Chemical effects of repeated equilibrations of variable-charge soils with phosphogypsum solution

Surface applications of gypsum or phosphogypsum (PG) to acid soils have resulted in improved crop growth as a result of amelioration of subsoil acidity; however, the mechanism involved is not clearly understood. The effects of repeated equilibrations of soils with PG solution on soil solution and extractable ion composition were investigated in this study. Cultivated and woodland Cecil Bt horizon (0.6-0.8 m depth) soil materials (clayey, kaolinitic, thermic Typic Kanhapludult) were sequentially equilibrated five times in the laboratory with 2 g L-1 PG solution at 40% (v/w) soil moisture content. With an increase in the number of equilibrations, the pH of the displaced soil solution increased in the cultivated soil, and initially decreased and then increased slightly in the woodland soil. On the basis of predictions of saturation indices by using the MINTEQ speciation model, the composition of displaced soil solutions showed no evidence of CaSO4 precipitation regardless of the number of equilibrations. The cumulative sorption of Ca and SO4 after five equilibrations with PG solution was 2.80 and 1.69 cmolc kg-1, respectively, in the cultivated soil, and 4.43 and 4.41 cmolc kg-1, respectively, in the woodland soil. For the cultivated soil, 83 and 71% of the final cumulative sorption of Ca and SO4 was reached after the second equilibration while, for the woodland soil, the values were 45 and 46%, respectively. The retention of Ca and SO4 following the equilibration of the soils with PG solution was attributable to a salt-sorption mechanism, as evident from the sorption of Ca and SO4 in equal proportions with negligible release of cations or anions into the displaced soil solution. Successive equilibration of the soils with PG solution resulted in a decrease in extractable Al, an increase in extractable Ca, and a net increase in cation-exchange capacity. These changes were much greater in the woodland soil than in the cultivated soil.