a fragipan (e.g. Fragiaquepts, Fragic Epiaquepts, or Fragic Xerochrepts), a duripan (e.g. Durochrepts), or plinthite (e.g. Plinthaquepts) are considered. Carbonates within the soil profile of Inceptisols or a high base status define the Eutrochrepts. Inceptisols low in bases are common on the great group level (e.g. Dystropepts) and on the subgroup level (e.g. Dystric Eutrochrepts, Dystric Xerochrepts).

Shallow soil profiles are found in several subgroups, for instance, Lithic Ustochrepts, Lithic Cryaquepts, and Lithic Endoquepts. Vertic characteristics such as cracks and the extensibility of the mineral component of the soil define several subgroups of Inceptisols (e.g. Vertic Ustochrepts, Vertic Eutrochrepts, or Vertic Humitropepts).

Fluvial parent material is considered in several subgroups, for example, Fluventic Humitropepts, Fluventic Ustochrepts, or Fluventic Xerumbrepts. Inceptisols formed on volcanic material (e.g. volcanic glass) which does not meet the criteria of the Andisol order are considered by the formative element 'vitrandic' (e.g. Vitrandic Humitropepts, Vitrandic Durochrepts) or 'andic' if the fine-earth fraction exhibits a bulk density of 1.0 g/cm³ or less (e.g. Andic Durochrepts, Andic Fragiochrepts).

12.7.5) Distinguishing Characteristics

Inceptisols include soils that have some subsoil development but lack features of other soil orders. They are excluded from the Aridisol order by soil moisture regime, from the Vertisol order by lack of argillipedoturbative features, and from the Andisol order by andic parent material. In temperate climate and increased precipitation Mollisols or Alfisols are formed. In tropical and subtropical climate Ultisols or Oxisols are formed.

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12.8) Mollisols

Summary:

Vegetation: prairie, grassland

Climate: variety of soil temperature regimes (cryic to hypothermic)

Soil moisture regime: variety of soil moisture regimes - aquic, udic, ustic, or xeric; average annual precipitation between 200 to 800 mm

Major soil property: organic matter content, high base saturation,

Diagnostic horizons: argillic, cambic (natric, calcic, petrocalcic, gypsic, albic, duripan)

Epipedon: mollic

Major processes: melanization, decomposition, humification, pedoturbation

Characteristics: highly fertile soils

12.8.1) 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. ridgetops, windward slopes).

Parent Material: Mollisols occur on deposits and landscapes with a wide range of ages. Many Mollisols are formed on deposits associated with glaciation (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.

12.8.2) 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.

12.9.3) 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.

12.10.4) 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:

Mollic epipedon

Base saturation of 50 % or more in all horizons to a depth of 180 cm or a lithic or paralithic contact if shallower

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 occurence. 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 m

12) Soil Orders

12.1) Alfisols

12.2) Andisols

12.3) Aridisols

12.4) Entisols

12.5) Gelisols

12.6) Histosols

12.7) Inceptisols

12.8) Mollisols

12.9) Oxisols

12.10) Spodosols

12.11) Ultisols

12.12) Vertisols

12.13) Summaries / Orders

12) Soil Orders

12.1) Alfisols

12.2) Andisols

12.3.1) Aridisols

12.4.1) Entisols

12.5.1) Gelisols

12.6) Histosols

12.7) Inceptisols

12.8) Mollisols