Comparing the ability of Soil Taxonomy (2014) and WRB (2015) to distinguish lithologic discontinuity and an abrupt textural change in major soils of Iran

A “considerable increase in clay content within a very short distance” and “significant change of particle size distribution or mineralogy” are diagnostic soil properties defined as “abrupt textural change” and “lithologic discontinuity” in the most soil classification systems, i.e., Soil Taxonomy (ST) and World Reference Base for Soil Resources (WRB). This study investigates the ability of the latest versions of ST (2014) and WRB (2015) to classify soils that have an abrupt textural change and lithologic discontinuity. Six study sites were selected from different regions (north, middle north, central, south, southeast and west) of Iran. One representative pedon was selected, described, and sampled in each study site. The physicochemical properties of the different genetic horizons of each pedon were determined and the soils were classified according to the ST (2014) and WRB (2015) systems. In the ST system, the abrupt textural change characteristic was only used for some of the great groups or subgroups. Furthermore, the “Abruptic” qualifier was not considered for all of the reference soil groups (RSGs) in the WRB system. Although, the presence of lithologic discontinuity at taxon name was totally neglected by the ST system, the WRB system showed this property with the “Raptic” qualifier. This seems to be a limitation for both of the soil classification systems, thus the management of soils with abrupt textural change and lithologic discontinuity still faces an important challenge. To overcome this problem and to harmonize both the classification systems, defining new great groups such as Palegypsids and new subgroups including Lithic Calcixerepts, Calcic Palexerolls, Abruptic Argigypsids, Calcic Argigypsids, Calcic Paleustalfs, and the “Raptic” subgroup for all taxa in the ST system is highly recommended. The addition of “Abruptic” qualifier for the RSGs such as Chernozems, Calcisols, and Gypsisols seems necessary in the WRB system.