Effect of soil amendment with bauxite Bayer process residue (red mud) on the availability of phosphorus in very sandy soils

The chemical properties of red mud, a by-product 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 of soils so as to reduce leaching of P. To investigate the effect of added red mud on the availability of existing P in sandy soils an incubation experiment was conducted using 3 types of red mud (untreated red mud, red mud amended with 5% gypsum, and red mud leached with dilute acid to remove all soluble salts) mixed with 12 podsol topsoils at rates of 0, 2.5, 5, 10, 20, and 40 t/ha. The mixtures were incubated wet for 28 days in the dark at 20degreesC, both with and without a microbial inhibitor. The decrease in 0.5 M sodium bicarbonate extractable phosphorus (bic-P) was 40-60% of initial bic-P at 40 t/ha of all 3 red muds after wet incubation without microbial suppressant. This decreased to 20-40% after drying or with addition of the microbial suppressant. The decrease in bic-P was not due to the increase in pH due to red mud application and appears to involve both chemical and microbial actions. The increase in soil pH of up to 3 units would be beneficial for pasture production at most rates of red mud application and the associated increase in electrical conductivity is not sufficient to affect plant growth.