Soil classification

Soil classification deals with the systematic categorization of soils based on distinguishing characteristics as well as criteria that dictate choices in use.

Samples of soils belonging to different soil types


Soil classification is a dynamic subject, from the structure of the system itself, to the definitions of classes, and finally in the application in the field. Soil classification can be approached from the perspective of soil as a material and soil as a resource.


Engineers, typically geotechnical engineers, classify soils according to their engineering properties as they relate to use for foundation support or building material. Modern engineering classification systems are designed to allow an easy transition from field observations to basic predictions of soil engineering properties and behaviors.

The most common engineering classification system for soils in North America is the Unified Soil Classification System (USCS). The USCS has three major classification groups: (1) coarse-grained soils (e.g. sands and gravels); (2) fine-grained soils (e.g. silts and clays); and (3) highly organic soils (referred to as "peat"). The USCS further subdivides the three major soil classes for clarification. It distinguishes sands from gravels by grain size, and further classifying some as "well-graded" and the rest as "poorly-graded". Silts and clays are distinguished by the soils' Atterberg limits, and separates "high-plasticity" from "low-plasticity" soils as well. Moderately organic soils are considered subdivisions of silts and clays, and are distinguished from inorganic soils by changes in their plasticity properties (and Atterberg limits) on drying. The European soil classification system (ISO 14688) is very similar, differing primarily in coding and in adding an "intermediate-plasticity" classification for silts and clays, and in minor details.

Other engineering soil classification systems in the United States include the AASHTO Soil Classification System, which classifies soils and aggregates relative to their suitability for pavement construction, and the Modified Burmister system, which works similarly to the USCS, but includes more coding for various soil properties.[1]

A full geotechnical engineering soil description will also include other properties of the soil including color, in-situ moisture content, in-situ strength, and somewhat more detail about the material properties of the soil than is provided by the USCS code. The USCS and additional engineering description is standardized in ASTM D 2487.[2]

Soil science

Soil texture triangle showing the USDA classification system based on grain size

For soil resources, experience has shown that a natural system approach to classification, i.e. grouping soils by their intrinsic property (soil morphology), behaviour, or genesis, results in classes that can be interpreted for many diverse uses. Differing concepts of pedogenesis, and differences in the significance of morphological features to various land uses can affect the classification approach. Despite these differences, in a well-constructed system, classification criteria group similar concepts so that interpretations do not vary widely. This is in contrast to a technical system approach to soil classification, where soils are grouped according to their fitness for a specific use and their edaphic characteristics.

Natural system approaches to soil classification, such as the French Soil Reference System (Référentiel pédologique français) are based on presumed soil genesis. Systems have developed, such as USDA soil taxonomy and the World Reference Base for Soil Resources[3][4], which use taxonomic criteria involving soil morphology and laboratory tests to inform and refine hierarchical classes. Another approach is numerical classification, also called ordination, where soil individuals are grouped by multivariate statistical methods such as cluster analysis. This produces natural groupings without requiring any inference about soil genesis.

In soil survey, as practiced in the United States, soil classification usually means criteria based on soil morphology in addition to characteristics developed during soil formation. Criteria are designed to guide choices in land use and soil management. As indicated, this is a hierarchical system that is a hybrid of both natural and objective criteria. USDA soil taxonomy provides the core criteria for differentiating soil map units. This is a substantial revision of the 1938 USDA soil taxonomy which was a strictly natural system. The USDA classification was originally developed by Guy Donald Smith, former director of the U.S. Department of Agriculture's soil survey investigations.[5] Soil taxonomy based soil map units are additionally sorted into classes based on technical classification systems. Land Capability Classes, hydric soil, and prime farmland are some examples.

The European Union uses the World Reference Base for Soil Resources (WRB), currently the Update 2015[3] of the third edition 2014 (see the list of soils there). Previously, the earlier editions of the WRB were used. According to the first edition of the WRB (1998)[6], the booklet "Soils of the European Union"[7] was published by the former Institute of Environment and Sustainability (now: Land Resources Unit, European Soil Data Centre/ESDAC).

In addition to scientific soil classification systems, there are also vernacular soil classification systems. Folk taxonomies have been used for millennia, while scientifically based systems are relatively recent developments.[8] Knowledge on the spatial distribution of soils has increased dramatically. SoilGrids is a system for automated soil mapping based on models fitted using soil profiles and environmental covariate data. On a global scale, it provides maps at 1.00–0.25 km spatial resolution [9]. Whether sustainability might be the ultimate goal for managing the global soil resources, these new developments require studied soils to be classified and given its own name [10].


The US Occupational Safety and Health Administration (OSHA) requires the classification of soils to protect workers from injury when working in excavations and trenches. OSHA uses 3 soil classifications plus one for rock, based primarily on strength but also other factors which affect the stability of cut slopes:[11]

  • Stable Rock: natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed.
  • Type A - cohesive, plastic soils with unconfined compressive strength greater than 1.5 ton per square foot (tsf)(144 kPa), and meeting several other requirements (which induces a lateral earth pressure of 25 psf per ft of depth[12])
  • Type B - cohesive soils with unconfined compressive strength between 0.5 tsf (48 kPa) and 1.5 tsf (144 kPa), or unstable dry rock, or soils which would otherwise be Type A (lateral earth pressure of 45 psf per ft of depth[12])
  • Type C - granular soils or cohesive soils with unconfined compressive strength less than 0.5 tsf (48 kPa) or any submerged or freely seeping soil or adversely bedded soils (lateral earth pressure of 80 psf per ft of depth[12])
  • Type C60 - A subtype of Type C soil, though is not officially recognized by OSHA as a separate type, induces a lateral earth pressure of 60 psf per ft of depth[13][14]

Each of the soil classifications has implications for the way the excavation must be made or the protections (sloping, shoring, shielding, etc.) that must be provided to protect workers from collapse of the excavated bank.[15][16]

See also


  1. ^ "Donald M. Burmister". Archived from the original on 2012-04-15. Retrieved 2014-06-11.
  2. ^ Classification of Soils for Engineering Purposes: Annual Book of ASTM Standards, D 2487-83, 04, American Society for Testing and Materials, 1985, pp. 395–408, archived from the original on 2010-09-14, retrieved 2015-05-08
  3. ^ a b IUSS Working Group WRB (2015). "World Reference Base for Soil Resources 2014, Update 2015" (PDF). World Soil Resources Reports 106, FAO, Rome. Archived (PDF) from the original on 2018-08-29. Retrieved 2018-10-10.
  4. ^ H.-P. Blume, P. Schad (2015). "90 Years of Soil Classification of the IUSS" (PDF). IUSS Bulletin 126, 38–45. Archived from the original (PDF) on 2018-09-06. Retrieved 2018-12-09.
  5. ^ Donovan, Alan (1981-08-29). "Guy D. Smith, 73, USDA Soil Expert, Dies". Washington Post. ISSN 0190-8286. Archived from the original on 2017-11-15. Retrieved 2017-11-15.
  6. ^ ISSS-ISRIC-FAO (1998). "World Reference Base for Soil Resources". World Soil Resources Reports 84, FAO, Rome. Archived from the original on 2018-12-16. Retrieved 2018-12-13.
  7. ^ G. Tóth, L. Montanarella, V. Stolbovoy, F. Máté, K. Bódis, A. Jones, P. Panagos, M. Van Liedekerke (2008). "Soils of the European Union" (PDF). European Commission, Luxembourg.CS1 maint: multiple names: authors list (link)
  8. ^ Soil classification systems Archived 2006-09-07 at the Wayback Machine. Url last accessed 2006-04-18
  9. ^ Hengl, T; et al. (2017). "Global gridded soil information based on machine learning". PLOS ONE. 12. doi:10.1371/journal.pone.0169748.
  10. ^ Certini, G; Scalenghe, R (2019). "Unnamed soils, lost opportunities". Environmental Science & Technology. 53 (15): 8477–8478. Bibcode:2019EnST...53.8477C. doi:10.1021/acs.est.9b03050.
  11. ^ "Safety and Health Regulations for Construction Subpart P, Excavations, Appendix A". 9 August 1994. Archived from the original on 5 March 2016. Retrieved 2015-05-07.
  12. ^ a b c "Safety and Health Regulations for Construction Subpart P, Excavations, Appendix C". 9 August 1994. Archived from the original on 28 August 2017. Retrieved 2017-08-28.
  13. ^ "Corrugated Sheeting, Tabulated Data" (PDF). Pacific Shoring, LLC. January 1, 2012. Archived (PDF) from the original on August 22, 2016. Retrieved August 8, 2016.
  14. ^ "Soil classification. - Occupational Safety and Health Administration". Archived from the original on 2016-08-27. Retrieved 2016-08-08.
  15. ^ "Sampling and Analytical Methods | Classification of Soils for Excavations, ID-194". Archived from the original on 2014-03-19. Retrieved 2014-06-11.
  16. ^ "Safety and Health Regulations for Construction Subpart P, Excavations, Appendix B". 9 August 1994. Archived from the original on 15 July 2014. Retrieved 2014-06-11.


Current international system

  • Buol, S.W., Southard, R.J., Graham, R.C., and McDaniel, P.A. (2003). Soil Genesis and Classification, 5th Edition. Iowa State Press - Blackwell, Ames, IA.
  • Driessen, P., Deckers, J., Spaargaren, O., & Nachtergaele, F. (Eds.). (2001). Lecture notes on the major soils of the world. Rome: FAO.
  • IUSS Working Group WRB: World Reference Base for Soil Resources 2014, Update 2015. World Soil Resources Reports 106, FAO, Rome 2015. ISBN 978-92-5-108369-7 (PDF 2,3 MB).

Current national systems

  • Agriculture Canada Expert Committee on Soil Survey. (1987). The Canadian system of soil classification (2nd ed.). Ottawa: Canadian Government Publishing Centre.
  • Avery, B. W. (1980). Soil classification for England and Wales: higher categories. Cranfield, England: Cranfield University, Soil Survey & Land Research Centre/National Soil Resources Institute.[1]
  • Baize, D., & Girard, M. C. (Eds.). (1995). Référentiel pédologique 1995. Paris: Institut National de la Recherche Agronomique.
  • Baize, D., & Girard, M. C. (Eds.). (1998). A sound reference base for soils: The "Référentiel Pédologique" (English translation by Hodgson J.M., Eskenazi N.R., & Baize D. ed.). Paris: Institut National de la Recherche Agronomique.
  • Baize, D., & Girard, M. C. (Eds.). (2008). Référentiel pédologique, troisième édition. Association française pour l’étude du sol (Afes). Versailles, France.
  • Hewitt, A. E. (1992). Soil classification in New Zealand: legacy and lessons. Australian Journal of Soil Research, 30, 843-854.
  • Hewitt, A. E. (2010). New Zealand soil classification, third edition. Manaaki Whenua - Landcare Research. Lincoln, Canterbury, New Zealand.
  • Isbell, R. F. and the National Committee on Soil and Terrain. (2016). The Australian soil classification, second edition. CSIRO. Clayton South, Victoria, Australia.
  • Soil Classification Working Group. (2018). Soil classification: a natural and anthropogenic system for South Africa, third edition. Agricultural Research Council; Institute for Soil, Climate and Water. Pretoria, RSA.
  • Soil Survey Staff. (1999). Soil taxonomy: a basic system of soil classification for making and interpreting soil surveys (2nd ed.). Washington, DC: US Department of Agriculture Soil Conservation Service.

Current technical systems

Technical soil classification systems focus on representing some specific facet or quality of the soil, rather than a direct pedogenetic classification. Such technical classifications are developed with specific applications in mind, such as soil-water relationships, land quality assessment or geotechnical engineering.

  • Boorman, D. B., Hollis, J. M., & Lilly, A. (1995). Hydrology of soil types: a hydrologically-based classification of the soils of the United Kingdom (No. 126): UK Institute of Hydrology.[2]
  • Klingebiel, A. A., & Montgomery, P. H. (1961). Land capability classification. Washington, DC: US Government Printing Office.
  • Sanchez, P. A., Palm, C. A., & Buol, S. W. (2003). Fertility capability soil classification: a tool to help assess soil quality in the tropics. Geoderma, 114(3-4), 157-185.
  • American Society for Testing and Materials, 1985, D 2487-83, Classification of Soils for Engineering Purposes: Annual Book of ASTM Standards. Vol. 04.08, pp 395–408.

Earlier systems of historical interest

  • Baldwin, M., Kellogg, C. E., & Thorp, J. (1938). Soil classification. In Soils and men: Yearbook of agriculture (pp. 979–1001). Washington, DC: U.S. Department of Agriculture.
  • Simonson, R. W. (1989). Historical aspects of soil survey and soil classification with emphasis on the United States, 1899-1970. Wageningen, NL: International Soil Reference and Information Centre (ISRIC).


  • Eswaran, H., Rice, T., Ahrens, R., & Stewart, B. A. (Eds.). (2002). Soil classification : a global desk reference. Boca Raton, Fla.: CRC Press.
  • Butler, B. E. (1980). Soil classification for soil survey. Oxford: Oxford Science Publications.Science, 96,
  • Cline, M. G. (1949). Basic principles of soil classification. Soil Science, 67(2), 81-91.
  • Cline, M. G. (1963). Logic of the new system of soil classification. Soil 17-22.
  • Webster, R. (1968). Fundamental objections to the 7th approximation. Journal of Soil Science, 19, 354-366.
  • Terzaghi Karl (1924). Soil Mechanics in Engineering Practice, Wiley-Interscience; 3 Sub-edition (January 1996, ISBN 0-471-08658-4)
  • kevin Hart (1923) . founded it

Numerical classification

  • McBratney, A. B., & de Gruijter, J. J. (1992). A continuum approach to soil classification by modified fuzzy k-means with extragrades. Journal of Soil Science, 43(1), 159-175.


  1. ^ "Soil classification system of England and Wales". Cranfield University, National Soil Resources Institute. Archived from the original on 2018-04-30. Retrieved 2019-01-06.
  2. ^ "Hydrology of soil types" (PDF). Cranfield University, National Soil Resources Institute. Archived (PDF) from the original on 2016-03-18. Retrieved 2019-01-06.

External links

AASHTO Soil Classification System

The AASHTO Soil Classification System was developed by the American Association of State Highway and Transportation Officials, and is used as a guide for the classification of soils and soil-aggregate mixtures for highway construction purposes. The classification system was first developed by Hogentogler and Terzaghi in 1929, but has been revised several times since.

Plasticity index of A-7-5 subgroup is equal to or less than the LL - 30. Plasticity index of A-7-6 subgroup is greater than LL - 30.


Agronomy is the science and technology of producing and using plants for food, fuel, fiber, and land restoration. Agronomy has come to encompass work in the areas of plant genetics, plant physiology, meteorology, and soil science. It is the application of a combination of sciences like biology, chemistry, economics, ecology, earth science, and genetics. Agronomists of today are involved with many issues, including producing food, creating healthier food, managing the environmental impact of agriculture, and extracting energy from plants. Agronomists often specialise in areas such as crop rotation, irrigation and drainage, plant breeding, plant physiology, soil classification, soil fertility, weed control, and insect and pest control.

Australian Soil Classification

The Australian Soil Classification is the classification system currently used to describe and classify soils in Australia. It is a general-purpose, hierarchical classification system, and consists of five categorical levels from the most general to the most specific: order, suborder, great group, subgroup, and family. An interactive, online key is available. The Australian Soil Classification supersedes other classification systems previously developed for Australian soils, including the Factual Key (1960) and the Handbook of Australian Soils (1968).

Canadian system of soil classification

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.


Edaphology (from Greek ἔδαφος, edaphos, "ground",-λογία, -logia) is one of two main divisions of soil science, the other being pedology. Edaphology is concerned with the influence of soils on living things, particularly plants. Edaphology includes the study of how soil influences humankind's use of land for plant growth as well as man's overall use of the land. General subfields within edaphology are agricultural soil science (known by the term agrology in some regions) and environmental soil science. (Pedology deals with pedogenesis, soil morphology, and soil classification.)

In Russia, edaphology is considered equivalent to pedology, but is recognized to have an applied sense consistent with agrophysics and agrochemistry outside Russia.

FAO soil classification

The Food and Agriculture Organization of the United Nations (FAO) developed a supra-national classification, also called World Soil Classification, which offers useful generalizations about pedogenesis in relation to the interactions between the main soil-forming factors. It was first published in form of the UNESCO Soil Map of the World (1974) (scale 1 : 5 M.). Many of the names offered in that classification are known in many countries and do have similar meanings.

Originally developed as a legend to the Soil Map of the World, the classification was applied by United Nations sponsored projects. Many countries modified this system to fit their particular needs.

The Soil Units (106) were mapped as Soil Associations, designated by the dominant soil unit:

with soil phases (soil properties, such as saline, lithic, stony),

with three textural classes (coarse, medium, and fine)

three slopes classes superimposed (level to gently undulating, rolling to hilly, and steeply dissected to mountainous)Soil Units form 26 World Classes. The FAO soil map was a very simple classification system with units very broad, but was the first truly international system, and most soils could be accommodated on the basis of their field descriptions. The FAO soil map was intended for mapping soils at a continental scale but not at local scale.

In 1998 this system was replaced by the World Reference Base for Soil Resources.


Luvisols are a group of soils, comprising one of the 32 Reference Soil Groups in the international system of soil classification, the World Reference Base for Soil Resources (WRB). They are widespread, especially in temperate climates, and are generally fertile. Luvisols are widely used for agriculture.


Pedalfer is composed of aluminum and iron oxides. It is a subdivision of the zonal soil order comprising a large group of soils in which sesquioxides increase relative to silica during soil formation. Pedalfers usually occur in humid areas. It is not used in the current United States system of soil classification but the term commonly shows up in college geology texts.

Pedalfers have three subdivisions of which one is Lateritic soils.

Pedalfer is a formative element in the United States soil taxonomic system for the Alfisols soil order. Alf is the formative element in the Alfisol name, and refers to aluminium (Al) and iron (Fe).


Pedocal is a subdivision of the zonal soil order. It is a class of soil which forms in semiarid and arid regions. It is rich in calcium carbonate and has low soil organic matter. With only a thin A horizon (topsoil), and intermittent precipitation calcite, other soluble minerals ordinarily removed by water may build up in the B horizon (subsoil) forming a cemented layer known as caliche. It is not used in the current United States system of soil classification but the term commonly shows up in college geology texts.


Pedology (from Greek: πέδον, pedon, "soil"; and λόγος, logos, "study") is the study of soils in their natural environment. It is one of two main branches of soil science, the other being edaphology. Pedology deals with pedogenesis, soil morphology, and soil classification, while edaphology studies the way soils influence plants, fungi, and other living things. The quantitative branch of pedology is called pedometrics.


In soil science, Podzols are the typical soils of coniferous or boreal forests. They are also the typical soils of eucalypt forests and heathlands in southern Australia. In Western Europe, Podzols develop on heathland, which is often a construct of human interference through grazing and burning. In some British moorlands with Podzolic soils, Cambisols are preserved under Bronze Age barrows (Dimbleby, 1962).

Polish Soil Classification

The Polish Soil Classification (Polish: Systematyka gleb Polski) is a soil classification system used to describe, classify and organize the knowledge about soils in Poland.


A Regosol in the World Reference Base for Soil Resources (WRB) is very weakly developed mineral soil in unconsolidated materials. Regosols are extensive in eroding lands, in particular in arid and semi-arid areas and in mountain regions. Internationally, Regosols correlate with soil taxa that are marked by incipient soil formation such as Entisols in the USDA soil taxonomy or Rudosols and possibly some Tenosols in the Australian Soil Classification.

The group of Regosols is a taxonomic rest group containing all soils that could not be accommodated in any of the other groups. Excluded from the Regosols are weakly developed soils that classify as Leptosols (very shallow soils), Arenosols (sandy soils) or Fluvisols (in recent alluvial deposits). These soils have AC-profiles. Profile development is minimal as a consequence of young age and/or slow soil formation.

Land use and management of Regosols vary widely. Some Regosols are used for capital-intensive irrigated farming but the most common land use is low volume grazing. Regosols in mountain areas are best left under forest.

Regosols occur in all climate zones without permafrost and at all elevations. Regosols are particularly common in arid areas, in the dry tropics and in mountain regions.

Regosols cover an estimated 260 million hectares worldwide, mainly in arid areas in the mid-western United States, Northern Africa, the Near East and Australia. Some 50 million hectares of Regosols occur in the wet/dry tropics, most especially in northern Australia, and another 36 million hectares in mountain areas.


In the World Reference Base for Soil Resources and similar soil classification systems, a sapric is a subtype of a histosol where virtually all of the organic material has undergone sufficient decomposition to prevent the identification of plant parts. Muck is a sapric soil that is naturally waterlogged or is artificially drained.

Soil type

A soil type is a taxonomic unit in soil science. All soils that share a certain set of well-defined properties form a distinctive soil type. Soil type is a technical term of soil classification, the science that deals with the systematic categorization of soils. Every soil of the world belongs to a certain soil type. Soil type is an abstract term. In nature, you will not find soil types. You will find soils that belong to a certain soil type.

In hierarchical soil classification systems, soil types mostly belong to the higher or intermediate level. A soil type can normally be subdivided into subtypes, and in many systems several soil types can be combined to entities of higher category. However, in the first classification system of the United States (Whitney, 1909), the soil type was the lowest level and the mapping unit.

For the definition of soil types, some systems use primarily such characteristics that are the result of soil-forming processes (pedogenesis). An example is the German soil systematics. Other systems combine characteristics resulting from soil-forming processes and characteristics inherited from the parent material. Examples are the World Reference Base for Soil Resources (WRB) and the USDA soil taxonomy. Other systems do not ask whether the properties are the result of soil formation or not. An example is the Australian Soil Classification.

A convenient way to define a soil type is referring to soil horizons. However, this is not always possible because some very initial soils may not even have a clear development of horizons. For other soils, it may be more convenient to define the soil type just referring to some properties common to the whole soil profile. For example, WRB defines the Arenosols by their sand content. Many soils are more or less strongly influenced by human activities. This is reflected by the definition of many soil types in various classification systems.

Because soil type is a very general and widely used term, many soil classification systems do not use it for their definitions. The USDA soil taxonomy has six hierarchical levels that are named order, suborder, great group, subgroup, family, and series. The WRB calls the first level Reference Soil Group. The second level in WRB is constructed by adding qualifiers, and for the result (the Reference Soil Group plus the qualifiers), no taxonomic term is used.

USDA soil taxonomy

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.

Unified Soil Classification System

The Unified Soil Classification System (USCS) is a soil classification system used in engineering and geology to describe the texture and grain size of a soil. The classification system can be applied to most unconsolidated materials, and is represented by a two-letter symbol. Each letter is described below (with the exception of Pt):

If the soil has 5–12% by weight of fines passing a #200 sieve (5% < P#200 < 12%), both grain size distribution and plasticity have a significant effect on the engineering properties of the soil, and dual notation may be used for the group symbol. For example, GW-GM corresponds to "well-graded gravel with silt."

If the soil has more than 15% by weight retained on a #4 sieve (R#4 > 15%), there is a significant amount of gravel, and the suffix "with gravel" may be added to the group name, but the group symbol does not change. For example, SP-SM could refer to "poorly graded SAND with silt" or "poorly graded SAND with silt and gravel."

World Reference Base for Soil Resources

The World Reference Base for Soil Resources (WRB) is an international soil classification system for naming soils and creating legends for soil maps. The currently valid version is the Update 2015 of the third edition 2014. It is edited by a working group of the International Union of Soil Sciences (IUSS).

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