World Reference Base for Soil Resources

TL;DR: The World Reference Base for Soil Resources (WRB) is an international soil classification system for naming soils and creating legends for soil maps as discussed by the authors, which has two hierarchical levels: the first level presents 32 Reference Soil Groups (RSGs), which are identified using a key.

Abstract: World Reference Base for Soil Resources (WRB) is an international soil classification system for naming soils and creating legends for soil maps. It is edited by a Working Group of the International Union of Soil Sciences (IUSS). The currently valid edition is the update 2015 of the third edition 2014. WRB has two hierarchical levels: The first level presents 32 Reference Soil Groups (RSGs), which are identified using a key. RSGs are groups of soils that have undergone similar pedogenesis or have been formed from similar parent material or represent major ecological regions. In the second level, the soil names are constructed by adding a set of qualifiers to the name of the RSG. For the second level, 185 qualifiers are defined. Some can be combined with many RSGs, others with only a few or even with just one. The qualifiers available for use with a particular RSG are listed in the key, along with the RSG. They are divided into principal and supplementary qualifiers. The principal qualifiers are regarded to be the most significant for the further characterization of soils of the particular RSG. They are ranked and given in an order of importance. The supplementary qualifiers are not ranked but used in alphabetical order. The definitions of RSGs and qualifiers are to a certain extent based on diagnostics: Diagnostic materials are materials that significantly influence pedogenic processes. Diagnostic properties are typical results of soil-forming processes or reflect special conditions of soil formation. Diagnostic horizons are typical results of soil-forming processes, but with a minimum thickness and therefore recognizable as horizontal layers. For naming a soil, all applying qualifiers must be listed in the soil name. For map legends, generalization is required, and the number of qualifiers depends on the scale and the purpose of the map. Qualifiers may be combined with specifiers to form subqualifiers for a further speciation of the qualifiers. WRB provides codes for the RSGs, qualifiers, and specifiers and syntax rules for the combination of the codes. At the end of this article, examples are provided for naming soils and creating map legends, including the use of codes.