Gypsum As a Potent Salinity Agent – Geochemical Realization.

Gypsum, having quite a low solubility compared to soluble salts is not regarded as a salinity agent. Saturated gypsum solutions are widely used for germinating plants, and gypsum application to soils is a most common practice for treating sodic soils even in saline affected sites. In this research we studied salinity problems in gypsum-rich soils (up to 20% gypsum content) originated from an altered inland wetland in the Hula Valley, Israel. Analysis of data collected from field service laboratory of about 1000 surface soils sampled across the valley, has revealed that soil paste's electrical conductivity (EC) values ranged from 0.7 to 25 dS/m (5.5 ± 7.7 dS/m). Both Ca²⁺ and SO₄^2- were found to be major components in the paste-extracted soil solutions. Spatial analysis in the study area (~2500 ha) indicated that salinity severity as indicated by the paste extracts EC, was well correlated to gypsum controlled solutions. To bridge the gap between the results linking gypsum to salinity and the common notion of gypsum not being a saline agent, a combined experimental and computation approach was used to assess the effect of gypsum as a plausible potent salinity agent. A geochemical model was developed and experimentally tested showing that at equilibrium gypsum may add >10 dS/m over the contribution of the other salts in the soil solution. The Ca²⁺/SO₄^2- ratio in the soil solution was shown to depend on the soil/solution ratio and to be a major variable. The model is used to predict soil composition and salinity change based on the Ca²⁺/SO₄^2- ratio as soil dries due to evapotranspiration. Kinetic factors may aggravate the gypsum effect at time of soil drying, as soluble soil organic matter was shown to delay gypsum precipitation from oversaturated solutions for several days.