The two most widely used modern soil classi fi cation schemes are Soil Taxonomy (ST) and World Reference Base for Soi l Resources (WRB). The purpose of this paper is to clarify the differences and highlight the simi-larities between ST and WRB in their current state, with special emphasis on calcareous soils in ar id and semi-arid regions of Iran. Four study sites were selected from central, northeast, and southeast Iran, and four representative pedons were descri bed and classi fi ed for each site. Then, soil units from WRB were com-pared to those obtained by using ST at the family level. The results demons trated that WRB shows more accessory soil properties compared to ST; moreover, through the method of combining quali fi ers, the WRB system is a better option for naming calcare ous soils. De fining new great groups, including Natrisalids, Gypsisalids, Calcisalids, Gypsicalcids, and Calcixeralfs, and new subgroups consisting of the Calcic Natrisalids and Calcic Natrargids for ST, and the “Paralithic ” quali fi er for the WRB system, could increase the usefulness and correlatio n between these two soil classi fi cation systems. Moreo ver, the de fi nition of some reference soil groups (such as Vertisols), diagnostic horizons (such as salic), and/or quali fi ers (such as Luvic prefi x) should be revised to harmonize the classi fi cation systems.

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