Qualifiers in the international soil classification system WRB-2022: composition, connotation, functions

The International soil classification system – WRB – was published every eight years starting with 1998; in its each version, the number, essence and status of qualifiers were more or less different. In accordance with the WRB principles, qualifiers, both principal and supplementary, are based on soil forming processes implemented in “diagnostics”. Analysis of the list of qualifiers in the last WRB version of 2022 (281 qualifiers) revealed the priority of diagnostic horizons as criteria for selecting both categories of qualifiers, less numerous were soil chemical properties. Among anthropogenic qualifiers, the technogenic ones are more numerous and diverse than those related to agriculture. The number of qualifiers per Reference soil group, mainly supplementary, is the greatest in the widely spread soils: Technosols, Cambisols, and Gleysols; it is minimal in soils confined to certain geographical sites, f.i. Nitisols. In relation to the Reference soil groups, qualifiers may be separated into “universal” being in the lists of almost all groups (indicating texture, gley, plowing) and “unique”, which are inherent to specific soils. Principal qualifiers’ function is creation of a central image of a Reference soil group based on major soil forming processes, and their number should be limited. Based on the calculation of taxonomic distances, the pairs of Referential groups with the most similar set of main qualifiers were Stagnosols and Planosols, Calcisol and Gypsisol, Alisols and Acrisols, which is determined by the similarity of the processes forming them. Referential groups Histosols and Gleysols are characterized by the most unique set of main qualifiers. As for the names of qualifiers, they are constructed of formative elements taken from 26 languages, along with the dominant Latin and Greek. Qualifiers in the Podzols Reference group are discussed as an example of approaches to soil diagnostic in WRB and Russian soil classification.