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A Compendium of On-Line Soil Survey Information

Soil Classification for Soil Survey

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When mapping soils, we can record individual properties at each observation point. We soon discover, however, that sets of properties co-vary, and that we can recognise classes of soils.

PageTop Frequently Asked Questions

I have no idea about soil classification

I have prepared a short Introduction to Soil Classification (PDF, 33Kb) which you may find helpful.

Bob Ahrens of the NCSS (USA) has prepared a short article "Soil classification: Past & Present" (PDF, 405Kb) from the US perspective. This gives a nice historical overview, emphasizing the soil series as the fundamental object of classification.

Why so many systems?

"The purpose of any classification is so to organize our knowledge that the properties of objects may be remembered and their relationships may be understood most easily for a specific objective. The process involves formation of classes by grouping the objects on the basis of their common properties. In any system of classification, groups about which the greatest number, most precise, and most important statements can be made for the objective serve the purpose best."
-- Marlin G. Cline, "Basic Principles of Soil Classification", Soil Science 67, 1949:81-91.

So, different systems have different objectives. In particular, Soil Taxonomy was designed to group soil series in the USA in increasingly-general interpretive groups, whereas World Reference Base and its predecessor FAO Legend were designed to understand and organize world soil geography.

Yet another reason for different systems is different concepts of soils, classes of soils and in particular soil individuals. Thus the French emphasize horizon assemblages, and don't require that a horizon sequence have only one classification.

And of course, indigenous people, that is, people living on and using the land, often have a classification based on perceived differences that are important to their uses.

Soil classification was once closely tied to the presumed genesis of the soil, but this has been replaced with observable properties; however these often have a strong genetic link. This connection for the WRB and ST is explored in this article by Bockheim & Gennadiev.

How can I translate a classification to a different system?

Direct correspondence between classes is rare. However, many of the concepts from different systems are similar, and good ideas from one system often get taken up into another, so there is often a relatively good correlation. Most books which define a classification include correspondence tables.

Classes in different systems almost never correspond 1-to-1. That is, pedons of a sinlge class in one system may end up in different classes in another system. Generally, at lower levels this is not such a big problem, because the interpretations of similar low-level taxa will also be similar.

For individual profiles, it is best to re-classify directly in the target system, using its definitions.

The World Reference Base project is probably the best linga franca, for three reasons: (1) it is intended to cover all soils of the world, (2) it was built with participation from almost all "schools" of classification, and (3) it tries as much as possible to use similar definitions, limits etc. as other systems. So one approach is to correlate all systems to WRB. Then a given class in one system can be correlated to a WRB class, then that WRB class to the target system. Of course, if there is the one-to-many problem discussed above, the correspondence will not be perfect.

There are some specific correlations that have been published:

PageTop World Reference Base (WRB)

This international standard soil classification system was developed by an international collaboration coordinated by the International Soil Reference and Information Centre (ISRIC) and sponsored by the International Union of Soil Science (IUSS) and the FAO via its Land & Water Development division.

"A world reference system for soil resources is a tool for the identification of pedological structures and their significance. It serves as a basic language in soil science and facilitates (1) the scientific communication; (2) the implementation of soil inventories and transfer of pedological data, elaboration of different systems of classification having a common base, interpretation of maps, etc.; (3) the international use of pedological data, not only by soil scientists but also by other users of soil and land."

The WRB is a two-level classification:

  1. 30 Reference Soil Groups. Examples: Histols, Fluvisols, Luvisols
  2. Subdivisions of the Reference Soil Groups, using any defined combination of 121 qualifiers. Examples: Leptic Umbrisols, Chromi-Vertic Luvisols. It is possible to use either a single qualifier (the most important) or all relevant qualifiers.

    The subdivisions do not take into account all possible differences among soil map units. In particular: climate, parent material, vegetation, depth of water table or drainage, and physiographic features such as slope, geomorphology or erosion are not considered as such, except insofar as they have affected soil morphology. These features can be used locally to defined mapping phases, but they are not considered soil properties to be classified as such.

    Some detailed internal properties are also not considered at this level of detail, namely, substratum layers, thickness and morphology of solum or individual horizons. These can be used to define series or forms locally, for detailed soil survey.

The WRB borrows heavily from modern soil classification concepts, including Soil Taxonomy, the legend for the FAO Soil Map of the World 1988, the Référentiel Pédologique, and Russian concepts. The emphasis is on soil morphology, and a major difference with Soil Taxonomy is that soil climate is not part of the system, except in so far as the effects of climate affect soil properties. As far as possible, diagnostic criteria match those of existing systems, so that correlation with national and previous international systems is as straightforward as possible.

The WRB is not intended to be used in semi-detailed or detailed mapping; many detailed soil properties that are important for land use and soil behaviour are not specified in sufficient detail in the two levels of the WRB. For detailed mapping and site characterisation, local organisations or survey projects are expected to use locally-defined soil series, soil forms, or similar. The WRB is used to group these locally-defined soils for correlation and communication.

However, many transboundary or even national projects began to use the WRB as a de-facto legend, in response the FAO in 2010 published Guidelines for constructing small-scale map legends using the WRB (also accessible from the WRB home page).


The WRB has been translated into German, French, Spanish, Italian, Polish, Vietnamese, Rumanian, Japanese and Lithuanian; Chinese and Russian are in progress.

Working Groups

These groups have been set up to deal with specific classification questions in the framework of the WRB


Some teaching material has been developed using the WRB as an organizing principle.

PageTop Soil Taxonomy

This system was first published in 1975, and has undergone 8 revisions since. It is a hierarchical classification that tries to group similar soils into increasingly general categories. It was designed to support soil survey in the USA, specifically the correlation of soil series and the provision of map unit names at various levels of cartographic detail. It tries to classify all World soils, but the main aim has always been to group soils of the USA.

NRCS Soil Classification home page

Supplementary sites

Updating Soil Taxonomy

Soil Taxonomy is still being revised, as new knowledge about soils is recorded, and as field scientist, especially in the USA, try to organise existing information in a "more perfect hierarchy". Several working groups have been formed to recommend changes.

PageTop FAO Soil Map of the World

This system was originally intended only as a legend for the Soil Map of the World, 1:5M, and was originally published between 1974 and the early 1980's, along with the map sheets. Since then it has become more of a classification system and in fact is now subsumed into the WRB. There have been two versions. The most recent is the 1988 Revised Legend.

Reference: FAO, 1988. FAO/Unesco Soil Map of the World, Revised legend, with corrections and updates. World Soil Resources Report 60, FAO, Rome. Reprinted with updates as Technical Paper 20, ISRIC, Wageningen, Netherlands, 1997. ISBN 90-6672-057-3

PageTop National systems

France United Kingdom Germany Russia South Africa
Australia Brasil Canada Switzerland Norway  

SectionTop France

SectionTop Great Britain (England, Wales, Scotland)

Nothing now, the BSSS has taken their on-line information away.

SectionTop Germany

SectionTop Switzerland

SectionTop Norway

SectionTop Australia

The Australian system is a highly-developed, detailed hierarchical classification with special emphasis on highly-weathered soils and soils of arid and semi-arid regions. It is based on a large amount of high-quality data.

SectionTop Canada

The Canadian system is a highly-developed, detailed hierarchical classification with special emphasis on boreal and prarie soils. It is based on a large amount of high-quality data.

SectionTop South Africa

SectionTop Brasil

SectionTop Russia

PageTop Numerical Classification (Ordination)

An alternative to hierarchical classification systems based on expert judgement about pedogenesis is an objective classification based on the actual differences between individuals. These are usually called "ordination" because they attempt to impose an order on the actual data. The usual idea is to minimize within-class variance, and maximize between-class variance, according to some objective criterion. It is somewhat similar to unsupervised classification in image processing: the information, rather than the pre-conceptions of the classifie, guides the classification.

PageTop Interpretive Classifications

In an interpretive classification, soils are classified according to their suitability for general or specific land uses.

Land Capability Classification Hydrological Function Wetlands Agricultural & Horticultural Problems & Limitations

SectionTop Land Capability Classification

This was first developed in the USA and has been adapted in many countries. Its aim is to rank all soils from 'best' to 'worst' according to the degree of relatively permanent physical limitations to productive land use (agriculture, grazing, forestry)

SectionTop Hydrological Classification

These group soils according to their influence on hydrologic processes.

SectionTop Wetland Classification

These classify soils according to their internal hydrologic regime

SectionTop Agricultural & Horticultural

SectionTop Problems & Limitations

Soils can be classified by their main constraints to management for multiple uses. This is implicit in some aspects of general-purposes classifications such as WRB, but it may be helpful to highlight the problems directly. But, to paraphrase the Venezuelan saying, "there are no difficult soils, only incompetant land users".

PageTop Technical & Functional systems

In a technical system, there is no attempt to classify into natural groupings. Soils are classified directly by a chosen set of characteristics that are thought to relate directly to a certain use or group of uses.

Engineering Suitability Topsoil

SectionTop Engineering Classifications

Civil engineers view soil as a foundation or building material. They have developed several classifications of soil as an earthy material. The classes are directly related to the manner of using the soil for civil engineering. These technical systems are designed to predict the engineering properties and behavior of a soil based on a few simple laboratory or field tests.

The most common is the Unified Soil Classification, with three major groups: (1) coarse grain, (2) fine grain, and (3) highly organic. These groups are then subdivided as follows:

  1. Coarse grain: gravels, sands
  2. Fine grain: silts, clays, organics silts & clays
  3. Organic soils

These are then subdivided according to their plasticity (for fine soils and coarse soils with some fines) and homogeneity (for coarse soils).

SectionTop Suitability Classifications

These group soils by specific properties important for a land use or set of land uses. They are directly usable by land managers working in a specific geographic area and for specific land uses.

SectionTop Topsoil Classifications

Soil classification tends to ignore or downplay the diversity of topsoil characteristics, mainly because they can change fairly rapidly under human influence. However, the topsoil determines to a large extent soil-related Land Qualities, especially for infiltration, erosion, crusting and other surface processess.

To fill this gap, a draft proposal has been developed by ISRIC and FAO. This is an important development, and soil scientists working with surface processes should test the classification to see if it is a useful stratification for their purposes.

PageTop Technogenic & Anthropogenic Soils

These soils pose special problems for traditional classification systems.

PageTop Indigenous Classifications


Author: D G Rossiter URL:
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