Gypsum amendment of alkaline bauxite residue - Plant available aluminium and implications for grassland restoration

An inherent property of bauxite refining residue is high pH and associated elevated levels of solubilised aluminium. High solubilised Al content of the residue can inhibit plant growth and under natural conditions significant reduction of aluminium levels is extremely slow. Various procedures have been employed globally to reduce the alkaline properties of bauxite residue. At the Aughinish Alumina Ltd. refinery gypsum is applied as an amendment to reduce pH and sodicity of the residue for restoration. An additional benefit of this procedure is a significant decrease in plant available Al in the residue and low herbage concentrations. Furthermore, deficiencies of nutrients such as Ca, a symptom of Al toxicity, are not evident. To avoid excessive Al uptake in restored residue it is recommended that adequate calcium and other nutrients are added to the residue and nutrient cycling is encouraged as part of the restoration programme.