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

Abstract

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.

Gypsum, a Tricky Material

“…7). Th is behavioral change is not considered in the defi nitions of gypsic and petrogypsic horizons within the current soil classifi cation systems (Herrero, 2004). We lack evidences about if this change is reversible.…”

Gypsum Soils—Their Morphology, Classification, Function, and Landscapes

“…However, the mechanical action of gypsum should not be confused with the clay-flocculation effect of Ca 2þ produced through the dissolution of gypsum. While precipitation of calcium carbonate is described as having an aggregant effect, gypsum precipitation has a displacive action (Watson, 1985) on surrounding materials, as shown by experiments conducted on quartz sand (Plet-Lajoux et al, 1971), montmorillonite (Halitim et al, 1983), and as discussed in field observations by Herrero (2004) and Basyoni and Mousa (2009). Displacive effects do not occur in the early stages of gypsum crystal growth in preexisting voids, as with the presence of vermiform gypsum associated with root pores and other voids in gypsiferous soils or infillings by gypsum lentils with bow-shaped arrangement observed in longitudinal thin sections of earth worm channels.…”

“…However, the possibility that "pedogenic gypsum and silica have developmental stages that are similar to the stages of carbonate morphology" (Birkeland, 1999) is questionable. Herrero (2004) addressed the distinction between calcium carbonate and gypsum and determined that a "close parallelism . is unsatisfactory".…”

“…The silt size or larger for pedogenic gypsum crystals (Doner and Lynn, 1989;Poch et al, 1998) and solubility of gypsum (about 2.4 g L À1 in distilled water at 25 C) do not lend themselves to support continued infilling and eventual engulfment of pore space in clastic parent sediments. In addition, the cementation model is ill-suited for gypseous soils (Artieda, 1996;Herrero and Porta, 2000) because most or all of the groundmass in these soils consists of gypsum crystals in direct contact with each other (Herrero, 2004). In contrast, pedogenic carbonate can range from clay-size to sand-size material.…”

Polygenetic saline gypsiferous soils of the Bam region, Southeast Iran

“…Regarding the diagnostic saliferous-gypsiferous horizon in these soils, this study illustrates that it is necessary to include micromorphological characteristics of the petrogypsic horizon in its definition in Soil Taxonomy (i.e. like argillic horizon) as it is also recommended by Herrero (2004). The distribution pattern of gypsum in petrogypsic horizon with low gypsum content should be indicated, as the cementing agent is not dominantly gypsum.…”