Rate of dissolution of gypsum from different sources and its effect on water infiltration of soil

Abstract

Five sources of gypsum (pure, mined and three industrial) were pressed into pellets and their dissolution rate when suspended in water compared by measuring electrical conductivity of the stirred solution. Pellets of industrial phosphogypsum from Phalaborwa, Potchefstroom and Richards Bay all dissolved more rapidly than mined gypsum from Kimberley but more slowly than pure gypsum. Electron microscopy of partially dissolved pellet surfaces indicated surface texture—as influenced by crystallinity of the gypsum—as the probable factor controlling dissolution rate. The mined and industrial gypsum samples were applied in their original condition (Phalaborwa is granular) at a rate of 10 000 kg ha⁻¹ to the surface of a dispersive, crusting (but non-sodic) soil placed in trays tilted at a 5% slope and exposed to distilled water applied as simulated rain (44 mm h⁻¹). The fraction of the 50-mm rain which infiltrated was: control 17%, Kimberley 42%, Phalaborwa 44%, Richards Bay 62% and Potchefstroom 68%. Electrical conductivity of the drainage water was: Kimberley 110; Phalaborwa 160; Richards Bay 200 and Potchefstroom 240 mS m⁻¹. Both granule size and crystallinity limit the effectiveness of gypsum as a crust inhibitor. Phalaborwa proved to be as effective as Potchefstroom during a second rain applied after the soil surface had dried. This was ascribed to the evaporative recrystallization of dissolved gypsum as a surface efflorescence. Because their attrition by dissolution and erosion will be slower, at least some coarse fragments may be desirable to prolong the benefit from dressing the soil surface with gypsum.