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Reactions of Gypsum or Phosphogypsum in Highly Weathered Acid Subsoils

Univ. of Florida, Inst. of Food and Agric. Sciences, Citrus Research and Education Center, Lake Alfred, FL 33850

M. E. Sumner and W. P. Miller

Dep. of Agronomy, Univ. of Georiga, Athens, GA 30602

*Corresponding author.

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.

Contribution from the Dep. of Agronomy, Univ. of Georgia and Georgia Agric. Exp. Stn.

Received for publication April 26, 1989.