Editing Soil classification (section)

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

Inscriptions at the [[Temple of Edfu|temple of Horus at Edfu]] outline a soil classification used by Tanen to determine what kind of temple to build at which site.<ref>{{cite journal | doi=10.1016/j.geoderma.2022.116191 | title=Sacred soils of ancient Egypt | year=2022 | last1=Retallack | first1=Gregory J. | journal=Geoderma | volume=428 | page=116191 | bibcode=2022Geode.42816191R | s2cid=253191277 | doi-access=free }}</ref> Ancient Greek scholars produced a number of classification based on several different qualities of the soil.<ref> https://cpb-us-e1.wpmucdn.com/blogs.uoregon.edu/dist/d/3735/files/2013/07/antiquity-2008-1l4ladp.pdf </ref>

=== Engineering ===
[[Geotechnical engineering|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. [[Sand|sands]] and [[Gravel|gravels]]); (2) fine-grained soils (e.g. [[Silt|silts]] and [[Clay|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, classifying some as "well-graded" and the rest as "poorly-graded". Silts and clays are distinguished by the soils' [[Atterberg limits]], and thus the soils are separated into "high-plasticity" and "low-plasticity" soils. 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 [[Donald Burmister|Burmister]] system, which works similarly to the USCS but includes more coding for various soil properties.<ref>{{cite web |url=http://www.civil.columbia.edu/ling/burmister/burmister.html |title=Donald M. Burmister |publisher=Civil.columbia.edu |access-date=2014-06-11 |archive-url=https://web.archive.org/web/20120415003511/http://www.civil.columbia.edu/ling/burmister/burmister.html |archive-date=2012-04-15 |url-status=live }}</ref>

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.<ref>{{Citation |publisher=American Society for Testing and Materials |year=1985 |series=D 2487-83 |title=Classification of Soils for Engineering Purposes: Annual Book of ASTM Standards |volume=04 |issue=8 |pages=395–408 |url=http://www.astm.org/Standards/D2487.htm |access-date=2015-05-08 |archive-url=https://web.archive.org/web/20100914111626/http://www.astm.org/Standards/D2487.htm |archive-date=2010-09-14 |url-status=live }}</ref>

=== Soil science ===
{{main|Soil science}}
[[Image:SoilTextureTriangle.svg|thumb|Soil texture triangle showing the [[USDA]] classification system based on grain size]]
[[File:Global soils map USDA.jpg|thumb|upright=1.8|Map of global soil regions from the [[USDA soil taxonomy#Soil Order|USDA]]]]

For soil resources, experience has shown that a natural system approach to [[Taxonomy|classification]], i.e. grouping soils by their intrinsic property ([[soil morphology]]), behaviour, or [[Soil formation|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 [[Edaphology|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]],<ref name=WRB4ed>{{Cite web|url = https://wrb.isric.org/files/WRB_fourth_edition_2022-12-18.pdf|title = World Reference Base for Soil Resources, fourth edition|author=IUSS Working Group WRB|year = 2022|publisher = International Union of Soil Sciences, Vienna|access-date = 2023-09-27}}</ref><ref name=90years>{{Cite web|url = https://iuss.boku.ac.at/files/iuss126_def_72dpi.pdf|title = 90 Years of Soil Classification of the IUSS|author = H.-P. Blume, P. Schad|year = 2015|publisher = IUSS Bulletin 126, 38–45|access-date = 2018-12-09|archive-url = https://web.archive.org/web/20180906161731/https://iuss.boku.ac.at/files/iuss126_def_72dpi.pdf|archive-date = 2018-09-06|url-status = dead}}</ref> which use [[Taxonomy (biology)|taxonomic]] criteria involving soil morphology and laboratory tests to inform and refine [[hierarchy|hierarchical]] classes. Another approach is numerical classification, also called [[ordination (statistics)|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 D. Smith|Guy Donald Smith]], director of the [[United States Department of Agriculture|U.S. Department of Agriculture's]] soil survey investigations.<ref>{{Cite news|url=https://www.washingtonpost.com/archive/local/1981/08/29/guy-d-smith-73-usda-soil-expert-dies/962ae425-d6cb-40c0-af27-c0fcb1700b2d/|title=Guy D. Smith, 73, USDA Soil Expert, Dies|last=Donovan|first=Alan|date=1981-08-29|newspaper=Washington Post|access-date=2017-11-15|language=en-US|issn=0190-8286|archive-url=https://web.archive.org/web/20171115082844/https://www.washingtonpost.com/archive/local/1981/08/29/guy-d-smith-73-usda-soil-expert-dies/962ae425-d6cb-40c0-af27-c0fcb1700b2d/|archive-date=2017-11-15|url-status=live}}</ref> Soil taxonomy based soil map units are additionally sorted into classes based on technical classification systems. [[Land use capability map|Land Capability Classes]], [[hydric soil]], and [[prime farmland]] are some examples. <!-- consider moving this paragraph to soil survey article-->

The European Union uses the [[World Reference Base for Soil Resources]] (WRB), currently the fourth edition is valid.<ref name=WRB4ed/> According to the first edition of the WRB (1998),<ref name=WRB98>{{Cite web|url = http://www.fao.org/docrep/w8594e/w8594e00.HTM|title = World Reference Base for Soil Resources|author = ISSS-ISRIC-FAO|year = 1998|publisher = World Soil Resources Reports 84, FAO, Rome|access-date = 2018-12-13|archive-url = https://web.archive.org/web/20181216015002/http://www.fao.org/docrep/w8594e/w8594e00.HTM|archive-date = 2018-12-16|url-status = live}}</ref> the booklet "Soils of the European Union"<ref name=SEU>{{cite web |url = https://esdac.jrc.ec.europa.eu/ESDB_Archive/eusoils_docs/other/EUR23439.pdf|title = Soils of the European Union|author=G. Tóth, L. Montanarella, V. Stolbovoy, F. Máté, K. Bódis, A. Jones, P. Panagos, M. Van Liedekerke|year = 2008|publisher = European Commission, Luxembourg}}</ref> 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 taxonomy|Folk taxonomies]] have been used for millennia, while scientifically based systems are relatively recent developments.<ref>[http://forages.oregonstate.edu/is/ssis/main.cfm?PageID=162 Soil classification systems] {{webarchive|url=https://web.archive.org/web/20060907150151/http://forages.oregonstate.edu/is/ssis/main.cfm?PageID=162 |date=2006-09-07 }}.  Url last accessed 2006-04-18</ref> 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&nbsp;km spatial resolution.<ref>{{cite journal |last1=Hengl |first1=T |display-authors=etal |title= Global gridded soil information based on machine learning |journal= PLOS ONE |date=2017 |volume=12 |issue=2 |pages=e0169748 |doi=10.1371/journal.pone.0169748|pmid=28207752 |pmc=5313206 |doi-access=free }}</ref> 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.<ref>{{cite journal |last1=Certini |first1=G |last2=Scalenghe |first2=R |title= Unnamed soils, lost opportunities |journal= Environmental Science & Technology  |date=2019 |volume=53 |issue=15 |pages=8477–8478 |doi=10.1021/acs.est.9b03050|pmid=31293149 |bibcode=2019EnST...53.8477C |doi-access=free }}</ref>

===OSHA===
The U.S. [[Occupational Safety and Health Administration]] (OSHA) requires the classification of soils to protect workers from injury when working in excavations and trenches. OSHA uses three soil classifications plus one for rock, based primarily on strength but also other factors which affect the stability of cut slopes:<ref name=OSHA-pa>{{cite web |url=http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10931 |title=Safety and Health Regulations for Construction Subpart P, Excavations, Appendix A |publisher=Osha.gov |date=9 August 1994 |access-date=2015-05-07 |archive-url=https://web.archive.org/web/20160305203236/https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10931 |archive-date=5 March 2016 |url-status=live }}</ref>

* {{vanchor|Stable Rock}}: natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed.
* {{vanchor|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<ref name=OSHA-pc>{{cite web |url=http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10933 |title=Safety and Health Regulations for Construction Subpart P, Excavations, Appendix C |publisher=Osha.gov |date=9 August 1994 |access-date=2017-08-28 |archive-url=https://web.archive.org/web/20170828230854/https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10933 |archive-date=28 August 2017 |url-status=live }}</ref>)
* {{vanchor|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<ref name=OSHA-pc />)
* {{vanchor|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<ref name=OSHA-pc />)
* {{vanchor|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<ref name="corrug">{{cite web|url=http://pacificshoring.com/store/media/pdf/corrugated-sheeting.pdf|title=Corrugated Sheeting, Tabulated Data|date=January 1, 2012|publisher=Pacific Shoring, LLC|access-date=August 8, 2016|archive-url=https://web.archive.org/web/20160822075112/http://pacificshoring.com/store/media/pdf/corrugated-sheeting.pdf|archive-date=August 22, 2016|url-status=live}}</ref><ref>{{cite web|url=https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=21360|title=Soil classification. - Occupational Safety and Health Administration|website=www.osha.gov|access-date=2016-08-08|archive-url=https://web.archive.org/web/20160827032637/https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=21360|archive-date=2016-08-27|url-status=live}}</ref>

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.<ref>{{cite web |url=http://www.osha.gov/dts/sltc/methods/validated/id194/id194.html |title=Sampling and Analytical Methods &#124; Classification of Soils for Excavations, ID-194 |publisher=Osha.gov |access-date=2014-06-11 |archive-url=https://web.archive.org/web/20140319035208/https://www.osha.gov/dts/sltc/methods/validated/id194/id194.html |archive-date=2014-03-19 |url-status=live }}</ref><ref name=OSHA-pb>{{cite web |url=http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10932 |title=Safety and Health Regulations for Construction Subpart P, Excavations, Appendix B |publisher=Osha.gov |date=9 August 1994 |access-date=2014-06-11 |archive-url=https://web.archive.org/web/20140715151544/https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10932 |archive-date=15 July 2014 |url-status=live }}</ref>