World Reference Base for Soil Resources 2014: International Soil Classification System for Naming Soils and Creating Legends for Soil Maps – Updated 2015
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Every soil in the world can be allocated to one of the 32 Reference Soil Groups as defined in this document, and can further be characterized by a set of qualifiers. The resulting soil name provides information on soil genesis, soil ecological function and soil properties relevant for land use and management. The same system, refined slightly, may be used to name the units of soil map legends, thereby providing comprehensive spatial information. By accommodating national soil classification systems, the World Reference Base facilitates the worldwide correlation of soil information.
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World Reference Base for Soil Resources 2014 - Food and Agriculture Organization of the United Nations
Cover photographs (left to right):
Ekranic Technosol – Austria (©Erika Michéli)
Reductaquic Cryosol – Russia (©Maria Gerasimova)
Ferralic Nitisol – Australia (©Ben Harms)
Pellic Vertisol – Bulgaria (©Erika Michéli)
Albic Podzol – Czech Republic (©Erika Michéli)
Hypercalcic Kastanozem – Mexico (©Carlos Cruz Gaistardo)
Stagnic Luvisol – South Africa (©Márta Fuchs)
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ISBN 978-92-5-108369-7 (print)
E-ISBN 978-92-5-108370-3 (PDF)
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© FAO, 2014
Recommended citation:
IUSS Working Group WRB. 2014. World Reference Base for Soil Resources 2014. International soil classification system for naming soils and creating legends for soil maps. World Soil Resources Reports No. 106. FAO, Rome.
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Contents
Foreword
Acknowledgements
List of acronyms
Chapter 1. Background and basics
1.1 History
1.2 Major changes in WRB 2014
1.3 The object classified in the WRB
1.4 Basic principles
1.5 Architecture
1.6 Topsoils
1.7 Translation into other languages
Chapter 2. The rules for classifying soils and creating map legends
2.1 General rules
2.2 Rules for classifying soils
2.3 Rules for creating map legends
2.4 Subqualifiers
2.5 Buried soils
Chapter 3. Diagnostic horizons, properties and materials
Diagnostic horizons
Diagnostic properties
Diagnostic materials
Chapter 4. Key to the Reference Soil Groups with lists of principal and supplementary qualifiers
Chapter 5. Definitions of Qualifiers
References
Annex 1. Description, distribution, use and management of Reference Soil Groups
Annex 2. Summary of analytical procedures for soil characterization
Annex 3. Recommended codes for the Reference Soil Groups, qualifiers and specifiers
Annex 4. Soil particle-size and texture classes
Foreword
The first edition of the World Reference Base for Soil Resources (WRB) was released at the 16th World Congress of Soil Science at Montpellier in 1998. At the same event, it was also endorsed and adopted as the system for soil correlation and international communication of the International Union of Soil Sciences (IUSS). The second edition of the WRB was released at the 18th World Congress at Philadelphia in 2006.
After an additional eight years of intensive worldwide testing and data collection, the third edition of the WRB is presented. This publication builds on and reflects the valuable work of the authors of the earlier drafts and editions of the WRB, as well as the experiences and contributions of many soil scientists who participated in the work of the IUSS Working Group on the WRB.
The WRB is a soil classification system for naming soils and creating soil map legends. It is hoped that this publication will contribute to the understanding of soil science in the general public and in the scientific community.
The publication has been made possible by the sustained efforts of a large group of expert authors, as well as the cooperation and logistic support of the IUSS and the Food and Agriculture Organization of the United Nations (FAO).
Peter Schad (Chair)
Cornie van Huyssteen (Vice-Chair)
Erika Michéli (Secretary)
IUSS Working Group WRB
Ronald Vargas
Land and Water Development Division
Food and Agriculture Organization of the United Nations (FAO)
Acknowledgements
This edition has been compiled under the leadership of Peter Schad (Technische Universität München, Freising, Germany), Cornie van Huyssteen (University of the Free State, Bloemfontein, South Africa) and Erika Michéli (Szent István University, Gödöllő, Hungary).
The fundamental decisions have been made by the members of the WRB Board: Lúcia Anjos (Brazil), Carlos Cruz Gaistardo (Mexico), Seppe Deckers (Belgium), Stefaan Dondeyne (Belgium), Einar Eberhardt (Germany), Maria Gerasimova (Russia), Ben Harms (Australia), Arwyn Jones (European Commission), Pavel Krasilnikov (Russia), Thomas Reinsch (United States of America), Ronald Vargas (FAO), and Ganlin Zhang (China). Language editing was performed by Ben Harms (Australia).
The current third edition received contributions from many scientists, among them are: David Badia Villas (Spain), Frank Berding (The Netherlands), Hans-Peter Blume (Germany), Vanda Buivydaite (Lithuania), Wolfgang Burghardt (Germany), Przemysław Charzynski (Poland), Joe Chiaretti (United States of America), Juan Comerma (Venezuela), Carmelo Dazzi (Italy), Mahmut Dingil (Turkey), Arnulfo Encina Rojas (Paraguay), Márta Fuchs (Hungary), Luise Giani (Germany), Sergey Goryachkin (Russia), Alfred Hartemink (United States of America), Juan José Ibañez Martí (Spain), Plamen Ivanov (Bulgaria), Reinhold Jahn (Germany), Jerôme Juilleret (Luxembourg), Cezary Kabała (Poland), Andrzej Kacprzak (Poland), Arno Kanal (Estonia), Nikolay Khitrov (Russia), Roger Langohr (Belgium), Xavier Legrain (Belgium), Andreas Lehmann (Germany), Peter Lüscher (Switzerland), Gerhard Milbert (Germany), Brian Murphy (Australia), Freddy Nachtergaele (FAO), Otmar Nestroy (Austria), Åge Nyborg (Norway), Tatiana Prokofieva (Russia), David Rossiter (The Netherlands), Daniela Sauer (Germany), Jaroslava Sobocká (Slovakia), Karl Stahr (Germany), Leigh Sullivan (Australia), Wenceslau Teixeira (Brazil), Łukasz Uzarowicz (Poland).
The Working Group is greatly indebted to two brilliant soil scientists who made tremendous contributions to the development of the WRB and have sadly passed away: Rudi Dudal (Belgium, 1926–2014) was the lead author of the Soil Map of the World, and Otto Spaargaren (The Netherlands, 1944–2015) was for a long time the leading scientist of the WRB Working Group.
Finally, the Working Group wishes to express its gratitude to the FAO for its support and for making possible the printing and distribution of this publication.
List of acronyms
Aldith Aluminium extracted by a dithionite-citrate-bicarbonate solution
Alox Aluminium extracted by an acid ammonium oxalate solution
Alpy Aluminium extracted by a pyrophosphate solution
CaCO3 Calcium carbonate
CEC Cation exchange capacity
COLE Coefficient of linear extensibility
EC Electrical conductivity
ECe Electrical conductivity of saturation extract
ESP Exchangeable sodium percentage
FAO Food and Agriculture Organization of the United Nations
Fedith Iron extracted by a dithionite-citrate-bicarbonate solution
Feox Iron extracted by an acid ammonium oxalate solution
Fepy Iron extracted by a pyrophosphate solution
HCl Hydrochloric acid
ISRIC International Soil Reference and Information Centre
ISSS International Society of Soil Science
IUSS International Union of Soil Sciences
KOH Potassium hydroxide
KCl Potassium chloride
Mndith Manganese extracted by a dithionite-citrate-bicarbonate solution
NaOH Sodium hydroxide
NH4OAc Ammonium acetate
ODOE Optical density of the oxalate extract
RSG Reference Soil Group
SAR Sodium adsorption ratio
Siox Silicon extracted by an acid ammonium oxalate solution
SiO2 Silica
SUITMA Soils in Urban, Industrial, Traffic, Mining and Military Areas (IUSS working group)
TRB Total reserve of bases
UNESCO United Nations Educational, Scientific, and Cultural Organization
USDA United States Department of Agriculture
WRB World Reference Base for Soil Resources
Chapter 1
Background and basics
1.1 HISTORY
From its beginnings to the second edition 2006
The World Reference Base (WRB) is based on the Legend (FAO-Unesco, 1974) and the Revised Legend (FAO, 1988) of the Soil Map of the World (FAO-Unesco, 1971-1981). In 1980, the International Society of Soil Science (ISSS, since 2002 the International Union of Soil Sciences, IUSS) formed a Working Group ‘International Reference Base for Soil Classification’ for further elaboration of a science based international soil classification system. This Working Group was renamed ‘World Reference Base for Soil Resources’ in 1992. The Working Group presented the first edition of the WRB in 1998 (FAO, 1998) and the second edition in 2006 (IUSS Working Group WRB, 2006). In 1998, the ISSS Council endorsed the WRB as its officially recommended terminology to name and classify soils.
A detailed description of the WRB history before 2006 is given in the second edition of the WRB (IUSS Working Group WRB, 2006).
From the second edition 2006 to the third edition 2014
The second edition of the WRB was presented at the 18th World Congress of Soil Science 2006 in Philadelphia, USA (book: IUSS Working Group WRB, 2006; file: ftp://ftp.fao.org/agl/agll/docs/wsrr103e.pdf. After publication, some errors and needs for improvement were identified, and an electronic update was published in 2007 http://www.fao.org/fileadmin/templates/nr/images/resources/pdf_documents/wrb2007_red.pdf.
The second edition was translated into several languages. The Russian translation is of the 2006 printed version; whereas all other translations are of the 2007 electronic update: Arabic, German, Polish, Slovak, Spanish, and Turkish.
Since 2006, several WRB field correlation tours were organized to test the second edition:
2007: Germany (special topic: Technosols and Stagnosols)
2009: Mexico
2010: Norway
2011: Poland
2012: Australia (Victoria and Tasmania)
2013: Russia (ultra-continental permafrost soils in Sakha)
The field tours associated with the meetings of the IUSS Commission on Soil Classification in Chile (2008) and the United States (Nebraska and Iowa, 2012) were additional tests of the second edition and also the tours offered with the 19th World Congress of Soil Science 2010 in Australia.
The second edition of the WRB is a system to classify soils. Soon after its publication, there was an identified demand to create map legends using the WRB. For this purpose, the ‘Guidelines for constructing small-scale map legends using the WRB’ were published in 2010 http://www.fao.org/nr/land/soils/soil/wrb-documents/en/. They were recommended for map scales of 1 : 250 000 or smaller. The version for classifying soils (2006/07) and the version for creating map legends (2010) are based on the same definitions, but use different qualifier sequences and different rules for using the qualifiers (see below).
Now, after 8 years, a third edition has been prepared.
1.2 MAJOR CHANGES IN WRB 2014
The major changes are:
•The qualifier sequences and the rules for qualifier usage are now suitable for both classifying soils and creating map legends. They are now subdivided into principal qualifiers (ranked for every Reference Soil Group, RSG, in order of relevance) and supplementary qualifiers (not ranked).
•The only change at the Reference Soil Group (RSG) level is to replace Albeluvisols by Retisols. Retisols have a broader definition and include the former Albeluvisols.
•Fluvisols have moved down in the key to be the second last RSG. The Umbrisols are now placed directly after Phaeozems. The following RSGs switched their positions: Solonetz and Vertisols, Durisols and Gypsisols, Cambisols and Arenosols. The soils characterized by an argic horizon now have the following order: Acrisols – Lixisols – Alisols – Luvisols.
•The definition of Gleysols has been broadened.
•The definition of Acrisols, Alisols, Luvisols and Lixisols has been narrowed by setting the lower depth limit for the occurrence of the argic horizon uniformly to 100 cm. This implicitly widens the definition of Arenosols.
•There are now two different types of base saturation in use. First, the effective base saturation is used to separate Acrisols from Lixisols, Alisols from Luvisols and the Dystric qualifier from the Eutric qualifier. It is defined in WRB as exchangeable(Ca + Mg + K + Na) / exchangeable(Ca + Mg + K + Na + Al); exchangeable bases by 1 M NH 4 OAc (pH 7), exchangeable Al by 1 M KCl (unbuffered). Second, the base saturation (pH 7) is used for all other purposes. It is defined in WRB as exchangeable(Ca + Mg + K + Na) / CEC (pH 7); CEC and exchangeable bases by 1 M NH 4 OAc (pH 7).
•Three new diagnostic horizons have been defined. The chernic horizon replaces the voronic horizon and is required for Chernozems. The pretic horizon allows a better accommodation of ‘Terra preta de Indio’ within the Anthrosols. The protovertic horizon (the former vertic properties) describes layers with weakly expressed shrink-swell features.
•The anthric, takyric and yermic horizons have been changed to diagnostic properties.
•’Retic properties’ are a newly introduced diagnostic property in order to characterize Retisols. ‘Albeluvic glossae’ replaces ‘albeluvic tonguing’. ‘Shrink-swell cracks’ are a new diagnostic property that is useful for the definition of Vertisols and related soils.
•Some new names have been created: ‘protocalcic properties’ (instead of ‘secondary carbonates’), ‘sideralic properties’ (instead of ‘ferralic properties’). The ‘gleyic colour pattern’ and the ‘stagnic colour pattern’ are now ‘gleyic properties’ and ‘stagnic properties’ repectively. The ‘abrupt textural change’ has been renamed ‘abrupt textural difference’; and ‘lithological discontinuity’ is now ‘lithic discontinuity’.
•The albic horizon has been redefined as ‘albic material’.
•‘Soil organic carbon’ has been introduced to separate pedogenetic organic carbon from organic carbon that satisfies the diagnostic criteria of artefacts. ‘Dolomitic material’ is a new diagnostic material. ‘Hypersulfidic material’ and ‘hyposulfidic material’ are introduced as specific varieties of sulfidic material.
•‘Technic hard rock’ has been renamed ‘technic hard material’.
•Major improvements have been made in the definitions of the argic and natric horizons, in the depth criteria of the mollic and umbric horizons and in the separation between organic and mineral materials.
•Several new qualifiers have been added to give more information about some important soil properties. Precise rules have been introduced for the use of specifiers to define subqualifiers.
•The WRB should be able to express characteristics regarded as important in national systems. Some amendments have been made to allow for the better representation of soil units in the WRB, for example from the Australian and the Brazilian systems.
•Some parts of the world had not previously been well represented in the WRB system before, e.g. ultra-continental permafrost soils. The system has been enlarged to allow a better classification of these soils.
•Efforts have been made to improve the clarity of definitions and terminology.
1.3 THE OBJECT CLASSIFIED IN THE WRB
Like many common words, ‘soil’ has several meanings. In its traditional meaning, soil is the natural medium for the growth of plants, whether or not it has discernible soil horizons (Soil Survey Staff, 1999).
In the 1998 WRB, soil was defined as:
"… a continuous natural body which has three spatial and one temporal dimension. The three main features governing soil are:
•It is formed by mineral and organic constituents and includes solid, liquid and gaseous phases.
•The constituents are organized in structures, specific for the pedological medium. These structures form the morphological aspect of the soil cover, equivalent to the anatomy of a living being. They result from the history of the soil cover and from its actual dynamics and properties. Study of the structures of the soil cover facilitates perception of the physical, chemical and biological properties; it permits understanding the past and present of the soil, and predicting its future.
•The soil is in constant evolution, thus giving the soil its fourth dimension, time."
Although there are good arguments to limit soil survey and mapping to identifiable stable soil areas with a certain thickness, the WRB has taken the more comprehensive approach to name any object forming part of the epiderm of the earth (Sokolov, 1997; Nachtergaele, 2005). This approach has a number of advantages; notably that it allows for the tackling environmental problems in a systematic and holistic way, and avoids sterile discussion on a universally agreed definition of soil and its required thickness and stability. Therefore, the object classified in the WRB is: any material within 2 m of the Earth’s surface that is in contact with the atmosphere, excluding living organisms, areas with continuous ice not covered by other material, and water bodies deeper than 2 m¹. If explicitly stated, the object classified in the WRB includes layers deeper than 2 m.
The definition includes continuous rock, paved urban soils, soils of industrial areas, cave soils as well as subaqueous soils. Soils under continuous rock, except those that occur in caves, are generally not considered for classification. In special cases, the WRB may be used to classify soils under rock, for example for palaeopedological reconstruction of the environment.
1.4 BASIC PRINCIPLES
General principles
•The classification of soils is based on soil properties defined in terms of diagnostic horizons, diagnostic properties and diagnostic materials, which to the greatest extent possible should be measurable and observable in the field. Table 1 provides an overview of the diagnostics uned in the WRB.
•The selection of diagnostic characteristics takes into account their relationship with soil forming processes. An understanding of soil-forming processes contributes to a better characterization of soils but these processes should not, as such, be used as differentiating criteria.
•To the extent possible at a high level of generalization, diagnostic features that are of significance for soil management are selected.
•Climate parameters are not applied in the classification of soils. It is understood that they should be used for interpretation purposes, in combination with soil properties, but they should not form part of soil definitions. The classification of soils is therefore not subordinated to the availability of climate data. The name of a certain soil will not become obsolete due to global or local climate change.
•The WRB is a comprehensive classification system that enables accommodation of national soil classification systems.
•The WRB is not intended to be a substitute for national soil classification systems, but rather to serve as a common denominator for communication at the international level.
•The WRB comprises two levels of categorical detail:
-the First Level having 32 Reference Soil Groups (RSGs);
-the Second Level, consisting of the name of the RSG combined with a set of principal and supplementary qualifiers.
•Many RSGs in the WRB are representative of major soil regions so as to provide a comprehensive overview of the world’s soil cover.
•Definitions and descriptions reflect variations in soil characteristics that occur both vertically and laterally in the landscape.
•The term Reference Base is connotative of the common denominator function of the WRB: its units (RSGs) have sufficient width to facilitate harmonization and correlation with existing national systems.
•In addition to serving as a correlation between existing classification systems, the WRB also serves as a communication tool for compiling global soil databases and for the inventory and monitoring of the world’s soil resources.
•The nomenclature used to distinguish soil groups retains terms that have been used traditionally or that can be introduced easily into current language. They are defined precisely, in order to avoid the confusion that occurs where names are used with different connotations.