Compost, limestone, and gypsum effects on calcium and aluminum transport in acidic minespoil

Combining limestone (CaCO₃) application with compost or gypsum (CaSO₄·2H₂O) can substantially increase subsoil Ca and decrease subsoil Al saturation in highly weathered acidic soils because of increased Ca mobility and formation of nontoxic Al-organic matter complexes. However, little is known about the effect of these surface amendments on subsoil chemistry in acidic minespoils that are high in SO₄. This study used small segmented laboratory columns to analyze the effect of surface incorporated compost, CaCO₃, and gypsum on Ca and Al chemistry in the highly acidic minespoil material (pH ≈ 2.5) below the zone of incorporation. Compost did not affect subsoil Al and Ca chemistry but caused a small increase in subsoil pH. Because of the high acidity in the spoil material CaCO₃ solubility in the amended layer was high and was not increased by compost. Adding gypsum increased Ca leaching into the subsoil compared with CaCO₃ alone, but extractable Ca was higher only after extensive leaching. Increases in extractable Ca in the subsoil were strongly correlated to decreases in extractable Al and Fe, indicating cation-exchange processes. However, significant losses of Al in unamended spoil columns and the small impact of CaCO₃ and gypsum on total Al leaching indicated that Al chemistry was also influenced by a solid phase. Solubility calculations pointed to a jurbanitelike solid phase buffering Al activity in the subsoil, even after extensive leaching. Because of this buffering effect and the high acidity of the spoil material, none of the surface treatments would be expected to alleviate subsoil phytotoxicity in highly acidic minespoil material.