Amelioration of soil chemical constraints for revegetation on gold oxide processing residues

Gold oxide processing residues are highly alkaline, sodic and saline and initially require amelioration for plant establishment. The aim of this study was to determine the efficacy of soil covers and residue ameliorants in alleviating chemical constraints for revegetation. The effects of residue covers (10 cm topsoil plus 0, 15 or 30 cm gravelly sub-soil referred to as gravel), combined with gypsum (30 and 60 t ha−1), and compost (0 and 50 m3 ha−1) on chemical properties (pH, electrical conductivity (EC), exchangeable Na percentage (ESP), cation exchange capacity (CEC), total N, extractable N, P, K, S, cations and micronutrients) of gold oxide processing residue were studied in a large-scale field trial at Boddington, Western Australia in a Mediterranean climate. After the first season's rainfall, the EC1:5 decreased substantially (from 4.5–5.0 to about 1.0 dS m−1) across all treatments in the 0–10 cm residue layer (RES1), while topsoil and gravel materials which had low initial salinity (0.04–0.07 dS m−1) increased their EC1:5 to 0.40–0.45 dS m−1 in the 10 cm above the residue interface in the 2nd hot-dry summer season. pH of both RES1 and the 10–20 cm layer of residue (RES2) dropped from 9.0 to 8.0 over the trial period but topsoil pH remained at 5.2–5.5, regardless of treatments. Exchangeable Na percentage in RES1 and RES2 decreased, but increased from 17 to 70% in topsoil placed directly on the residue surface, compared with no increase in topsoil where there was a gravel layer separating topsoil from residue. Fertiliser added to topsoil raised extractable nutrients to levels adequate for plant growth. Nutrient elements did not differ with gypsum rate and compost amendment except for S, on account of the gypsum applied, and extractable P and Mn, which were elevated by compost. These findings indicate the efficacy of gravel plus fertilised topsoil cover on residue treated with surface broadcast gypsum to alleviate the adverse properties of alkaline, saline, sodic gold oxide processing residue as a medium for plant establishment and early growth.