Revisiting the definitions of gypsic and petrogypsic horizons in Soil Taxonomy and World Reference Base for Soil Resources

Gypsic and petrogypsic horizons occur in large areas of arid and semi-arid regions of the world. The occurrence of gypsum in soils is considered a key feature by most soil classification and mapping systems, that have coined specific names for these soils and horizons. However, the current methodology for description and definition of gypsic and petrogypsic horizons in the two most popular soil taxonomic systems “Soil Taxonomy” and “World Reference Base for Soil Resources” (WRB) doesn’t address sufficiently the advances in knowledge of the constitution, genesis and behavior of gypseous horizons. Some of their basic statements, like the presence of secondary gypsum or the degree of cementation are often ambiguous in the field. Further, the definitions of gypsic and petrogypsic horizons should not be interlocked, each definition should be based on field characteristics linked with microscopic and hydric properties that control the durability and the life-supporting capability of gypseous soils. The avoiding of confusion between gypsum-rich and calcium carbonate-rich horizons when grouping soil taxa or diagnostic horizons by means of soil-forming processes is stressed.