Effect of application of bauxite residue (red mud) to very sandy soils on subterranean clover yield and P response

Bauxite residue (red mud) is the byproduct from treatment of crushed bauxit e with caustic soda to produce alumina. When dried the residue is alkaline and has a high capacity to retain phosphorus (P). The residue is added to p astures on acidic sandy soils to increase the capacity of the soils to reta in P so as to reduce leaching of P into waterways and so reduce eutrophicat ion of the waterways. This paper examines how red mud influences the effectiveness of P from sing le superphosphate for producing subterranean clover (Trifolium subterraneum ) dry herbage, in the year of application and in the years after applicatio n (residual value). Red mud was applied at 0, 2, 5, 10, 20, and 40 t/ha and the P was applied at 0, 5, 10, 20, 40, 80, and 160 kg P/ha. In the year of application and the year after application of red mud, dry m atter yields were doubled on the soil treated with 20 t/ha of red mud compa red with the untreated control. Improvements in production were initially g reater in the red mud treatments than in the lime treatment (2 t lime/ha). Red mud increased the maximum yield plateau for P applied in current and pr evious years. When P was applied to freshly applied red mud, more P needed to be applied to produce the same yield as the amount of red mud applied in creased. Red mud increased soil pH, and the increases in yield are attribut ed to removing low soil pH as a constraint to pasture production. This init ial need for higher amounts of fertiliser P when increasing amounts of red mud were applied may be due to increased P sorption caused by increased pre cipitation of applied P when the fertiliser was in close contact with the f reshly alkaline red mud. When P was freshly applied to red mud that had been applied to the soil 12 months ago, yield response and P content increased. This was attributed to the reduction in sorption of P due to red mud being neutralised by the soil and because sorption of P already present in the soil reduced the capacity of the red mud to sorb freshly applied fertiliser P. Residues of P in the soil and pH were also increased with application of red mud. In the years after application of red mud and lime, relative to P applied t o nil red mud and nil lime treatment, the effectiveness of fertiliser P app lied to the red mud and lime treatments increased. This was so as determine d using plant yield, P concentration in plant tissue, and soil P test.