Growth of Lolium perenne , Chemical, and Microbial Properties in a Newly Developed Engineered Soil Based on Bauxite Residue as Affected by pH and Exchangeable Na

A new engineered soil product was produced using acidified bauxite residue as its major component. The effect of pH of manufacture (8.7, 8.3, 8.1, 7.3, 6.9, 6.7, and 6.4) on properties of the material and ryegrass (Lolium perenne L. cv RPR) growth over a 26-week period were investigated. As pH was lowered there were statistically significant decreases in effective cation exchange capacity (ECEC), exchangeable Ca and Na, exchangeable sodium percentage (ESP), extractable Mn, Ni, and Pb, soluble organic C, microbial biomass C, and basal respiration, while extractable As, Cr, V, and Se significantly increased. Dry matter yield at harvests 1, 2, and 3 and total shoot mass were significantly lower in pH 8.1, 8.3 and 8.7 treatments than the others. Initial exchangeable Na was above 50 mmolc kg−1 (ESP > 24–26%) for the former three treatments and at or below 30 mmolc kg−1 (ESP < 21%) in the others. Herbage Na concentrations at harvest 1 showed a statistically significant trend of 8.1, 8.3, 8.7 > 7.3 > 6.9, 6.7, 6.4, which is the opposite of the trend for dry matter yields. Herbage Na concentrations were 17.6 g kg−1 at pH 8.7 and 12.9 g kg−1 at pH 6.4, while equivalent shoot dry matter yields were 1.61 and 2.58 g pot−1. It was concluded that the main factor limiting ryegrass growth at higher pH values was high exchangeable Na. As pH decreased concentrations of tissue Na, N, S, Ca, As, Mo, V, Se, and Al in ryegrass decreased, while those of Mn, Fe, Ni, Pb, and B tended to increase.