Reactions of Gypsum or Phosphogypsum in Highly Weathered Acid Subsoils

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Abstract
Crop responses to gypsum or phosphogypsum (PG) amendment to acid soils have been highly variable in different soils. Therefore, soil properties influence the mechanisms of amelioration of acid‐soil infertility by gypsum or PG. The effects of gypsum or 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 (Bladen: Typic Albaquult; Cecil and Wedowee: Typic Hapludults). 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 respective ions by the soil. The sorption of Ca and SO4 was considerably greater in the woodland soils than in cultivated soils. The sorption of Ca from PG solution was equivalent to the increase in negative charge in the Cecil and Wedowee soils. In the Bladen soil, the increase in surface charge was lower than the sorption of Ca. The increase in exchangeable cations in the Cecil and Wedowee soils, after equilibration with PG solution, was very similar to sorption of SO4. Equilibration of the soils with PG or gypsum solution decreased the exchangeable Al by 40, 21, and 6 to 7% in the Cecil, Wedowee, and Bladen soils, respectively. 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.