DISSOLUTION RATES OF SELECTED AUSTRALIAN GYPSUM SOURCES

Six samples of gypsum (2 mined materials, 3 from industrial by-product s and analytical grade gypsum for comparison) were evaluated for their rates of dissolution. The rate of dissolution of gypsum and the subse quent movement of dissolved ions away from the site of dissolution det ermine the effectiveness of gypsum as a soil amendment. Compressed pel lets were produced with similar surface areas and densities and their rates of dissolution assessed using electrical conductivity measuremen ts of a stirred solution in which the pellets were suspended in a perf orated nylon container. The rate of dissolution was significantly slow er in the mined material and in one of the phosphogypsum samples when compared with the other sources of gypsum. Scanning electron microscop y of phosphogypsum samples exhibiting different dissolution rates sugg est that these differences may in part be attributed to the average si ze and morphology of the gypsum crystallites. It is suggested that the internal surface area is smaller in the case of the least soluble ind ustrial gypsum and consequently its rate of dissolution is reduced. Th ese observed differences in dissolution rates would affect the efficac y of these materials in reducing clay dispersion and improving hydraul ic conductivity of soils, and should therefore be taken into account w hen assessing a particular product for a specific agricultural use.