Abstract

Summary. Six samples of gypsum (2 mined materials, 3 from industrial by-products and analytical grade gypsum for comparison) were evaluated for their rates of dissolution. The rate of dissolution of gypsum and the subsequent movement of dissolved ions away from the site of dissolution determine the effectiveness of gypsum as a soil amendment. Compressed pellets were produced with similar surface areas and densities and their rates of dissolution assessed using electrical conductivity measurements of a stirred solution in which the pellets were suspended in a perforated nylon container. The rate of dissolution was significantly slower in the mined material and in one of the phosphogypsum samples when compared with the other sources of gypsum. Scanning electron microscopy of phosphogypsum samples exhibiting different dissolution rates suggest that these differences may in part be attributed to the average size and morphology of the gypsum crystallites. It is suggested that the internal surface area is smaller in the case of the least soluble industrial gypsum and consequently its rate of dissolution is reduced. These observed differences in dissolution rates would affect the efficacy of these materials in reducing clay dispersion and improving hydraulic conductivity of soils, and should therefore be taken into account when assessing a particular product for a specific agricultural use.