USDA soil taxonomy

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USDA soil taxonomy (ST) developed by United States Department of Agriculture and the National Cooperative Soil Survey provides an elaborate classification of soil types according to several parameters (most commonly their properties) and in several levels: Order, Suborder, Great Group, Subgroup, Family, and Series . The classification was originally developed by Guy Donald Smith, former director of the U.S. Department of Agriculture's soil survey investigations. [1]



A taxonomy is an arrangement in a systematic manner; the USDA soil taxonomy has six levels of classification. They are, from most general to specific: order, suborder, great group, subgroup, family and series. Soil properties that can be measured quantitatively are used in this classification system – they include: depth, moisture, temperature, texture, structure, cation exchange capacity, base saturation, clay mineralogy, organic matter content and salt content. There are 12 soil orders (the top hierarchical level) in soil taxonomy. [2] [3] The names of the orders end with the suffix -sol. The criteria for the different soil orders include properties that reflect major differences in the genesis of soils. [4] The orders are:

The percentages listed above [5] are for land area free of ice. "Soils of Mountains", which constitute the balance (11.6%), have a mixture of those listed above, or are classified as "Rugged Mountains" which have no soil.

The above soil orders in sequence of increasing degree of development are Entisols, Inceptisols, Aridisols, Mollisols, Alfisols, Spodosols, Ultisols, and Oxisols. Histosols and Vertisols may appear in any of the above at any time during their development.

The soil suborders within an order are differentiated on the basis of soil properties and horizons which depend on soil moisture and temperature. Forty-seven suborders are recognized in the United States. [6]

The soil great group category is a subdivision of a suborder in which the kind and sequence of soil horizons distinguish one soil from another. About 185 great groups are recognized in the United States. Horizons marked by clay, iron, humus and hard pans and soil features such as the expansion-contraction of clays (that produce self-mixing provided by clay), temperature, and marked quantities of various salts are used as distinguishing features. [6]

The great group categories are divided into three kinds of soil subgroups: typic, intergrade and extragrade. A typic subgroup represents the basic or 'typical' concept of the great group to which the described subgroup belongs. An intergrade subgroup describes the properties that suggest how it grades towards (is similar to) soils of other soil great groups, suborders or orders. These properties are not developed or expressed well enough to cause the soil to be included within the great group towards which they grade, but suggest similarities. Extragrade features are aberrant properties which prevent that soil from being included in another soil classification. About 1,000 soil subgroups are defined in the United States.sfn|Donahue|Miller|Shickluna|1977|p=409}}

A soil family category is a group of soils within a subgroup and describes the physical and chemical properties which affect the response of soil to agricultural management and engineering applications. The principal characteristics used to differentiate soil families include texture, mineralogy, pH, permeability, structure, consistency, the locale's precipitation pattern, and soil temperature. For some soils the criteria also specify the percentage of silt, sand and coarse fragments such as gravel, cobbles and rocks. About 4,500 soil families are recognised in the United States. [7]

A family may contain several soil series which describe the physical location using the name of a prominent physical feature such as a river or town near where the soil sample was taken. An example would be Merrimac for the Merrimack River in New Hampshire. More than 14,000 soil series are recognised in the United States. This permits very specific descriptions of soils. [8]

A soil phase of series, originally called 'soil type' describes the soil surface texture, slope, stoniness, saltiness, erosion, and other conditions. [8]

Soil Orders

Global distribution of Soil Orders in the USDA soil taxonomy system. A much larger version of the map is also available. Global soil regions.jpg
Global distribution of Soil Orders in the USDA soil taxonomy system. A much larger version of the map is also available.
Name of Soil Orders in Soil Taxonomy with Their Major Characteristics
NameMajor CharacteristicsNameMajor Characteristics
Alfisols Must have argillic, natric, or kandic horizon; High-to-medium base saturation; Moderately weathered; Commonly form under boreal or broadleaf forests; Rich in iron and aluminum; Common in humid areas, semi-tropics, and mediterranean climates; 9.6% of global & 14.5% of U.S. ice-free land Andisols Form from volcanic ejecta, dominated by allophane or Al-humic complexes; Must have andic soil properties: high in poorly crystalline Fe and Al minerals, high in phosphorus, low bulk density, and high proportions of glass and amorphous colloidal materials, such as allophane, imogolite and ferrihydrite; High Organic Matter content, sometimes melanic epipedon; 0.7% of global & 1.7% of U.S. ice-free land
Aridisols Dry soil (i.e., must have aridic moisture regime); Ochric epipedon is common; Sometimes argillic or natric horizon; Must have some diagnostic subsurface horizon; Commonly in deserts; 12.7% of global & 8.8% of U.S. ice-free land Entisols Least soil profile development; Ochric epipedon is common; No B horizons; most common order by surface area (16.3% of global & 12.2% of U.S. ice-free land)
Gelisols Soils with permafrost within 100 cm or cryoturbation (frost churning) within 100 cm plus permafrost within 200 cm; Commonly at high latitudes and elevations; 8.6% of global & 7.5% of U.S. ice-free land Histosols Must have histic epipedon; Usually aquic soil moisture regime; No diagnostic subsurface horizons; Rapid decomposition when aerated; Peat or bog; >20% organic matter; Organic soil materials extending down to an impermeable layer or with an organic layer that is more than 40 cm thick and without andic properties Commonly in wetlands (swamps, marshes, etc.); 1.2% of global & 1.3% of U.S. ice-free land
Inceptisols Similar to Entisol, but beginning of a B horizon is evident; No diagnostic subsurface horizons; On landscapes continuously eroded or young deposits; Cambic, sulfuric, calcic, gypsic, petrocalcic, or petrogypsic horizon, or with a mollic, umbric, or histic epipedon, or with an exchangeable sodium percentage of >15% or fragipan; 9.9% of global & 9.1% of U.S. ice-free land Mollisols Must have mollic epipedon; High base saturation of >50%; Dark soils; Some with argillic or natric horizons; Common in grasslands; 6.9% of global & 22.4% of U.S. ice-free land
Oxisols Most soil profile development; Must have oxic horizon within 150 cm of soil surface; Low nutrient availability; No argillic horizon; Highly weathered; Dominated by end-member clays, Al and Fe oxides; Commonly in old landscapes in tropics; 7.6% of global & <0.01% of U.S. ice-free land Spodosols Must have spodic horizon within 2 m of soil surface and without andic properties; Usually have albic horizon; High in Fe, Al oxides and humus accumulation; Acidic soils; Common in coniferous or boreal forests; 2.6% of global and 3.3% of U.S. ice-free land
Ultisols Must have argillic or kandic horizon; Low base saturation of <35% at 2 m depth or 75 cm below a fragipan; Common in subtropical regions; often known as red clay soils; 8.5% of global & 9.6% of U.S. ice-free land Vertisols Usually mollic epipedon; High in shrinking and swelling clays; >30% clay to a depth of 50 cm; Deep cracks (called gilgai) form when soil dries; Form from parent material high in clay (e.g., shales, basins, exposed Bt horizons of old soils); 2.4% of global & 1.7% of U.S. ice-free land

Example of classification of a soil type

Order: Entisols

Suborder: Fluvents
Great Group: Torrifluvents
Subgroup: Typic Torrifluvents
Family: Fine-loamy, mixed, superactive, calcareous, Typic Torrifluvents
Series: Jocity, Youngston.

Another Example

Order: Alfisols

Suborder: Xeralfs
Great Group: Durixeralfs
Subgroup: Abruptic Durixeralfs
Family: Fine, Mixed, Active, thermic Abruptic Durixeralfs
Series: San Joaquin (soil)

Link to Official Series Description:

Soil temperature regimes

Global Distribution of soil temperature regimes Global Distribution of Soil Temperature Regimes.jpg
Global Distribution of soil temperature regimes

Soil temperature regimes, such as frigid, mesic, and thermic, are used to classify soils at some of the lower levels of the Soil Taxonomy. The cryic temperature regime distinguishes some higher-level groups. These regimes are based on the mean annual soil temperature (MAST), mean summer temperature, and the difference between mean summer and winter temperatures all at a soil depth of 50 cm. It is normally assumed that the MAST (in °C) equals the sum of the mean annual air temperature plus 2°C. If the difference between mean summer and winter temperatures is less than 6 °C, then add "Iso" at the front of the name of the Soil Temperature Class.

Soil temperature regimeTemperature range
Pergelic~ -8°C to -4°C
Subgelic~ -4°C to 0°C
Frigid~ 0°C to 8°C
Mesic8°C to 15°C
Thermic15°C to 22°C
Hyperthermic22°C or higher

Soil moisture regimes

Map of United States Distribution of Soil Moisture Regimes United States Soil Moisture Regimes.jpg
Map of United States Distribution of Soil Moisture Regimes
Map of Global Distribution of Soil Moisture Regimes Global Distribution of Soil Moisture Regimes.jpg
Map of Global Distribution of Soil Moisture Regimes

The soil moisture regime, often reflective of climatic factors, is a major determinant of the productivity of terrestrial ecosystems, including agricultural systems. The soil moisture regimes are defined based on the levels of the groundwater table and the amounts of soil water available to plants during a given year in a particular region. Several moisture regime classes are used to characterize soils.

Soil Moisture RegimeMajor Characteristics
AquicSoil is saturated with water and virtually free of gaseous oxygen for sufficient periods of time, such that there is evidence of poor aeration (gleying and mottling); Common in wetlands
Udic Soil moisture is sufficiently high year-round in most years to meet plant requirement; Common in humid regions
Ustic Soil moisture is intermediate between Udic and Aridic regimes; generally, plant-available moisture during the growing season, but severe periods of drought may occur; Common in semi-arid regions
AridicSoil is dry for at least half of the growing season and moist for less than 90 consecutive days; Common in arid (desert-like) regions
XericSoil moisture regime is found in Mediterranean-type climates, with cool, moist winters and warm, dry summers. Like the Ustic Regime, it is characterized as having long periods of drought in the summer

See also

Related Research Articles

Oxisol A soil type known for occurring in tropical rain forests

Oxisols are a soil order in USDA soil taxonomy, best known for their occurrence in tropical rain forest, 15–25 degrees north and south of the Equator. In the World Reference Base for Soil Resources (WRB), they belong mainly to the ferralsols, but some are plinthosols or Nitisols. Some oxisols have been previously classified as laterite soils.

Alfisol Soil type

Alfisols are a soil order in USDA soil taxonomy. Alfisols form in semi-arid to humid areas, typically under a hardwood forest cover. They have a clay-enriched subsoil and relatively high native fertility. "Alf" refers to aluminium (Al) and iron (Fe). Because of their productivity and abundance, the Alfisols represent one of the more important soil orders for food and fiber production. They are widely used both in agriculture and forestry, and are generally easier to keep fertile than other humid-climate soils, though those in Australia and Africa are still very deficient in nitrogen and available phosphorus. Those in monsoonal tropical regions, however, have a tendency to acidify when heavily cultivated, especially when nitrogenous fertilizers are used.

Mollisol soil type in USDA soil taxonomy

Mollisols are a soil order in USDA soil taxonomy. Mollisols form in semi-arid to semi-humid areas, typically under a grassland cover. They are most commonly found in the mid-latitudes, namely in North America, mostly east of the Rocky Mountains, in South America in Argentina (Pampas) and Brazil, and in Asia in Mongolia and the Russian Steppes. Their parent material is typically base-rich and calcareous and include limestone, loess, or wind-blown sand. The main processes that lead to the formation of grassland Mollisols are melanisation, decomposition, humification and pedoturbation.

The udic moisture regime is common to soils of humid climates which have well-distributed rainfall, or which have enough rain in summer so that the amount of stored moisture plus rainfall is approximately equal to, or exceeds, the amount of evapotranspiration. Water moves down through the soil at some time in most years.

Vertisol Clay-rich soil, prone to cracking

In both the World Reference Base for Soil Resources (WRB) and the USDA soil taxonomy, a Vertisol is a soil in which there is a high content of expansive clay minerals, many of them known as montmorillonite, that form deep cracks in drier seasons or years. In a phenomenon known as argillipedoturbation, alternate shrinking and swelling causes self-ploughing, where the soil material consistently mixes itself, causing some Vertisols to have an extremely deep A horizon and no B horizon.. This heaving of the underlying material to the surface often creates a microrelief known as gilgai.

In USDA soil taxonomy, a Psamment is defined as an Entisol which consists basically of unconsolidated sand deposits, often found in shifting sand dunes but also in areas of very coarse-textured parent material subject to millions of years of weathering. This latter case is characteristic of the Guiana Highlands of northern South America. A Psamment has no distinct soil horizons, and must consist entirely of material of loamy sand or coarser in texture. In the World Reference Base for Soil Resources (WRB), Psamments are known as Arenosols.

Entisol soil type

In USDA soil taxonomy, Entisols are defined as soils that do not show any profile development other than an A horizon. An entisol has no diagnostic horizons, and most are basically unaltered from their parent material, which can be unconsolidated sediment or rock. Entisols are the second most abundant soil order, occupying about 16% of the global ice-free land area.

The paleopedological record is, essentially, the fossil record of soils. The paleopedological record consists chiefly of paleosols buried by flood sediments, or preserved at geological unconformities, especially plateau escarpments or sides of river valleys. Other fossil soils occur in areas where volcanic activity has covered the ancient soils.

This is an index of articles relating to soil.

Acrisol soil type

An Acrisol is a Reference Soil Group of the World Reference Base for Soil Resources (WRB). It has a clay-rich subsoil and is associated with humid, tropical climates, such as those found in Brazil, and often supports forested areas. In the USDA soil taxonomy, Acrisols correspond to the Humult, Udult and Ustult suborders of the Ultisols and also to Oxisols with a kandic horizon and to some Alfisols. The Acrisols low fertility and toxic amounts of aluminium pose limitations to its agricultural use, favouring in many places its use for silviculture, low intensity pasture and protected areas. Crops that can be successfully cultivated, if climate allows, include tea, rubber tree, oil palm, coffee and sugar cane.


Paleopedology is the discipline that studies soils of past geological eras, from quite recent (Quaternary) to the earliest periods of the Earth's history. Paleopedology can be seen either as a branch of soil science (pedology) or of paleontology, since the methods it uses are in many ways a well-defined combination of the two disciplines.

Gleysol type of wetland soil

A Gley is a wetland soil that, unless drained, is saturated with groundwater for long enough periods to develop a characteristic gleyic colour pattern. This pattern is essentially made up of reddish, brownish, or yellowish colours at surfaces of soil particles and/or in the upper soil horizons mixed with greyish/blueish colours inside the peds and/or deeper in the soil. Gleysols are also known as Gleyzems, meadow soils, Aqu-suborders of Entisols, Inceptisols and Mollisols, or as groundwater soils and hydro-morphic soils.

Inceptisol USDA soil classification

Inceptisols are a soil order in USDA soil taxonomy. They form quickly through alteration of parent material. They are more developed than Entisols. They have no accumulation of clays, iron oxide, aluminium oxide or organic matter. They have an ochric or umbric horizon and a cambic subsurface horizon.

Ustochrepts are a great group of soils, in the USDA soil taxonomy. They are classed in the sub-order Ochrepts, in the order Inceptisols

The Canadian System of Soil Classification is more closely related to the American system than any other, but they differ in several ways. The Canadian system is designed to cover only Canadian soils. The Canadian system dispenses with the sub-order hierarchical level. Solonetzic and Gleysolic soils are differentiated at the order level.

Haplocambids is a soil Taxonomy great group. Soil with 0-5 slopes over 5 °C temperature and loam soil structure is Haplocambids. Almost cold condition and high altitude soil classified into this group. These soil types are the most commonly occurring of the Cambids. The soils are characterized by minimal horizon expression. Most Haplocambids have a redistribution of carbonates below the cambic horizon. The amount of carbonates, however, is insufficient to meet the definition of a calcic horizon, or the upper boundary is more than 100 cm below the soil surface. These soils occur on a variety of landscapes, commonly on those that are younger than late Pleistocene in age. Haplocambids are divided into 22 suborders.

Nitisol soil type

A Nitisol in the World Reference Base for Soil Resources (WRB) is a deep, red, well-drained soil with a clay content of more than 30% and a blocky structure. Nitisols correlate with the Kandic Alfisols, Ultisols and Inceptisols of the USDA soil taxonomy.

Ustic soil type

Ustic is a class of soil moisture regime. It is one of a range of different soil moisture regimes, such as: aquic moisture regime, aridic moisture regime, udic moisture regime and xeric moisture regime. The ustic moisture regime is intermediate between the aridic regime and the udic regime.

The Polish Soil Classification is a soil classification system used to describe, classify and organize the knowledge about soils in Poland.


  1. Donovan, Alan (1981-08-29). "Guy D. Smith, 73, USDA Soil Expert, Dies". Washington Post. ISSN   0190-8286 . Retrieved 2017-11-15.
  2. The Soil Orders Archived 12 January 2010 at the Wayback Machine , Department of Environmental Sciences, University of Virginia, retrieved 23 October 2012.
  3. Donahue, Miller & Shickluna 1977, pp. 411–32.
  4. Soil Survey Staff (1999). Soil taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys. 2nd edition. Natural Resources Conservation Service. U.S. Department of Agriculture Handbook 436 (PDF). United States Dept. of Agriculture, Naturel Resources Conservation Service. Retrieved November 22, 2019.
  5. The Twelve Soil Orders: Soil Taxonomy Archived 26 March 2012 at the Wayback Machine , Soil & Land Resources Division, College of Agricultural and Life Sciences, University of Idaho
  6. 1 2 Donahue, Miller & Shickluna 1977, p. 409.
  7. Donahue, Miller & Shickluna 1977, pp. 409–10.
  8. 1 2 Donahue, Miller & Shickluna 1977, p. 410.