The International Soil Classification System WRB, Third Edition, 2014

Abstract

The international soil classification system World Reference Base for Soil Resources (WRB) is edited by a Working Group of the International Union of Soil Sciences and published by the Food and Agriculture Organization of the United Nations. The third edition was released in 2014. The WRB has two hierarchical levels. The first level allocates the soils of the world to 32 Reference Soil Groups (RSGs), which are identified using a key. Many RSGs represent specific soil-forming processes, are representative of major soil regions, or reflect special parent materials. In the second level, the soil names are constructed by adding a set of qualifiers to the name of the RSG. In total, 186 qualifiers are defined. Some can be combined with many RSGs, others with only a few or even with just one. For every RSG, a list of the possible qualifiers is provided. These possible qualifiers are subdivided into principal and supplementary qualifiers. Principal qualifiers are regarded as being the most significant for a further characterization of soils of the particular RSG. They are ranked and given in an order of importance. Supplementary qualifiers give some further details about the soil. They are not ranked but are used in alphabetical order. The WRB uses diagnostic horizons, diagnostic properties, and diagnostic materials. Diagnostic horizons and properties reflect widespread, common results of the processes of soil formation or indicate specific conditions of soil formation. In addition, diagnostic horizons require a certain thickness, thus forming a recognizable layer in the soil. Diagnostic materials are materials that significantly influence pedogenetic processes or are indicative of them. The definitions of many RSGs and qualifiers refer to the presence or absence of certain diagnostics at a certain depth. In addition, many definitions refer to individual features such as the base saturation or clay content. To name a soil, the RSG has to be provided with all applying principal and supplementary qualifiers. For map legends, the number of qualifiers depends on scale. Detailed rules are established to achieve comprehensive names for map units at different scale levels.