REACTIONS OF GYPSUM OR PHOSPHOGYPSUM IN HIGHLY WEATHERED ACID SUBSOILS

The effects of gypsum or phosphogypsum (PG) amendments on soil-solution composition and surface reactions were studied in subsoils (0.6-0.8 m) from cultivated and woodland sites representing three soil series. Soils were equilibrated in the laboratory with either deionized water or saturated solutions (2.0 g L-1) of either PG or reagent-grade CaSO4.2H2O (gypsum) at 20, 40, and 60% (v/w) soil moisture contents. Soil solution was displaced by centrifugation. Depletion of Ca and SO4 from gypsum or PG solution during equilibration was due to sorption of the resepective ions by the soil. The sorption of Ca and So4 was considerably greater in woodland soils than in cultivated soils. Equilibration of the soils with PG or gypsum solution decreased the exchangeable Al. The amelioration of acid-soil infertility by gypsum or PG was due to a combination of SO4-induced surface charge development, increased cation retention, and a decrease in exchangeable Al. The ameliorative effects of acid-soil infertility by gypsum or PG amendment were greater in soils dominant in variable-charge characteristics than in those dominant in permanent-charge characteristics. -from Authors