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Soil classification is the orderly arrangement of soils according to some known or inferred characteristics. Soils are classified to organize information regarding their properties and behavior so that they can be used for different purposes with the maximum benefit under sustainable management. Systematic soil classifications began in the last part of the nineteenth century and were mainly based on ideas of soil genesis and soil forming factors as influenced by the philosophy of Dokuchaiev. Dokuchaiev himself proposed a soil classification system in 1879 before presentation of his Ph.D. thesis on the Russian Chernozems in 1883 (Gimenez 2011). There are two types of soil classification: process oriented and property oriented. The process-oriented classification systems are known as genetic soil classification systems (Bokheim and Gennadiyev 2000). Soil classification systems based on observed properties are known as natural classification systems, and those on inferred properties are technical systems. There are regional and international soil classification systems. Examples of regional classification systems include Australian soil classification system (Isbell 1996, 2002), the Canadian soil classification system (Agriculture Canada Expert Committee on Soil Survey 1987), soil classification of England and Wales (Avery 1980), French soil classification system (Baize and Girard 1995), and so on. Among international systems, Soil Taxonomy (USDA 1960, 1975) and World Reference Base for Soil Resources (FAO 1998) are popularly used worldwide at present. These systems have their own terminology and diagnostic features for different taxa.

The Soil Taxonomy is a revolutionary system in that it has abandoned all the classical, indigenous, and folk soil names as laterites, podzols, groundwater podzols, prairies, chernozems, rendzinas, black cotton soils, tropical red earths, and Solonchaks used in the earlier Russian pedology literature and used in the old American system of soil classification (Baldwin et al. 1938; Thorp and Smith 1949). It has coined a new set of soil nomenclatures and managed to keep a link in the naming of higher to lower categories, so that the higher category affiliation and properties are immediately conceived from the lower category name. World Reference Base retained a few old names such as Chernozems and Solonchaks and coined some new ones such as Anthrosols, Technosols, and so on. As there are different systems of soil classification in different parts of the world, there are different nomenclatures of soil. The same soil is called in different names in different systems (such as Oxisols in Soil Taxonomy and Ferralsols in WRB). Histosols is a taxon used in both Soil Taxonomy and WRB but their diagnostic characteristics are different. This may make considerable confusion in identification and interpretation of soil (Krasilnikov et al. 2010). Therefore, correlation between Soil Taxonomy and WRB systems is often necessary.

4.1 Soil Taxonomy Is the Most Popularly Used Soil Classification System

The Soil Survey Staff of the United States Department of Agriculture (USDA 1960) published a soil classification system named “Soil Classification, A Comprehensive System.” It was popularly known as “The 7th Approximation” because it was the 7th revision before its publication. Initially, there were ten soil orders—Alfisols, Aridisols, Entisols, Inceptisols, Histosols, Mollisols, Oxisols, Spodosols, Ultisols, and Vertisols. USDA (1975) published “Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Survey” and included an additional soil order Andisols. Another soil order Gelisols was further included latter (USDA and NRCS 1998), so that the total number of soil orders, the highest category in the hierarchy, is 12 at present.

Soil Taxonomy classifies a “soil individual” according to its observable and measurable properties. A soil individual is the area of soil in a landscape that has similar pedon (Chap. 1) characteristics. The properties that are observed and measured include morphological, physical, chemical, and mineralogical properties. The morphological properties are horizon differentiation, soil depth, etc.; the physical properties include color, texture, structure, compaction, etc.; and the chemical and mineralogical properties include organic matter, pH, base saturation percentage, clay, iron and aluminum oxides, and silicate clays of different horizons or layers. The lower boundary of the pedon for soil classification in this system is 200 cm from soil surface unless restricted by the presence of hard bed rock within this depth. If bed rock is present within this depth the upper unconsolidated part is examined for classification. This part of soil is then called the control section. There are several diagnostic criteria for identification of different categories. Certain diagnostic horizons are used to identify the higher categories. Soil moisture and temperature regimes are used for differentiation of some taxa.

The diagnostic horizons and some other diagnostic features are briefly mentioned below. The main characteristics of different orders and suborders will also be indicated, but for details of diagnostic features and identification of different taxa, the reader is referred to Soil Genesis and Classification, 4th and 5th edition (Buol et al. 1997, 2003, respectively), Soil Taxonomy, 2nd edition (Soil Survey Staff 1999), Keys to Soil Taxonomy, 9th and 11th edn. (USDA and NRCS 2003; Soil Survey Staff 2010, respectively).

4.2 There Are Some Diagnostic Horizons in Soil Taxonomy

Soil taxonomy uses some diagnostic horizons for different categories. The horizons that form in the surface are called epipedons (Gk. epi  =  over; pedon  =  soil) and those in the subsurface soil are endopedons. An epipedon is the surface soil layer where the materials are well weathered and unconsolidated and darkened by organic matter. This may include O horizon where it is present, or the A horizon and part of the eluvial E horizons, provided they are considerably darkened by organic matter. Diagnostic features of several epipedons are given below:

Epipedons

Diagnostic features

Anthropic epipedons

Surface horizons showing the signs of long-term human disturbance such as plowing and manuring. It resembles a mollic epipedon but contains high citric acid soluble P2O5. If the soil is not irrigated, all parts of the epipedon are dry for 9 months or more in normal years

Histic epipedons

Usually wet surface horizons rich in organic matter (at least 12% organic carbon if the soil has no clay; at least 18% organic carbon if the soil has 60% clay, and an intermediate proportional organic carbon for intermediate amounts of clay). It is generally 20–40 cm thick. When plowed, the soil contains 8% or more organic carbon if there is no clay and at least 16% organic carbon if there is 60% or more clay

Melanic epipedons

Thick (>30 cm), black horizons rich in organic matter (more than 6% organic carbon) that has developed from andic (volcanic ash) materials high in allophane

Mollic epipedons

Soft surface horizon that contains sufficient organic matter (>0.60%) to give it a dark color. Base saturation is >50%. It remains moist at least 3 months a year, when the soil is 5 °C or higher to a depth of 50 cm

Ochric epipedons

Light-colored surface mineral horizon. Color values more than 5 dry or more than 3 moist, contains less organic carbon than 0.60%, hard or very hard when dry. It is too thin, too light in color, or too low in organic carbon to be a mollic or umbric horizon

Plaggen epipedons

This is also a surface horizon influenced by long-term human activity, but it contains many artifacts such as brick chips and pottery. It is 50 cm or more thick

Umbric epipedons

It has general characteristics of mollic epipedon but has it less than 50% base saturation. It develops under comparatively higher rainfall from parent materials poor in Ca and Mg

There are also some subsurface horizons that are diagnostic of different taxa in Soil Taxonomy. They are formed below the surface of the soil through eluviation, illuviation, and other translocation and transformation processes. These horizons generally include E and B horizons. Sometimes subsurface horizons may be exposed to the surface due to removal of the surface soil by erosion or human development activities. Different diagnostic subsurface horizons and their major features are presented below:

Subsurface horizons

Diagnostic features

Agric horizon

It forms under the plow layer and contains significant amounts of silt, clay, and humus accumulated as thick, dark lamellae

Albic horizon

It is a light-colored leached horizon with color values 5 dry or more or 4 moist or more. This horizon is created by eluviation of clay and oxides of Fe and Al. It is typical of E horizon

Argillic horizon

It has accumulation of silicate clays due to illuviation or formation in place. It has 3% more clay than the eluvial layer containing <15% clay, or 8% more clay if the eluvial layer has >40% clay. The argillic horizon contains 1.2 times clay of the eluvial layers having 15–40% clay. It should be at least one-tenth thick of all overlying horizons or more than 15 cm whichever is thinner. It is observed in B horizons of humid regions

Calcic horizons

This is a thick horizon (15 cm or more) that has an accumulation of carbonates, commonly of Ca or Mg, in excess of 15% calcium carbonate equivalent. It contains 5% more carbonate than underlying horizons

Cambic horizon

This horizon has a texture of very fine sand, or loamy fine sand, or finer. It may show accumulation of some clay, or sesquioxides but not too high to be an argillic or spodic horizon, respectively

Duripan

A hard pan formed by cementation with silica. Air dry fragments from more than half of the horizon do not slake in water or HCl

Fragipan

This is a horizon of high bulk density and is brittle when moist. The high bulk density is due to extreme compaction that restricts penetration of roots and water

Glossic horizon

This is a thin (5 cm or more thick) transitional horizon between E and B. Albic materials constitute 15–85%, and remaining materials are like those of the underlying horizon, usually argillic, kandic, natric or fragipan horizon

Gypsic horizon

This horizon is 15 cm or more thick and has an accumulation of calcium sulfate. It has at least 5% more calcium sulfate than the underlying material

Kandic horizon

This horizon shows an accumulation of low-activity clay (kandic group, e.g., kaolinite) and has less than 16 cmole kg−1 clay CEC at pH 7

Natric horizon

It has characteristics similar to an argillic horizon, except that it has prismatic or columnar structure and more than 15% exchangeable sodium percentage

Oxic horizon

This horizon is of sandy loam or finer texture and has a thickness of at least 30 cm. It contains highly weathered materials including 1:1 clays and oxides of Fe and Al and has CEC of 16 cmole kg−1 clay or less. It contains less than 10% weatherable minerals in the sand fraction

Placic horizon

This is an iron/manganese pan, reddish brown to black and hard, which is thin (2–10 cm) and remain within 50 cm from soil surface

Petrocalcic horizons

Horizons having similar characteristics of calcic horizons but cemented are called petrocalcic horizon

Petrogypsic horizon

Strongly cemented gypsic horizons are known as petrogypsic horizons

Salic horizon

It is at least 15-cm-thick horizon having accumulation of secondary soluble salts. The electrical conductivity of 1:1 soil water extract exceeds 30 dS m−1 more than 90 days in a year

Sombric horizon

This horizon resembles umbric epipedon in color and base saturation but has formed by accumulation of humus

Spodic horizon

This horizon shows an accumulation of colloidal organic matter and aluminum oxide. It may or may not accumulate iron oxide

Sulfuric horizon

This horizon is at least 15 cm thick and contains sulfuric materials that gives a pH <3.5 and shows yellow mottles of jarosite

4.3 Soil Moisture Regimes Indicate Soil Moisture Status

Some soil moisture regimes are used in distinguishing suborders in an order or other categories. The soil moisture regimes are identified on the basis of the level of moisture saturation (or cumulative and consecutive periods of dryness and moistness) of the soil moisture control section (defined latter). In determining soil moisture regimes, a soil is considered dry when the soil water potential is less than −1,500 kPa and moist when the soil water potential is greater than −1,500 kPa. Since water potential of −1,500 kPa is considered to be the wilting point for most crops, a soil is considered dry when its water potential is less than −1,500 kPa.

Soil moisture control section: Soil moisture control section is a part of the soil which lies between an upper and a lower boundary in the vertical direction.

Upper boundary: The depth to which the dry soil (soil water potential  <  −1,500 kPa) is wetted by 2.5 cm of water in 24 h.

Lower boundary: The depth to which the dry soil is wetted by 7.5 cm of water in 48 h.

These limits depend on soil texture, structure, pore-size distribution, presence of pans, etc. Usually the soil moisture control section varies from 10 to 30 cm below the soil surface if the soil is fine-loamy to clayey, from 20 to 60 cm if the texture is coarse loamy, and from 30 to 90 cm if the texture is sandy. The soil moisture regimes used in identifying different categories of soil taxonomy are presented below:

Soil moisture regimes

Characteristics

Aquic

For considerable periods of the year, the soils are water saturated and remain in a state of reduction due to the absence of dissolved oxygen in ground water, for example, in tidal marshes, landlocked depressions

Aridic/torric

Aridic/torric regimes are used for the same moisture conditions but in different taxa. The soil moisture control section is dry for more than half of the cumulative days per year and moist in some or all parts for  <90 consecutive days, for example, in soils of arid climate and the dryness is due to soil physical conditions

Udic

The soil moisture control section is not dry in any part for as long as 90 consecutive days and dry for <45 consecutive days in the 4 months following the summer solstice, for example, in soils of humid climate

Ustic

This moisture regime is intermediate between the aridic regime and udic regime. Here plant growth is restricted for some part of the year by low moisture supply

Xeric

The soil moisture control section is dry in all parts for 45 or more days in the 4 months following the summer solstice and moist in all parts for 45 or more days in the 4 months following the winter solstice. This soil moisture regime is typical of Mediterranean climates

4.4 Soil Temperature Regimes Differ in Mean Annual and Mean Seasonal Soil Temperatures

Soil temperature regimes are based on mean annual, mean summer, mean winter, and the differences between mean summer and mean winter soil temperatures either at a depth of 50 cm or at a densic (relatively unaltered materials that are non-cemented and rupture resistant), lithic (a coherent underlying material), or paralithic (unaltered materials that are an extremely weakly to moderately rupture resistant) contact which one is shallower. The following soil temperature regimes are identified:

Soil temperature regimes

Characteristics

Cryic

1. In mineral soils, the mean summer soil temperature is:

If the soil is not saturated with water in some part of the summer and has an O horizon <8 °C

If the soil is not saturated with water in some part of the summer and has no O horizon <15 °C

If the soil is saturated with water in some part of the summer and has an O horizon or histic epipedon <6 °C

If the soil is saturated with water in some part of the summer and has no O horizon <13 °C

2. In organic soils, the mean annual soil temperature is <6 °C

Frigid

A soil warmer in summer than a cryic regime, but its mean annual temperature is lower than 8 °C and the difference between mean summer and mean winter soil temperatures is more than 6 °C either at a depth of 50 cm from the soil surface or at a densic, lithic, or paralithic contact, whichever is shallower

Mesic

The mean annual soil temperature is 8 °C or higher but <15 °C, and the difference between mean summer and winter temperature is >6 °C

Thermic

The mean annual soil temperature is 15 °C or higher but <22 °C, and the difference between mean summer and winter temperature is >6 °C

Hyperthermic

The mean annual soil temperature is 22 °C or higher, and the difference between mean summer and winter temperature is >6 °C

Isofrigid

The mean annual soil temperature is >8 °C, and the difference between mean summer and winter temperature is <6 °C

Isomesic

The mean annual soil temperature is 8 °C or higher but <15 °C, and the difference between mean summer and winter temperature is <6 °C

Isothermic

The mean annual soil temperature is 15 °C or higher but <22 °C, and the difference between mean summer and winter temperature is <6 °C

Isohyperthermic

The mean annual soil temperature is 22 °C or higher, and the difference between mean summer and winter temperature is <6 °C

4.5 There Are Some Other Diagnostic Features in Soil Taxonomy

Other diagnostic features including abrupt textural changes (contrasting textures between adjacent horizons), anhydrous conditions (active layer in soils of cold deserts and dry permafrost), coefficient of linear extensibility (COLE—ratio of the difference between the moist length and dry length of a clod to its dry length), durinodes (weakly cemented to indurated nodules), fragic soil properties (properties of a fragipan, but the thickness and volume do not meet the requirements of a fragipan), n value (relation between the percentage of water in a soil under field conditions and its percentage of inorganic clay and humus), plinthite (iron-rich mixture of clay and other minerals in platy, polygonal, and reticulate patterns), resistant minerals (durable minerals in sand fraction), slickensides (polished soil surfaces created by mass slides of soil), and weatherable minerals are sometimes used for identification of some taxa.

4.6 There Are Six Categories in Soil Taxonomy

In this hierarchical system, soil orders are divided into suborders, suborders into great groups, and so on. The hierarchy and number of known soil taxa in each category are shown below:

Order

12 orders

Suborder

64 suborders

Great group

319 great groups

Subgroup

2,484 subgroups

Family

>8,000 families

Series

Unknown number of series

The highest category in Soil Taxonomy is the soil order. Orders are distinguished by the presence or absence of diagnostic horizons and other diagnostic features, indicating the predominant soil forming processes. An order includes soils whose properties suggest that they are similar in their genesis. Subdivisions of orders on the basis of genetic homogeneity are suborders. Suborders of an order are differentiated by the presence or absence of properties associated with wetness, soil moisture regimes, major parent material, and vegetation. Subdivisions of suborders are great groups. They are identified according to similar kind, arrangement, and degree of expression of horizons, giving emphasis on upper sequum, base status, soil temperature, and moisture regimes. Subdivisions of great groups are subgroups. One subgroup has the central concept of a great group; it is given the name “Typic” before the great group name. Other subgroups in a great group are differentiated on characteristics that are intergrades between those of the central concept and those of the orders, suborders, or great groups. Soils of a subgroup having similar physical and chemical properties affecting their response to management and especially to the penetration of plant roots are grouped into a family. Differences in texture, mineralogy, temperature, and soil depth are bases for family differentiation. Soil series is the lowest but most specific category of soil taxonomy. A soil series includes soil individuals that have similar (within an acceptable range) pedon characteristics. Pedon characteristics are horizon arrangement, color, texture, structure, consistence, mottling, pH, etc. A soil series is usually given a name after the place name where the polypedon is found for the first time. Similar polypedons belong to the same soil series. It is extremely difficult to know the number of soil series so far identified. New soil series are continually being added as more detailed study is being done of soils throughout the world.

Soil Taxonomy has a systematically derived nomenclature in categories from order to subgroup. Within this hierarchy, the name of a taxon indicates the major characteristics of the soil and the higher categories to which it belongs. An example is given below:

Order::

Entisol (Soils with little or no horizon differentiation)

Suborder::

Aquent (wet Entisol)

Great group::

Sulfaquent (wet Entisol that has sulfidic materials)

Subgroup::

Typic Sulfaquent (soils that meet the central concept of Sulfaquent)

Family and series names are, however, somewhat arbitrary.

4.7 Each Soil Order Has Its Own Characteristic Features

4.7.1 Alfisols Are Well-Developed Soils with High Base Status

Alfisols are fine textured soils with high content of exchan-geable bases. They have mostly developed in the humid temperate and also in the humid tropical regions under deciduous forests. They have accumulation of clay in the B horizon to form argillic, kandic, or natric horizon, with BSP >35% in the lower part or below the argillic or kandic horizon. They may contain petrocalcic horizons and duripan, fragipan, and plinthite. There is enough moisture for plant growth for 3 consecutive months during the growing seasons. There is relatively little accumulation of organic matter in mineral horizons. These soils are highly fertile and are extensively cultivated with widely diverse cropping patterns as favored by climatic conditions. Some are used for hay, pasture, range, and forests. Alfisols comprise 9.6% of the ice-free land of the earth. Alfisols have five suborders. They are:

Aqualfs::

 Aqualfs are Alfisols that have aquic conditions (shallow groundwater table that saturates soil with water) 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. Aqualfs are abundant in humid regions and are primarily used for rice cultivation. They are fairly fertile, and other crops including corns (maize), soybeans can be grown if artificially drained. Nearly all Aqualfs are believed to have supported forest vegetation in the past.

Cryalfs::

 Cryalfs are more or less freely drained Alfisols of the cold regions (cryic soil temperature regime) and occur mostly at high elevations, as in the Rocky Mountains in the Western United States. They normally have a udic moisture regime. Most of the Cryalfs are used as forest because of their short, cool growing season.

Udalfs::

 Udalfs are the more or less frequently drained Alfisols that have udic soil moisture regime and a frigid, mesic, isomesic, or warmer temperature regime. Udalfs are very extensive in the United States and in Western Europe. Most Udalfs with a mesic or warmer temperature regime have or had deciduous forest vegetation, and many of the frigid temperature regimes have or had mixed coniferous and deciduous trees.

Ustalfs::

 Ustalfs have an ustic soil moisture regime and a frigid, mesic, isomesic, or warmer temperature regime. Ustalfs are the Alfisols of subhumid to semiarid regions. They occur in the United States, Africa, India, South America, Australia, and southeastern Asia. Sorghum, wheat, and cotton are commonly cultivated with irrigation.

Xeralfs::

 Xeralfs have xeric soil moisture regime common of regions that have Mediterranean climate. They are found in South Africa, Chile, Western Australia, Southern Australia, and the Western United States. They are dry for extended periods in summer, but enough is available in winter. Small grains and other annuals are common crops without irrigation. Grapes and olives are also common crops where the climate is thermic. With irrigation, a wide variety of crops can be grown.

4.7.2 Andisols Are Soils with Andic (Volcanic Ash) Properties

Andisols are characterized by andic materials. Andic materials include volcanic ash, pumice, and cinders deposited during volcanic eruptions. These materials undergo transformation to amorphous or poorly crystallized silicate minerals, including allophane, imogolite, and ferrihydrite. Andisols are young soils and have not had enough time to be highly weathered. These soils are fine textured and have a high content of fresh weatherable minerals and a high cation exchange capacity. They may also contain considerable organic matter as aluminum–humus complex. They have low bulk densities. These soils are widely distributed in all geographical regions near sources of volcanoes. Andisols are generally fertile and are used for agriculture unless restricted by slope, altitude, soil moisture and temperature regimes, etc. One of the most important characteristics of Andisols is their high capacity to fix phosphorus on the surface of the amorphous minerals (Cordova et al. 1996). This is perhaps the principal chemical constraint of Andisols. Some Andisols are left under tundra and forests. Andisols cover more than 124 million ha or approximately 0.7% of the earth’s surface. Major areas of Andisols include Chile, Peru, Ecuador, Columbia, Central America, the USA, Kamchatka, Japan, the Philippines, Indonesia, and New Zealand. Andisols have seven suborders. They are:

Aquands::

Aquands are the Andisols with aquic conditions at or near the surface. These soils have dark-colored surface horizons that meet the requirements for a histic, umbric, or mollic epipedon. Aquic conditions 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::

Cryands 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 USA). They are usually occupied by cold tolerant forests.

Torrands::

Torrands are more or less well-drained Andisols of dry regions. They have an aridic/torric soil moisture regime and a frigid or warmer soil temperature regime. Natural vegetation is mostly desert shrubs. These soils are not extensive occurring mostly in the western part of North America, Hawaii or other Pacific regions. Most of the soils formed under grass or shrub vegetation.

Udands::

Udands are more or less well-drained Andisols of moist regions. They have a udic soil moisture regime. Udands are Andisols of the humid climates. They are the most extensive Andisols. These soils are moderately extensive on the Pacific Rim, including Washington, Oregon, and Hawaii in the USA. Most Udands formed under forest vegetation.

Ustands::

Ustands are defined as Andisols with ustic soil moisture regimes. These soils are distributed in the intertropical regions that experience seasonal precipitation distribution. They are found mostly in Mexico, Western USA, Pacific Islands, and the eastern part of Africa. Most Ustands formed under grass, shrub, or forest vegetation.

Vitrands::

Vitrands are relatively young Andisols that are coarse-textured soils and are dominated by volcanic glass. Most Vitrands are found near volcanoes. Vitrands are abundant in Oregon, Washington, and Idaho of the USA where they form mainly under coniferous forest vegetations. They are the Andisols that have a frigid or warmer soil temperature regime. They have a low water-holding capacity. Vitrands are restricted to ustic and udic soil moisture regimes.

Xerands::

Xerands are more or less well-drained Andisols that have a xeric soil moisture regime and a frigid, mesic, or thermic temperature regime. They are temperate Andisols with very dry summers and moist winters. Most Xerands formed under coniferous forest vegetation and some formed under grass or shrub vegetation.

4.7.3 Aridisols Are Soils of Drylands

Aridisols are soils of the arid regions including cold polar, cool temperate, and warm deserts. Aridisols may also occur in semiarid areas outside the zones broadly classified as arid, for example, in local conditions imposing aridity such as steep, south-facing slopes in Northern Hemisphere and in soils whose physical properties limit water infiltration or favor excessive drainage. Aridisols are classified on the basis of their soil moisture regime 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 (>8 °C) 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. However, shifting sands of deserts are not included in Aridisols.

There is low chemical weathering, low leaching, and sparse plant growth. Soil organic matter content is low so that ochric horizons abundantly develop in Aridisols. Salts (chlorides, sulfates, carbonates) released by limited chemical weathering are not usually translocated to considerable depths but are accumulated on the surface and, where there is some downward movement of water, in the B horizon. Aridisols have one or more of the following within 100 cm of surface a calcic, cambic, gypsic, natric, petrocalcic, petrogypsic, or salic horizon. An argillic horizon is found in some Aridisols. This is believed to have developed under a moister climate of the past. Aridisols are sparsely vegetated, mostly in xeric shrub lands with xerophytes, cactus, and thorns. They may be cultivated if irrigation can be given, but source of irrigation water is also scanty there. Aridisols comprise about 12% of the world’s ice-free land surface. Aridisols have seven suborders. They are:

Cryids::

Cryids are the Aridisols of the cold climates. These soils are characteristically developed at high elevations, dominantly in the mountain and basin areas of the USA and Asia and other parts of the world. Cryids commonly show evidence of periglacial features.

Salids::

Salids are Aridisols with accumulations that are more soluble than gypsum. The most common form is sodium chloride, but sulfates and others may also occur. These soils are common in depressions in the deserts or in closed basins in wetter areas bordering deserts. Some salts may be brought to the upper horizons by capillary rise of groundwater.

Durids::

Durids are the Aridisols that have an accumulation of silica. There is a duripan which is cemented partly with opal or chalcedony. The soils commonly have calcium carbonate. The duripan restricts movement of water and penetration of roots. These soils occur in the western part of the USA particularly in Nevada. They are not known to occur outside the USA.

Gypsids::

Gypsids are the Aridisols that have an accumulation of gypsum. These soils occur in Iraq, Syria, Saudi Arabia, Iran, Somalia, West Asia, and in some of the most arid regions of the USA. When the gypsic horizon occurs as a cemented impermeable layer, it is recognized as the petrogypsic horizon.

Argids::

Argids are the Aridisols that have accumulation of clay. These soils have an argillic or natric horizon. The presence of an argillic horizon is commonly attributed to a moister paleoclimate. Most Argids occur in North America with a few recognized in the deserts of North Africa or the Near East.

Calcids::

Calcids are the Aridisols that have accumulation of residual calcium carbonate or was added as dryfall. Precipitation is inadequate to leach or move the carbonates to great depths. These soils are extensive in the Western USA and other arid regions of the world.

Cambids::

These are the Aridisols with the least degree of soil development. They have a cambic horizon that has its upper boundary within 100 cm of the soil surface. These soils are the most common Aridisols in the USA and other parts of the world.

4.7.4 Entisols Are Young Soils That Lack Horizon Development

Entisols are defined as soils that have little or no sign of horizon differentiation. Most Entisols are basically unaltered from their parent materials. Actually they are affected to a limited extent by translocation processes. However, there is considerable darkening of the surface soil by organic matter. The presence of unweatherable parent materials, removal of soil materials by continuous erosion, continuous deposition of silts with floodwater in active floodplains, cold and dry climates, and insufficient time after rock exposure or sediment deposition are the causes of delayed soil development in Entisols. These soils are distributed over a wide geographic area and can be found in any climate and under any vegetation. Entisols along river floodplains are often intensively farmed and are some of the most agriculturally productive soils in the world. Most Entisols are used for pasture, rangeland, and forests. Entisols occupy about 16% of the global ice-free land surface. Entisols have four suborders. They are:

Aquents::

These are the wet Entisols. They may be found in tidal marshes, on deltas, on the margins of lakes where the soils are continuously saturated with water, on floodplains along streams where the soils are saturated at some time of the year, or in areas of wet, sandy deposits. Many Aquents have gleying with bluish or grayish colors and redoximorphic features. They may have any temperature regime. Most are formed in recent sediments and support vegetation that tolerates permanent or periodic wetness. Vast areas of alluvial Aquents are used for rice cultivation in South and Southeast Asia, including Bangladesh. Some Aquents have sulfidic materials (former acid sulfate soils).

Arents::

Arents are the Entisols that do not have horizons because they have been deeply mixed by plowing, spading, or other methods of moving by humans. Arents may have 3% or more, by volume, fragments of diagnostic horizons in one or more sub-horizons at a depth between 25 and 100 cm below the soil surface.

Fluvents::

Fluvents are mostly brownish to reddish soils that are formed in recent alluvial sediments, mainly on floodplains, fans, and deltas of rivers and small streams but not in back swamps where drainage is poor. Strata of clayey or loamy materials commonly have more organic carbon than the overlying, more sandy strata. Fluvents are often found associated with Aquents in floodplains. Rice and jute are grown in many Fluvents.

Psamments::

Psamments are Entisols that are very sandy at all layers. Some Psamments form in poorly graded but well-sorted sands on shifting or stabilized deposits, in cover sands, or in sandy parent materials that were sorted in an earlier geologic cycle. Psamments occur under any climate without permafrost within 100 cm of the soil surface. They can have any vegetation and can be cropped with irrigation. Psamments on old stable surfaces commonly consist of quartz sand. These soils are poorly fertile and dry and often show nutrient deficiencies.

4.7.5 Gelisols Are Soils of the Cold Zone

Gelisols are soils that contain gelic materials (mineral or organic soil materials that show cryoturbation, cryodesiccation, and/or ice segregation in the active layer). Gelisols are soils of very cold climates that contain permafrost within 2 m of the surface. Freezing and thawing in the active layer influence soil formation in Gelisols. Permafrost restricts the downward movement of water. Thus, there are few diagnostic horizons in Gelisols, if any. Cryoturbation results in irregular or broken horizons, organic matter accumulation on the permafrost table, oriented rock fragments, and silt caps on rock fragments. These soils are limited geographically to the high-latitude polar regions and localized areas at high mountain elevations. Gelisols are the permafrost-affected soils that occur throughout the zone of continuous permafrost in Antarctica (Bockheim 1995). Gelisols have three suborders. They are:

Histels::

Histels have organic horizons similar to Histosols except that they have permafrost within 2 m below the ground. 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 shallower. These soils occur predominantly in subarctic and low arctic regions of continuous or widespread permafrost. The natural vegetation in Histels is mostly mosses, sedges, and shrubs. The soils are used as wildlife habitat.

Turbels::

Turbels are Gelisols that commonly show cryoturbation and 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 common features in Turbels.

Orthels::

Orthels are soils that show little or no cryoturbation (less than one-third of the pedon). These soils occur primarily within the zone of discontinuous permafrost, in alpine areas where precipitation is greater than 1,400 mm per year. The natural vegetation is mostly lichens, mosses, sedges, shrubs, black spruce, and white spruce. The soils are used mostly as wildlife habitat. They occur throughout the Gelisol area in Alaska. The vegetation is mostly mosses, sedges, shrubs, and black spruce.

4.7.6 Histosols Are Soils Developed from Organic Soil Materials

Histosols are permafrost-free soils dominated by organic soil materials. Organic soil materials consist of organic debris accumulating at the surface in which the mineral component does not significantly influence the properties of soils. Organic soil materials have either:

  1. 1.

    Under water-saturated conditions 18% organic carbon (30% organic matter) or more if the mineral fraction has 60% or more clay, or 12% organic carbon (20% organic matter) if the mineral fraction has no clay, or a proportional intermediate organic carbon for intermediate content of clay

  2. 2.

    If never saturated with water for more than a few days, 20% or more organic carbon

Histosols typically form in settings where poor drainage inhibits the decomposition of plant and animal remains, allowing these organic materials to accumulate over time. Thus, they have developed in organic parent materials, and they are mostly soils that are commonly called bogs, moors, or peats and mucks. Peat is the name given to slightly decomposed organic material in soil, while muck is used for the rotten, highly decomposed material. The peat is used for fuel, potting soil in greenhouses and for packing. Histosols can be cultivated only if artificially drained. Histosols serve as important habitats for wetland plants and animals and as carbon reservoirs. Histosols are ecologically important because of the large quantities of carbon they contain. Typically, Histosols have very low bulk density (Chap. 5) and are poorly drained because of their occurrence in low-lying areas and high organic matter content. Most Histosols are acidic, and many are deficient in plant nutrients. Many Histosols are not suitable for cultivation because of poor drainage and low chemical fertility. However, many other Histosols formed on recent glacial lands can be very productive when drained. They can sometimes be used for orchards and vines if carefully managed. However, there is a great risk of wind erosion, shrinkage, subsistence, and compaction. Histosols occupy only 1.2% of the global ice-free land surface. Histosols have four suborders mostly distinguished on the basis of the state of organic matter and drainage. They are:

Fibrists::

Fibrists are the wet, slightly decomposed Histosols. The largest extent is in southern Alaska of the USA. Most of these soils support natural vegetation of widely spaced, small trees, shrubs, and grasses.

Folists::

Folists are the more or less freely drained Histosols that consist primarily of horizons derived from leaf litter, twigs, and branches resting on bedrock or on fragmental materials. Most of these soils support forest vegetation. Some of the soils mainly support grass. A few of the soils are used for specialty crops or for urban or recreational development.

Hemists::

Hemists are the wet Histosols in which the organic materials are moderately decomposed. They are extensive in Minnesota and Alaska. Most Hemists support natural vegetation and are used as woodland, rangeland, or wildlife habitat. Some have been cleared and drained and are used as cropland.

Saprists::

Saprists are the wet Histosols in which the organic materials are well decomposed. The largest extent in the USA is in Michigan, Florida, Wisconsin, Minnesota, and Alaska. Small areas are common on the Atlantic and gulf coasts. Many Saprists support natural vegetation and are used as woodland, rangeland, or wildlife habitat. Some of the soils, mostly those with a mesic or warmer temperature regime, have been cleared and drained and are used as cropland.

4.7.7 Inceptisols Are Soils That Show Beginning of Horizon Differentiation

Inceptisols are soils that exhibit only the beginning of soil profile development. They are weakly developed soils in that they have minimal horizon differentiation. They are more developed than Entisols and lack many characteristics of mature soils. Inceptisols may have many kinds of diagnostic horizons except argillic, natric, kandic, spodic, and oxic horizons. The most common horizon sequence is an ochric epipedon over a cambic horizon, with or without an underlying fragipan. Inceptisols typically have a cambic horizon, but one is not required if the soil has a mollic, umbric, histic, or plaggen epipedon or if there is a fragipan or duripan or any placic, calcic, petrocalcic, gypsic, petrogypsic, salic, or sulfuric horizon. Inceptisols are soils of humid and subhumid regions. Inceptisols are widely distributed and occur under a wide range of environmental settings. They are often found on fairly steep slopes, young geomorphic surfaces, wet sites, and on resistant parent materials. Inceptisols occupy 9.9% of the global ice-free land surface. Inceptisols have six suborders. They are:

Anthrepts::

Anthrepts are more or less freely drained Inceptisols that have either an anthropic or plaggen epipedon. Most have a cambric horizon. Anthrepts can have almost any temperature regime and almost any vegetation. Anthrepts are usually cultivated soils, but some Anthrepts have been diverted to other land uses.

Aquepts::

Aquepts are the wet Inceptisols. The water table remains at or near the surface for much of the year. Most Aquepts have formed in depressions, on nearly level plains, or on floodplains. Aquepts may have almost any particle-size class except fragmental. Many Aquepts in floodplains are used for cultivation of rice.

Cryepts::

Cryepts are Inceptisols of the cold regions such as high mountains or high latitudes. They do not have permafrost within 100 cm of the soil surface. They may be formed in loess, drift or alluvium (Chap. 3), or in solifluction (mass wasting of water-saturated soil material down the slope, over impermeable surface) deposits. Cryepts occur in the USA in the high mountains of the West, southern Alaska, as well as in other mountainous areas of the world. Vegetation is mostly conifers or mixed conifers and hardwood forests. Few soils are cultivated.

Udepts::

Udepts are mainly the more or less freely drained Inceptisols that have a udic or perudic soil moisture regime. They are Inceptisols of humid climates. They are found on nearly level to steeply sloping surfaces. Most of the soils were originally covered with forest vegetation, with some shrubs or grasses. The Udepts of the USA are most extensive in the Appalachian Mountains, on the Allegheny Plateau, and on the west coast. Many Udepts are now under cropping.

Ustepts::

Ustepts are mainly the more or less freely drained Inceptisols that have an ustic soil moisture regime. Rainfall occurs mainly during the summer. Some Ustepts are found in older deposits on steep slopes. Native vegetation is commonly grass but some supported trees. Most are used as cropland or pasture.

Xerepts::

Xerepts are mainly more or less freely drained Inceptisols that have a xeric soil moisture regime. They are Inceptisols of the temperate regions with very dry summers and moist winters. Xerepts are moderately extensive in the USA and are the most common in California, Oregon, Washington, Idaho, and Utah. The vegetation commonly is coniferous forest on soils with frigid or mesic temperature regimes and shrubs, grass, and widely spaced trees on the soils with a thermic temperature regime.

4.7.8 Mollisols Are Soils of the Grasslands

Mollisols are dark-colored, base-rich, mineral soils of the grasslands. They have a mollic epipedon. They may have an argillic, natric, calcic, or an albic horizon. Some have a duripan or a petrocalcic horizon. Mollisols do not have permafrost, organic soil materials and a spodic horizon. Mollisols may have any of the defined temperature regimes. Mollisols can have any soil moisture regime, but enough available moisture to support perennial grasses seems to be essential. Mollisols are used mainly for small grain in the drier regions and corn (maize) or soybeans in the warmer, humid region. Mollisols comprise 6.9% of the ice-free land of the earth.

Albolls::

Albolls are the Mollisols that have an albic horizon and fluctuating groundwater table. Most of these soils are saturated with water to or near the soil surface at some time during winter or spring in normal years. These soils developed mostly on broad, nearly level to sloping ridges, on back slopes, or in closed depressions. Most Albolls have developed under grass or shrub vegetation.

Aquolls::

Aquolls are the Mollisols that are wet and that have an aquic soil moisture regime. In these soils, the water table remains at or near the surface for much of the year. They have developed under grasses, sedges, and forbs, but a few have had forest vegetation. In the USA, Aquolls are most extensive in glaciated areas of the midwestern states where the drift was calcareous.

Cryolls::

Cryolls are more or less freely drained Mollisols of the cold region. They are abundant in the high mountains of the Western USA, on the plains and mountains of Eastern Europe, and in Asia. The vegetation of the Cryolls on the plains was mostly grasses. Cryolls in the mountains have either forest or grass vegetation. Cryolls in Alaska support spruce, birch, and aspen trees.

Rendolls::

Rendolls are shallow Mollisols over calcareous parent materials such as limestone, chalk, drift composted of limestone, or shell bars, of humid regions. These soils are extensive in some parts of the world and formed under forest vegetation or under grass and shrubs.

Udolls::

Udolls are Mollisols of humid climates mainly under tall grass prairie (an extensive, level or slightly undulating, treeless tract of land covered with coarse grasses) vegetation, but some could have supported boreal forests (Chap. 14) several thousand years ago. Most of these soils occur in the eastern part of the Great Plains.

Ustolls::

Ustolls are Mollisols of semiarid and subhumid climates having an ustic soil moisture regime. Rainfall occurs mainly during a growing season, often in heavy showers, but is erratic. Drought is frequent and sometimes may be severe. Natural vegetation in Ustolls may be grass in the Great Plains and forest in the mountains of Western USA.

Xerolls::

Xerolls are the temperate Mollisols with very dry summers and moist winters within a Mediterranean climate. Xerolls have a xeric soil moisture regime. Xerolls are extensive in parts of Turkey, northern Africa near the Mediterranean, and in some of the southern republics of the former USSR and in several states in the USA.

4.7.9 Oxisols Are Highly Weathered Tropical Soils with Enrichment of Kaolinite and Oxides of Fe, Al, and Mn

Oxisols develop under a climate characterized by small seasonal variation in soil temperature and no seasonal soil freezing, and high annual precipitation. They may have a wide range of soil moisture regimes from aridic to perudic. Oxisols with aridic soil moisture regimes are often considered as paleosols (Chap. 3). Usually, Oxisols develop under climatic conditions where precipitation exceeds evapotranspiration for some periods of the year to facilitate the removal of soluble weathering products and favors the residual concentration of kaolinite and sesquioxides, which are essential to form an oxic horizon. Oxisols have the upper boundary of an oxic horizon and no kandic horizon within 150 cm or 40% or more clay by weight in the fine-earth fraction. Oxisols do not have either of the following: (a) permafrost within 100 cm of the soil surface or gelic materials within 100 cm of the soil surface and (b) permafrost within 200 cm of the soil surface. Oxisols consist mainly of quartz; kaolinite; oxides of Fe, Mn, and Al; and organic matter. Oxisols are poorly fertile weathered soils occurring on gentle slopes of geologically old surfaces in tropical and subtropical regions. The natural vegetation ranges from tropical rainforests to desert savannas. Although many Oxisols are extremely infertile, some Oxisols may be made productive when cultivated with appropriate management. Oxisols comprise 7.5% of the global ice-free land surface. Oxisols have five suborders. They are:

Aquox::

Aquox are the Oxisols that have a water table at or near the surface for much of the year in shallow depressions and in seepage areas at the base of slopes. There is a tendency to accumulate iron in the form of secondary nodules, concretions, and plinthite.

Perox::

Perox are well-drained Oxisols with a perudic soil moisture regime. They are found in continuously humid climates, where precipitation exceeds evapotranspiration in all months.

Torrox::

Torrox are the Oxisols of the arid region. They have an aridic (torric) soil moisture regime. Torrox may become productive soils for a variety of crops if water and fertilizers are applied. They occur mainly in Southern Africa, Hawaii, and some areas of Australia.

Udox::

Udox are well-drained Oxisols with a udic soil moisture regime. These soils develop in humid areas. There is usually adequate rainfall in normal years to allow for continuous crop growing. Udox occur mostly in South America and in parts of Africa and Asia.

Ustox::

Ustox are the Oxisols that have an ustic soil moisture regime. These soils are found in semiarid and subhumid climates. There is at least 90 consecutive dry days in normal years. Available soil moisture is then very low, and cropping is not done in that time. One crop may be grown in the season when rainfall occurs. Two crops may be grown with irrigation in some areas. Ustox occur over a large portion of the interior of South America and in extensive areas of Africa.

4.7.10 Spodosols Are Soils with Accumulation of Amorphous Mixtures of Organic Matter and Aluminum in B Horizon

Spodosols typically form in coarse-textured parent materials and have a reddish-brown spodic horizon beneath a light-colored E horizon. Sometimes there is a fragipan or another sequum (a sequum is a couplet of an eluvial horizon above an illuvial horizon, usually an E and an underlying B horizon [(Schaetzl and Anderson 2005); many soil profiles in humid regions have an E–B sequum. Those soils that have two sequa are termed bisequal soils (Schaetzl 1996)] that have an argillic horizon below the spodic horizon. Some Spodosols have a placic horizon either on or within a spodic horizon or on a fragipan. Some Spodosols have thicker layers than a placic horizon that are cemented by spodic materials (humus–aluminum–clay complex) and organic matter. The particle-size class is mostly sandy, sandy-skeletal, coarse loamy, loamy-skeletal, or coarse-silty. Spodosols are most extensive in areas of cool, humid, or perhumid climates. They may also form, however, to a limited extent, in warm, humid tropical regions, where they occur mostly in areas of quartz-rich sands with fluctuating groundwater table. Most Spodosols in cool temperate regions are covered with coniferous or, less commonly, hardwood forests. Plenty of Spodosols are found in boreal forest regions. Some have been cleared for agriculture. Spodosols are naturally infertile, but some Spodosols may be made productive by good management. Spodosols occupy 2.6% of the global ice-free land surface. Spodosols have four suborders. They are:

Aquods::

Aquods are Spodosols that have an aquic soil moisture regime. They are poorly drained soils with a water table at or near the surface for much of the year. A wide variety of hydrophytic (water-loving) plants, ranging from sphagnum in cold areas to palms in the tropics, grow on these soils.

Cryods::

Cryods are Spodosols that have a cryic soil temperature regime. They are found in high latitude or high elevations. They are abundant in Alaska, in the mountains of Washington and Oregon of the USA, and Canada. Natural vegetation is mostly coniferous forest or alpine tundra.

Humods::

Humods are the relatively freely drained Spodosols that have a large accumulation of organic carbon in the spodic horizon. These soils have developed under coniferous forests and in Western Europe, commonly found in sandy materials where heather (a shrubland characterized by open, low growing woody vegetation) is dominant. In the tropics, most Humods have supported a rain forest.

Orthods::

Orthods are the relatively freely drained Spodosols that have a moderate accumulation of organic carbon in the spodic horizon. They are most extensive in the Northeastern United States and the Great Lakes states. Most Orthods are used as forest or have been cleared and are used as cropland or pasture. Orthods are naturally infertile, but they can be highly responsive to good management.

4.7.11 Ultisols Are Low Base Status Soils with an Argillic or a Kandic Horizon

Ultisols are red to yellow soils that are quite acidic, often having a pH of less than 5 and that develop in humid tropical (some in temperate) areas under forest vegetation. They are highly weathered soils and have an argillic or a kandic horizon with low base saturation, less than 35% BSP (by summation of exchangeable bases). The low base saturation status is mainly due to formation in parent material high in silica but low in bases. In some soils, the low base status results from intense leaching of parent material, while in others, a low base status and small quantities of weatherable minerals were initial parent material characteristics. They may have any soil temperature regime and any soil moisture regime except aridic. Leaching is high, and bases released by weathering usually are removed by leaching. The red and yellow colors result from the accumulation of iron oxide which is highly insoluble in water. Kaolinite, gibbsite, and aluminum-interlayered clays are common in the clay fraction. Major nutrients, such as calcium and potassium, are typically deficient in Ultisols. They are poorly fertile soils which may not be productive for most crops without addition of lime and fertilizers. Ultisols occupy 8.5% of the global ice-free land surface. Ultisols have five suborders. They are:

Aquults::

Aquults are the Ultisols that have a water table at or near the surface for much of the year. Aquults are found extensively on the coastal plains of the USA, particularly on the Atlantic and Gulf of Mexico. Most of the soils are forested.

Humults::

Humults are freely drained Ultisols rich in organic matter (>0.9% or more organic carbon in the upper 15 cm of the argillic or kandic horizon) of mid or low latitudes. Rainforests are the usual natural vegetation.

Udults::

These are more or less freely drained Ultisols that have a udic soil moisture regime. They develop in humid areas with well-distributed rainfall. Most of these soils have a forest vegetation, but some have a savanna.

Ustults::

These are freely drained Ultisols that have an ustic soil moisture regime and a relatively low content of organic carbon. These soils are generally found in semiarid and subhumid climates. The vegetation commonly consists of forest or savanna plants.

Xerults::

Xerults are freely drained Ultisols that have a xeric soil moisture regime. They are found in areas with very dry summers and moist winters typically of Mediterranean or temperate climates. Natural vegetation consisted mostly of coniferous forest plants.

4.7.12 Vertisols Are Soils That Crack Deeply and Widely Upon Drying

Vertisols are clayey soils that have deep, wide cracks for a considerable time of the year and have slickensides (a shiny surface of the cracks produced in soils containing a high proportion of swelling clays) within 100 cm of the mineral soil surface. They shrink when dry and swell when moistened. They are generally sticky in the wet season and hard in the dry season. Most Vertisols have an ustic soil moisture regime; some have an aridic and a udic regime. Vertisols generally have 50–70% clay with a relatively large proportion of fine clay in the clay fraction. The clays in Vertisols consist predominantly of 2:1 and 2:2 layer clay minerals, but some have considerable amounts of other clay minerals. The natural vegetation is predominantly grass, savanna, open forest, or desert shrub. Most Vertisols are well suited to farming if there is plenty of rainfall or irrigation water and if suitable management practices are followed. Because of the low permeability and tendency to remain waterlogged for long periods, Vertisols are often considered as problem soils (Chap. 11). Vertisols are extensive in some parts of the world. They were known as black cotton soils in India. Vertisols occupy 2.4% of the global ice-free land surface. Vertisols have six suborders. They are:

Aquerts::

Aquerts are the Vertisols that have aquic soil moisture regime. They have a water table at or near the surface for much of the year but are also dry enough for periods for cracks to open. They are found in low areas such as glacial lake plains, floodplains, stream terraces, and depressions.

Cryerts::

Cryerts are the Vertisols that have a cryic soil temperature regime. They are soils of the cold climate. They are fine textured soils and periodically shrink and swell forming cracks that commonly open in late summer. Cryerts occur on the cold prairies of Canada where they are commonly derived from lacustrine deposits. They also occur in the US Rocky Mountains.

Torrerts::

Torrerts are the Vertisols of arid climates. Their cracks commonly stay open for most of the year but may close for at least a few days during rains. Many of these soils are found in closed depressions that may be ponded from time to time by runoff from higher areas. Some Torrerts are found in the southwest of the USA. These soils are commonly used for rangeland.

Uderts::

Uderts are the Vertisols of humid areas. They have a udic soil moisture regime. The cracks may not open completely some years due to high precipitation. In the USA, the soils occur on gentle slopes and are derived dominantly from marine shales, marls, and alluvium. Many of these soils supported grass, but some support hardwood or pine forests.

Usterts::

These are the Vertisols in temperate areas that do not receive high amounts of rainfall during the summer. They have an ustic soil moisture regime. Cracks open and close once or twice during the year. They are found extensively in the USA, Australia, Africa, and India. If irrigated, Usterts can be used intensively, but large areas are used for grazing due to a lack of machinery to till soils.

Xererts::

Xererts are the Vertisols of Mediterranean climates, which have xeric soil moisture regime. These soils have cracks that regularly close and open each year. In the USA, most of the soils supported grasses.

4.8 FAO/UNESCO Soil Classification Is Now World Reference Base for Soil Resources

FAO/UNESCO published a soil map of the world at the scale of 1:5,000,000 (FAO 1971–1981) that needed a legend for different soil mapping units. This legend was treated as a soil classification system in different regions of the world. The legend for the map was further revised which contained 28 “major soil groupings” (FAO 1988). A new map of soil resources of the world at the scale of 1:25,000,000 was presented at the 14th International Congress of Soil Science, Kyoto, Japan in 1990. The FAO legend was later replaced by the World Reference Base for Soil Resources (FAO 1994, 1998). In these replacement and updates, some earlier units were renamed, some new units were added, and some were merged. In the updated version, there are 30 “reference groups.” Some group names resemble Soil Taxonomy, some have been taken from classical Russian and European names, and others have been synthesized. This is known as FAO/WRB soil classification system. The WRB reference groups have been adopted in 1998 by the International Union of Soil Sciences as the standard for soil correlation and nomenclature (FAO AGL 2003). The reader is referred to “World Reference Base for Soil Resources 2006,” World Soil Resources Report 103 by FAO (2006) for recent updates. Now, there are 32 “reference soil groups” (RSG). The names and characteristics of the RSG are given below:

Acrisols

Acrisols are soils that have a higher clay content in the subsoil than in the topsoil due to clay migration forming an argic sub-horizon with low-activity clays and low base saturation. Acrisols are found in humid tropical, humid subtropical, and warm temperate regions and are most extensive in Southeast Asia, the southern fringes of the Amazon Basin, the southeast of the United States of America, and in both East and West Africa. Sedentary farming can be done in Acrisols with complete fertilization and careful management.

Albeluvisols

Albeluvisols are soils that have an illuvial clay horizon with an irregular or broken upper boundary resulting in tonguing of bleached soil material into the illuviation horizon. Albeluvisols are distributed in Europe, North Asia, and Central Asia, with minor occurrences in North America. Albeluvisols are developed in the continental regions that had permafrost in the Pleistocene of northeast Europe, northwest Asia, and southwest Canada and also in the loess and cover sand areas and old alluvial areas in moist temperate regions, such as France, central Belgium, southeast of the Netherlands, and west of Germany. Albeluvisols are not much suitable for cropping due to their acidity, low nutrient levels, tillage and drainage problems, and for its short growing season and severe frost during the long winter. They are usually vegetated with forest.

Alisols

Alisols are soils that have a higher clay content in the subsoil than in the topsoil due to clay migration forming an argic subsoil horizon with low base saturation and high-activity clays. They occur predominantly in humid tropical, humid subtropical and warm temperate regions. Major occurrences of Alisols are found in Latin America (Ecuador, Nicaragua, Venezuela, Colombia, Peru and Brazil), West Indies (Jamaica, Martinique and Saint Lucia), West Africa, East Africa, Southeast Asia, and northern Australia. Alisols also occur in China, Japan, and the southeast of the United States of America. The undulating topography on which Alisols are usually found makes soil susceptible to erosion and truncation. Alisols are naturally poorly fertile and have toxic concentrations of Al.

Andosols

Andosols are soils that develop in volcanic ashes. Andosols may also develop in other silicate-rich materials under acid weathering in humid and perhumid climates. Andosols occur in volcanic regions all over the world. Important concentrations are found in South America, Central America, Mexico, United States of America, Japan, the Philippines, Indonesia, Papua New Guinea, New Zealand, Fiji, Vanuatu, New Caledonia, Samoa, Hawaii, Kenya, Rwanda, Ethiopia, Madagascar, Italy, France, Germany and Iceland. Andosols are generally fertile soils and have high agricultural potential. The strong phosphate fixation caused by active Al and Fe of Andosols is a problem. Ameliorative measures to reduce this effect include application of lime, silica, organic material, and phosphate fertilizer.

Anthrosols

Anthrosols are soils that have been modified profoundly through human activities, such as addition of organic materials or household wastes, irrigation, and cultivation. Anthrosols with plaggic horizons (horizon resulting from manuring) are most common in northwest Europe. Anthrosols with irragric horizons (horizon developed as a result of irrigation) are found in irrigation areas in dry regions, for example, in Mesopotamia, near oases in desert regions, and in parts of India. Anthrosols with an anthraquic horizon (puddle layer or plow pan) overlying a hydragric horizon (subsurface horizon having characteristics of wet cultivation) are characteristics of paddy soils which occupy vast areas in China and in parts of South and Southeast Asia (Bangladesh, Sri Lanka, Viet Nam, Thailand, and Indonesia). Anthrosols with hortic horizons (horizon resulting from deep cultivation and fertilizer application) are found all over the world where humans have fertilized the soil with household wastes and manure. Rye, oats, barley, potato, sugar beet, and summer wheat are common crops on European Anthrosols with a plaggic horizon. Rice is grown intensively in Anthrosols with anthraquic horizons. Puddling of wetland rice fields (involving destruction of the natural soil structure by intensive tillage when the soil is saturated with water) is done to reduce percolation.

Arenosols

Arenosols are sandy soils, including both soils developed in residual sands and soils developed in recently deposited sands such as dunes in deserts and beach lands. Arenosols are one of the most extensive RSG in the world. Vast expanses of deep Aeolian sands are found on the Central African plateau. Other areas of Arenosols occur in the Sahelian region of Africa, various parts of the Sahara, central and Western Australia, the Near East, and China. Most Arenosols occur in arid and semiarid regions, but they may be found in humid areas as well. Agricultural potential of Arenosols is limited because of their coarse texture, high percolation, and low water and nutrient retaining capacity. They are, however, easy to plow, easy of rooting and harvesting. Some Arenosols may used for cereals and vegetables with sprinkler and drip irrigation . Most Arenosols are left under natural vegetation.

Calcisols

Calcisols are soils with secondary accumulation of lime. Calcisols are common in highly calcareous parent materials and widespread in arid and semiarid environments. Many Calcisols occur together with Solonchaks that are actually salt-affected soils. Natural vegetation of Calcisols comprises of shrubs, grasses, and herbs. Drought-tolerant rainfed crops might be grown, but Calcisols may be made productive with irrigation. Extensive areas of Calcisols are used for production of irrigated winter wheat, melons, and cotton in the Mediterranean zone. Sorghum, alfalfa, and fodder crops are tolerant of high Ca levels. A high pH of Calcisols (usually >7.5) often limits micronutrient availability. Zinc deficiency is the most ubiquitous micronutrient problem affecting cereal crops, including corn (maize), rice, and wheat in Calcisols of Turkey (Yilmaz et al. 1997; Cakmak 2008), Syria, Iraq, India, and Pakistan.

Cambisols

Cambisols exhibit the beginning of soil formation. A weak horizon differentiation may be noticed in the subsoil. Transformation of parent material is evident from structure formation and mostly brownish discoloration, increasing clay percentage, and/or carbonate removal. Cambisols may occur in both dry climates and also in humid tropics and subtropics. The young alluvial plains and terraces of the Ganges–Brahmaputra system (India and Bangladesh in Southeast Asia) are probably the largest continuous surface of Cambisols in the tropics. Cambisols generally make good agricultural land and are used intensively. Cambisols with high base saturation in the temperate zone are among the most productive soils on earth. Cambisols on irrigated alluvial plains in the dry zone are used intensively for production of food and oil crops. Cambisols with groundwater influence in alluvial plains are highly productive paddy soils.

Chernozems

Chernozems are soils with a thick black surface layer that is rich in organic matter. Chernozems are distributed mainly in the middle latitude steppes (grassland plains without trees) of Eurasia and North America. Chernozems are regarded as the most productive soils and are mainly used for arable cropping. Wheat, barley, and corn (maize) are the principal crops grown, alongside other food crops and vegetables. Some Chernozems are used for livestock rearing. Maize is widely grown in the warm temperate belt.

Cryosols

Cryosols comprise mineral soils formed in a permafrost environment. Water is present primarily in the form of ice. Cryosols are widely known as permafrost soils. Cryosols are circumpolar in both the Northern and Southern Hemispheres. Major areas with Cryosols are the Russian Federation, Canada, China, Alaska, and in parts of Mongolia. Most areas of Cryosols in North America and Eurasia are in the natural state and support sufficient vegetation for grazing animals, such as caribou, reindeer, and musk oxen. Overgrazing leads rapidly to erosion and other environmental damage.

Durisols

Durisols are very shallow to moderately deep, moderately well-, to well-drained soils that contain a layer of cemented secondary silica within 100 cm of the soil surface. Extensive areas of Durisols occur in Australia, South Africa and Namibia, and the United States of America with minor occurrences in Central and South America. The agricultural use of Durisols is limited to extensive grazing (rangeland). Durisols in natural environments generally support enough vegetation to contain erosion, but elsewhere erosion of the surface soil is widespread.

Ferralsols

Ferralsols are the classical, deeply weathered, red or yellow soils of the humid tropics. These soils have a clay assemblage dominated by low-activity clays, mainly kaolinite, and a high content of sesquioxides. Ferralsols are abundant on the continental shields of South America (especially Brazil) and Africa (especially Congo, Democratic Republic of the Congo, southern Central African Republic, Angola, Guinea, and eastern Madagascar) and Southeast Asia. Most Ferralsols have great soil depth and good physical properties. They are well drained but may in times be droughty because of their low available water storage capacity. The chemical fertility of Ferralsols is poor, weatherable minerals are almost absent, and cation retention by the mineral soil fraction is weak. Under natural forest, the bulk of plant nutrients is contained in the biomass; available plant nutrients in the soil are concentrated in the soil organic matter. Productivity of these soils may be improved by manuring and mulching.

Fluvisols

Fluvisols are young soils in alluvial deposits. Fluvisols occur on all continents and in all climates. Major concentrations of Fluvisols are found along rivers and lakes, for example, in the Amazon basin; the Ganges Plain of India and Bangladesh; the plains near Lake Chad in Central Africa; the marshlands of Brazil, Paraguay, and northern Argentina; and in deltaic areas of the Ganges–Brahmaputra, Indus, Mekong, Mississippi, Nile, Niger, Orinoco, Plate, Po, Rhine and Zambezi. Some Fluvisols have a thionic horizon (sulfidic material; acid sulfate soils) are found in the coastal lowlands of Southeast Asia (Indonesia, Vietnam, and Thailand), West Africa (Senegal, Gambia, Guinea Bissau, Sierra Leone, and Liberia) and along the northeast coast of South America (French Guiana, Guyana, Suriname, and Venezuela). Paddy rice cultivation is widespread on tropical Fluvisols. Many dryland crops are grown on Fluvisols as well, normally with irrigation in the dry period.

Gleysols

Gleysols are wetland soils (Chap. 13) that are saturated with groundwater for long enough periods to develop a characteristic gleyic color pattern. The upper part of the soil profile may be reddish, brownish, or yellowish colors but essentially grayish/bluish colors beneath. Gleysols occur in nearly all climates, from perhumid to arid. The largest extent of Gleysols is in subarctic areas in the north of the Russian Federation (especially Siberia), Canada, and Alaska and in humid temperate and subtropical lowlands, for example in China and Bangladesh. Paddy rice is cultivated in many Gleysols. Adequately drained Gleysols can be used for arable cropping. The soil will be puddle if cultivated wet. Therefore, Gleysols in depression areas with unsatisfactory possibilities to lower the groundwater table are best kept under a permanent grass cover or swamp forest.

Gypsisols

Gypsisols are soils with substantial secondary accumulation of gypsum. These soils are found in the driest parts of the arid climate zone. They were earlier called desert soils. Major occurrences of Gypsisols are in Mesopotamia, near East and Central Asian republics, Libyan and Namib deserts, southeast and central Australia, and the United States of America. Gypsisols that contain only a low percentage of gypsum in the upper 30 cm can be used for the production of small grains, cotton, alfalfa, etc. Gypsisols in young alluvial and colluvial deposits have a relatively low gypsum content. Where such soils are in the vicinity of water resources, they can be very productive. Wheat, apricots, dates, corn (maize), and grapes can be grown satisfactorily if irrigated at high rates in combination with forced drainage.

Histosols

Histosols are formed in organic material. They include soils developed in predominantly moss peat in boreal, arctic and subarctic regions, moss peat, reeds/sedge peat (fen) and forest peat in temperate regions, and mangrove peat and swamp forest peat in the humid tropics. Vast majority of Histosols occurs in lowlands. Common names are peat soils, muck soils, and bog soils (Chap. 13). The majority of Histosols occur in the boreal, subarctic and low arctic regions of the Northern Hemisphere. Most of the remaining Histosols occur in temperate lowlands and cool montane areas; only one-tenth of all Histosols are found in the tropics. Extensive areas of Histosols occur in the United States of America and Canada, Western Europe, and northern Scandinavia. Tropical forest peat borders the Sunda Shelf in Southeast Asia. In the temperate zone, millions of hectares of Histosols have been opened for agriculture. In many instances, this has initiated the gradual degradation, and ultimately the loss, of the precious peat.

Kastanozems

Kastanozems are dry grassland soils of the short-grass steppe belt. They have a similar profile to that of Chernozems, but the humus-rich surface horizon is thinner and not as dark as that of the Chernozems. They show more prominent accumulation of secondary carbonates. Major areas of Kastanozems are in the Eurasian short-grass steppe belt (southern Ukraine, the south of the Russian Federation, Kazakhstan, and Mongolia); in the Great Plains of the United States of America, Canada, and Mexico; and in the Pampas and Chaco regions of northern Argentina, Paraguay, and southern Bolivia. Kastanozems are potentially rich soils but lack enough moisture for high yields. Irrigation is nearly always necessary for good crop production, but care must be taken to avoid secondary salinization of the surface soil. Small grains and irrigated food and vegetable crops are the principal crops grown. Extensive grazing is another important land use on Kastanozems.

Leptosols

Leptosols are very shallow soils over continuous rock and soils that are extremely gravelly and/or stony. Leptosols were earlier called lithosols. Some Leptosols on calcareous rocks belonged to rendzinas. Leptosols are the most extensive RSG on earth. Leptosols are found from the tropics to the cold polar tundra and from sea level to the highest mountains. Leptosols are particularly widespread in montane areas, notably in Asia and South America, in the Sahara and the Arabian deserts, the Ungava Peninsula of northern Canada, and in the Alaskan mountains. Leptosols support grasses and forests. Some Leptosols have been planted to teak and mahogany in Southeast Asia; in the temperate zone, they are mainly under deciduous mixed forests whereas acid Leptosols are commonly under coniferous forest.

Lixisols

Lixisols comprise soils that have a higher clay content in the subsoil than in the topsoil due to clay migration leading to an argic subsoil horizon. Lixisols have a high base saturation and low-activity clays at certain depths. Lixisols are found in seasonally dry tropical, subtropical, and warm temperate regions on Pleistocene and older surfaces. These soils occur in sub-Sahelian and East Africa, South and Central America, Indian subcontinent, Southeast Asia, and Australia. Many Lixisols bear natural savannah or open woodland vegetation. Tillage of wet soil or use of excessively heavy machinery compacts the soil and causes serious structure deterioration. Tillage and erosion control measures such as terracing, contour plowing, mulching, and use of cover crops help to conserve the soil.

Luvisols

Luvisols are soils that have an illuvial argic subsoil horizon with high-activity clays and a high base saturation. Luvisols extend mainly in temperate regions such as in the west and center of the Russian Federation, the United States of America, and Central Europe but also in the Mediterranean region and southern Australia. Most Luvisols are fertile soils and suitable for a wide range of agricultural uses. Luvisols with a high silt content are susceptible to structure deterioration where tilled when wet or with heavy machinery. Luvisols on steep slopes require erosion control measures. Luvisols in the temperate zone are widely grown to small grains, sugar beet, and fodder; in sloping areas, they are used for orchards, forests, and/or grazing.

Nitisols

Nitisols are soil that are deep, red, and well drained with diffuse horizon boundaries and a subsurface horizon with more than 30% clay and moderate to strong angular blocky structure. These soils develop in tropical regions. Weathering is relatively advanced. More than half of all Nitisols are found in tropical Africa, notably in the highlands of Ethiopia, Kenya, Congo, and Cameroon. Nitisols are also found in tropical Asia, South America, Central America, Southeast Africa, and Australia. Nitisols are among the most productive soils of the humid tropics. The deep and porous solum and the stable soil structure of Nitisols permit deep rooting and make these soils quite resistant to erosion. Nitisols are planted to plantation crops such as cocoa, coffee, rubber, and pineapple and are also widely used for food crop production on small holdings. There is high P sorption in some Nitisols, and P fertilizer application is profitable there.

Phaeozems

Phaeozems are soils of relatively wet grassland and forest regions in moderately continental climates. Like Chernozems and Kastanozems, they also have dark, humus-rich surface horizons, but they are more extensively leached and are less rich in bases. Phaeozems are found in the humid and subhumid Central Lowlands and eastern most parts of the Great Plains of the United States of America. Phaeozems are also found in the subtropical pampas of Argentina and Uruguay. Some Phaeozems are present in China and the Russian Federation. Phaeozems are porous, fertile soils and make excellent farmland. In the United States of America and Argentina, Phaeozems are in use for the production of soybean and wheat and other small grains. Phaeozems on the high plains of Texas produce good yields of irrigated cotton. Phaeozems in the temperate belt are planted to wheat, barley, and vegetables.

Planosols

Planosols are soils with a light-colored, surface horizon that shows signs of periodic water stagnation and abruptly overlies a dense, slowly permeable subsoil with significantly more clay than the surface horizon. Planosols occur in subtropical and temperate regions with clear alternation of wet and dry seasons, including Brazil, Paraguay, Argentina, East and Southern Africa, United States of America, Bangladesh, Thailand, and Australia. Land use on Planosols is normally less intensive than that on most other soils under the same climate conditions. Vast areas of Planosols are used for extensive grazing. Planosols in the temperate zone are mainly in grass, or they are planted to arable crops such as wheat and sugar beet. Planosols in Southeast Asia are widely planted to a single crop of paddy rice.

Plinthosols

Plinthosols are soils with plinthite (an Fe-rich or Mn-rich mixture of kaolinitic clay, gibbsite, etc., with quartz and other constituents forming a layer of hard nodules or a hardpan or irregular aggregates), petroplinthite (a continuous, fractured, strongly cemented to indurated nodules) or pisoliths (discrete strongly cemented to indurated nodules). Plinthosols are most common in the wet tropics, notably in the eastern Amazon Basin, the central Congo Basin, and parts of Southeast Asia and northern Australia. Poor natural soil fertility caused by strong weathering, waterlogging in bottomlands and drought cause serious limitations of these soils to agriculture.

Podzols

Podzols are soils with a typically ash-gray eluvial subsurface horizon, bleached by loss of organic matter and iron oxides, on top of a dark accumulation horizon with brown to black humus. Podzols mainly occur in humid areas in the boreal and temperate zones. Podzols are extensive in Scandinavia, the northwest of the Russian Federation, and Canada. The low nutrient status, low level of available moisture, and low pH make podzols unattractive soils for arable farming. Aluminum toxicity and P deficiency are common problems. They are mainly under natural forests in the temperate and boreal regions. Some Podzols have been cleared for cropping with irrigation, liming, and fertilization.

Solonchaks

Solonchaks are soils that have a high concentration of soluble salts. They are largely confined to the arid and semiarid climate zones and to coastal regions in all climates. Solonchaks are most extensive in the Northern Hemisphere, notably in the arid and semiarid parts of northern Africa, the Near East, the former Soviet Union, and Central Asia; they are also widespread in Australia and the Americas. Saline soils are regarded as problem soils (Chap. 11). Strongly salt-affected soils have little agricultural value. They are used for extensive grazing of sheep, goats, camels, and cattle, or lie idle. When they are reclaimed, they may give good yields of cereals and vegetables. Heavy irrigation to meet crop requirement and leaching requirement are needed.

Solonetz

Solonetz are soils with a high proportion of adsorbed Na and/or Mg ions. Solonetz that contain free soda (Na2CO3) are strongly alkaline (field pH  >8.5). These soils were earlier called alkali soils and sodic (Regional Salinity Laboratory 1954). Solonetz occur predominantly in areas with a steppe climate in particular in flat lands with impeded vertical and lateral drainage. Major Solonetz areas are found in Ukraine, Russian Federation, Kazakhstan, Hungary, Bulgaria, Romania, China, United States of America, Canada, South Africa, Argentina, and Australia. These soils are also regarded as problem soils, and their management for agriculture needs improvement of soil structure and porosity, lowering of pH by acidifying agents and lowering the ESP (Chap. 11). Reclaiming Solonetz involves cost, labor, and time.

Stagnosols

Stagnosols are soils with a perched water table showing redoximorphic features caused by surface water. Stagnosols are periodically wet and mottled in the topsoil and subsoil, with or without concretions and/or bleaching. Stagnosols are found in West and Central Europe, North America, southeast Australia, and Argentina. The agricultural suitability of Stagnosols is limited because of their oxygen deficiency resulting from stagnating water above a dense subsoil. They have to be drained for arable agriculture.

Technosols

Technosols are soils strongly influenced by human activity and containing many artifacts. They include soils from wastes (landfills, sludge, cinders, mine spoils and ashes), pavements with their underlying unconsolidated materials, soils with geo-membranes and constructed soils in human-made materials. Technosols are found worldwide around human habitation and development sites. They are more likely to be contaminated soils. Many Technosols have to be treated with care as they may contain toxic substances resulting from industrial processes. Many Technosols may be planted to forests.

Umbrisols

Umbrisols accommodate soils in which organic matter has accumulated within the mineral surface soil (in most cases with low base saturation) to the extent that it significantly affects the behavior and utilization of the soil. Umbrisols occur in cool, humid regions, mostly mountainous and with little or no soil moisture deficit. Umbrisols are common in the Andean ranges of Colombia, Ecuador, and, to a lesser extent, in Venezuela, Bolivia, and Peru. They also occur in Brazil, India, Nepal, China, and Myanmar. Many Umbrisols are under a natural or near-natural vegetation cover. Coniferous forest predominates in Brazil in the United States of America. Umbrisols in tropical mountain areas in South Asia and Oceania are under montane evergreen forest.

Vertisols

Vertisols are deeply and widely cracking soils with a high proportion of swelling clays. Most Vertisols occur in the semiarid tropics, with an average annual rainfall of 500–1,000 mm, but Vertisols are also found in the wet tropics, e.g. Trinidad (where the annual rainfall sum amounts to 3,000 mm). The largest Vertisol areas are on sediments that have a high content of smectitic clays. Vertisols are prominent in South Africa, Australia, southwest of the United States of America, Uruguay, Paraguay, and Argentina. Although these soils are also problem soils, they have considerable agricultural potential if proper management is adopted. Tillage is usually difficult in Vertisols, but they have good chemical fertility, and occurrence of many of them is on extensive level plains in humid areas. Many Vertisols are productive. The crops include wheat, barley, sorghum millet, cotton, chickpeas, flax, and sugarcane. Construction of roads and buildings and other structures on Vertisols is risky.

4.8.1 Correlation of Reference Soil Groups of WRB with Soil Taxonomy

Reference soil groups

Soil taxonomy equivalents

Acrisols

Ultisols

Albeluvisols

Alfisols

Alisols

Ultisols

Andosols

Andisols

Anthrosols

Entisols, Inceptisols

Arenosols

Entisols (Psamments)

Calcisols

Aridisols (Calcids)

Cambisols

Inceptisols

Chernozems

Mollisols

Cryosols

Gelisols

Durisols

Aridisols (Durids)

Ferralsols

Oxisols

Fluvisols

Entisols (Fluvents Fluvaquent)

Gleysols

Inceptisols, Entisols Alfisols, Mollisols

Gypsisols

Aridisols (Gypsids)

Histosols

Histosols, Gelisols (Histels)

Kastanozems

Mollisols (Ustolls, Xerolls)

Leptosols

Entisols

Lixisols

Alfisols

Luvisols

Alfisols

Nitisols

Inceptisols, Alfisols, Oxisols, Ultisols

Phaeozems

Mollisols (Udolls and Albolls)

Planosols

Alfisols, Ultisols, Mollisols

Plinthosols

Alfisols, Oxisols, Ultisols

Podzols

Spodosols

Regosols

Entisols

Solonchaks

Aridisols (Salids)

Solonetz

Aridisols (several other orders)

Stagnosols

Entisols, Inceptisols, Alfisols, Ultisols Mollisols

Technosols

Entisols

Umbrisols

Entisols, Inceptisols

Vertisols

Vertisols

Study Questions

  1. 1.

    What do you mean by a diagnostic horizon? Write the features of an anthropic, a plaggen, and a mollic epipedon. What are characteristics of argillic, albic, kandic, and calcic horizons? How does anthropic epipedon differ from plaggen epipedon?

  2. 2.

    What do you mean by soil moisture control section? How does torric regime differ from aridic regime? Which of ustic and udic soil moisture regime is more favorable for crop growth?

  3. 3.

    How do you distinguish between Entisols and Inceptisols? What soil orders are found in tropical areas? What soil orders are typical of humid temperate climate? What are the agricultural significance of Alfisols and Mollisols?

  4. 4.

    How do Chernozems, Phaeozems, and Kastanozems differ? How do you distinguish between Anthrosols and Technosols?

  5. 5.

    Make a list of reference soil groups suitable for cultivation of paddy rice. What are the land uses of Cryosols, Leptosols, Histosols, and Vertisols?