The effects of addition of bauxite red mud to soil on P uptake by plants

Abstract

The chemical properties of red mud, a byproduct of Bayer process refining of bauxite to alumina, make disposal of the material problematic. It is very alkaline (pH > 11), contains a large amount of sesquioxides, and thus has a very high P retention capacity. These characteristics have encouraged its use as a soil amendment to enhance P retention in sandy soils. A glasshouse experiment was carried out to investigate the effect of red mud on plant-available P. Leached red mud (LRM) (pH 7.24) was mixed at rates of 0, 5, 10, and 20 t/ha with a very sandy soil, provided with a full basal fertiliser and various rates of phosphate, and then sown with perennial ryegrass. Five harvests were obtained over a period of 245 days. At the end of the experiment the highest rate of addition of LRM gave an increase in soil pH of less than one unit and the electrical conductivity had not changed substantially or systematically. Bicarbonate-extractable P (bic-P) had decreased considerably from the initial values. However, there were no significant differences between bic-P values at the different levels of red mud application for the same rate of P application. Plant yield was not significantly different between treatments. Addition of red mud (a) decreased the P concentration of plants for the same amount of P applied; and (b) required a larger amount of bic-P to maintain a constant level of P in the plant. The red mud had adsorbed both applied and existing P and reduced the plant availability of bic-P. The economic impact of these processes needs evaluation.