Soil Classification for Soil Survey
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.
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.
"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.
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:
by Deckers, Nachtergaele and Spaargaren (who know what they are talking about); appears to be an informal working paper from the FAO
Compares the Canadian, USA (Soil Taxonomy 1994) and FAO 1988 systems: horizon definitions, diagnostic horizons, and highest-level taxa. (web version courtesy of Pedosphere.com)
Israeli, USDA Soil Taxonomy, FAO
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."
Includes world maps of the distribution of the reference groups, lists of publications, news about translations, errata etc. Check this for updates and news.
New version, released at the 2006 World Congress of Soil Science (PDF, 4.3 Mb)
This is a major update, including the Technosols, a revised qualifier system, many new qualifiers, and improved definitions.
The WRB is a two-level classification:
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.
From the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Hannover
These groups have been set up to deal with specific classification questions in the framework of the WRB
Sponsored by the IUSS and International Permafrost Association. Also deals with Cryosol genesis and their classification in Soil Taxonomy.
Some teaching material has been developed using the WRB as an organizing principle.
Describes the soils of the world by WRB group. Discusses major properties, use, and genesis. Edited by the "four wise men": Paul Driessen (Wageningen Agricultural University and ITC), Jozef Deckers (KU Leuven), Otto Spaargaren (ISRIC) and Freddy Nachtergaele (FAO). Available from the FAO Publications Catalogue" (ISBN 92-5-104637-9) or download (PDF, 13.1Mb):
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.
USDA-NRCS Agriculture Handbook No. 436 (the 'Big Green Book') has recently been revised and is now the 'Big Magenta Book'. This is a very large handbook, with example soils, maps, etc. From this site you can download the book, maps, and errata, all as PDF files.
As a service, I have stored a copy on the ITC server:
These are the keys and descriptions of diagnostic horizons etc. from Soil Taxonomy 2nd Edition, without any description of the soil classes. The most recent version is the 9th (2003)
Select the site closest to you:
"This forum is for topics and issues concerning the Soil Taxonomy and classification of the soils using Soil Taxonomy. Anyone may post a question or join in the discussion. Bob Engel, Soil Scientist at the National Soil Survey Center, the moderator for this forum."
This is an excellent place to voice your doubts and get them answered by the real ST experts
Includes representative soil series complete with landscape and profile pictures, and a simple explanation of the central concept of each order, from Paul McDaniel at the Soil Science Divison, University of Idaho in the beautiful Palouse region. Includes photos from the Marbut Memorial slide set (SSSA) and other sources, all of high quality.
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.
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
This is the system used in the original report and map sheets.
This includes a report that explains the evolution of the system.
|France||United Kingdom||Germany||Russia||South Africa|
"Une nouvelle méthode pour désigner les sols et les rattacher à un système de références. Outil irremplaçable pour effectuer des corrélations et transmettre une information riche. Remplace désormais la classification française des sols de 1967 (dite C.P.C.S.). Un langage clair et renouvelé, un outil pratique et efficace. Indispensable pour tous les scientifiques et ingénieurs qui ont à prendre en compte les sols ainsi que pour les enseignants des universités et grandes écoles agronomiques."
This is the updated version of the 1995 original, not yet translated to English.
"This is a new system for the naming of soils and linking them to a comprehensive reference base. This book takes stock of all that is presently known on the soils of Europe and further afield. Based on clarified and modernized concets, it proposes a clear and well-defined language. It goes beyond being a simple soil classification system and constitutes a coherent way of organizing all our knowledge. It is above all an efficient tool for conveying as much informatio as possible and for establishing correlations between different regions."
This is an approach to soil classification that is completely at odds with Soil Taxonomy. Instead of a rigid hierarchical classification system, we have a set of 102 reference solums (not polypedons!) which result from unique interpretations of morphology, genesis and function. A soil in the field can be assigned to one or more references; this fits with the idea of fuzzy classification. It merits close study.
Nothing now, the BSSS has taken their on-line information away.
A tantalizing but too-short article by Wolf Eckelmann of the Bundesanstalt für Geowissenschaften und Rohstoffe in Hannover, on the difficulties of merging the classification systems of the former DDR and BRD post-unification.
From the Soils and Biogeography group at University of Zurich, Department of Geography. Also includes profile description standards.
A key and list of Norwegian soil series, organized by detailed WRB second-level classification.
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.
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.
From Agriculture Canada; now also online.
From Agriculture Canada; link to the original document in PDF (41,974Kb!!)
From the University of Alberta's Introduction to Soils & Soil Resources course
This system has a major advantage for southern African conditions: it was developed on the basis of wide field experience, and the classes were set up to be interpretable at both levels. Many practical workers continue to use the red book, finding that the blue book introduced unncessary complications.
Select the popdown menu Soils and within that Soil classification
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.
From the Pedometrics group at University of Sydney. This applies to the case where classes are not sharply-defined and instead grade into each other, so that a given pedon can be fairly assigned to more than one class, each with a different degree of membership. Class memberships are then used for prediction of properties, as well as for spatial interpolation of classes and properties. The "extragrade" class is a catch-all for pedons that do not sufficiently resemble any main class. Well-explained, with all formulas. It has been implemented as a computer program.
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|
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)
By Douglas Helms, Natural Resources Conservation Service historian. Did you ever wonder who thought up those roman numerals? Or what the motivation for the system was? It's interesting to what degree any system is the product of its history and the context in which it was developed, and this article is quite revealing in this respect.
These group soils according to their influence on hydrologic processes.
These classify soils according to their internal hydrologic regime
Includes criteria, lists of series, historical evolution of concepts, field indicators
From CSIRO, "a user-friendly key to identify soils in vineyards by people who are not experts in soil classification". Very nice photos.
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".
From the FAO. Explains constraints for acid soils (with and without aluminium toxicity, acid sulphate soils), Calcareous soils, Histosols (peat soils), Salt-affected soils (saline and sodic soils), Sandy soils, Steeplands, and Vertisols. Note these same soils may be highly suitable in some cases!
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.
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:
These are then subdivided according to their plasticity (for fine soils and coarse soils with some fines) and homogeneity (for coarse soils).
The complete text is available online as HTML from the US Army's training website (registration required; restricted).
Includes photos of field tests, classification tables, and evaluation criteria for roads and airfields. There are comprehensive sections on the use of soils in engineering, including slope stabilisation & compaction.
Download PDF versions of the soil-related chapters from ITC:
Includes very nice tables of descriptive & interpretive properties of soils of various geologic origins.
From the Occupational Safety and Health Administration (USA): "Earth material that is excavated must be properly sloped or supported for construction and safety purposes. The factors and specifications that relate to this protection are outlined in 29 CFR 1926 Subpart P Appendix A. They include instructions for the proper sloping and shoring and bracing of the soil as determined by an analysis and classification of the material."
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.
This is a system of grouping Florida (USA) soils according to how they respond to certain forestry management activities. All the soils in Florida are classified in eight management groups.
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.
From the soil crusts project, mostly working in the upper Colorado River basin of the USA. Includes an extensive bibliography.
These soils pose special problems for traditional classification systems.
ICOMANTH is charged with defining appropriate classes in Soil Taxonomy for soils that have their major properties derived from human activities. It has recently been revived under the leadership of John Galbraith (Virginia Tech). Includes the contents of a CD-ROM, with photos and discussion of classification issues.
(copy stored here, courtesy of Prof. Dr. Wolfgang Burghardt, University of Duisburg-Essen)
von Michael Pachmajer, Johann Wolfgang Goethe-Universität, Frankfurt am M
Written by Christien H. Ettema while she was at the Institute of Ecology, University of Georgia (USA). By permission stored here at ITC for your information. Includes a nice introductory bibliography.
Niemeijer, David (1995) Indigenous Knowledge and Development Monitor
Concepcion, R.N. and Batjes, N.H. (1997); ISRIC Report 97/03.
|Author: D G Rossiter||
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