Cation and anion leaching and growth of Acacia saligna in bauxite residue sand amended with residue mud, poultry manure and phosphogypsum

Purpose: To examine (1) the effect of organic (poultry manure) and inorganic (residue mud and phosphogypsum) amendments on nutrient leaching losses from residue sand and (2) whether amendments improve the growth of plants in residue sand. Methods: Leaching columns were established using residue sand. The phosphogypsum-treated surface layer (0-15 cm) was amended with poultry manure and/or bauxite residue mud and the subsurface layer (15-45 cm) was either left untreated or amended with phosphogypsum. Results: Much of the Na+, K+, Cl− and SO⁴₂- was lost during the first four leachings. Additions of phosphogypsum to both surface and subsurface layers resulted in partial neutralization of soluble alkalinity. Mean pH of leachates ranged from 8.0 to 8.4, the major cation leached was Na+ and the major balancing anion was SO⁴₂-. Where gypsum was not applied to the subsurface, mean pH of leachates was 10.0-10.9, the main cation leached was still Na⁺ and the main balancing anions were a combination of SO⁴₂- and HCO₃⁻/CO₃²⁻. At the end of the experiment, concentrations of exchangeable Na+ in the subsurface layers were similar regardless of whether gypsum had been applied to that layer or not. Yields of Acacia saligna were promoted by additions of poultry manure to the surface layer but unaffected by gypsum incorporation into the subsurface layer. Conclusions: Lack of reaction of phosphogypsum with the subsurface layer is unlikely to be a major factor limiting revegetation of residue sand since in the absence of phosphogypsum the excess Na+ leaches with the residual alkalinity (HCO₃⁻/CO₃²⁻) rather than SO⁴₂-.