Gypsum and Soils: Concepts, Terms and Errors

The action of gypsum in soils is based primarily on the solubility of gypsum. Gypsum is a semi-soluble salt, which is more soluble than calcite but much less than many salts occurring in soils. However, because of its solubility, dissociation in water, and frequent occurrence in saline areas, gypsum is too often referred to as a salt that causes agricultural salinity. This is not true because gypsum does not significantly increase osmotic potential or ionic toxicity. Gypseous and gypsiferous are two terms often used for soils containing gypsum. According to the common definitions of these words, we propose to use gypsiferous when the soil contains some gypsum, but it does not control most properties of the soil. Thus, gypseous should be used for soils whose chemical and physical properties are due to the gypsum. Gypsum is not only somewhat soluble, it is also soft and fragile; two features related to the crystal structure of CaSO4•2H2O. The knowledge that water is an integral part of the crystal is often not considered. To routinely dry and pulverize soils containing gypsum for laboratory analyses, spreads suspect for all further analytical results. The classification of gypsum containing soils can become burdensome. Soil Taxonomy typically uses the noun as the prefix for Suborder such as Gypsids and for Great Groups, such as Gypsiargids or Gypsic Haplocambids. If the mineralogy of the soil is hypergypsic, it classification might be; Fine-gypseous, hypergypsic, non-active, thermic, Gypsic Haplocambids.