Editing Soil classification (section)

=== 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>