Comparison of single superphosphate and superphosphate coated with bauxiteresidue for subterranean clover production on phosphorus-leaching soils

Bauxite residue from alumina refining was used to coat granules of single s uperphosphate to reduce the leaching of phosphorus in coarse, sandy soils f or pastures in high rainfall areas of south-western Australia (> 800 mm ann ual average). The impact of coating the superphosphate on the leaching of p hosphorus was measured in a glasshouse experiment and the effectiveness of the fertiliser using dry herbage yield of subterranean clover (Trifolium su bterraneum) was measured in a field experiment. The glasshouse experiment m easured the effect of coating the superphosphate with bauxite residue at 0, 5, 10, 15, 20, 25, 30, 35, and 40% by weight. A coating of 25% (by weight) was chosen for the field experiment. In the glasshouse experiment, the coated granules were applied to columns o f soil, where subterranean clover was grown under leaching conditions. A co ating of 30%, by weight, reduced leaching of single superphosphate by about half. Increasing the coating of bauxite residue also increased the phospho rus uptake and increased the plant growth. In the field trial, the effectiveness of single superphosphate with a bauxi te residue coating of 25% by weight was increased on average by 100% in Yea r 1, 303% in Year 2, and 158% in Year 3, relative to freshly applied single superphosphate. The bauxite residue coating also increased the phosphorus content of the herbage in a similar manner to the increases in yield. Limit ed soil phosphorus tests showed only minor increases in the residues of pho sphorus where the superphosphate had been coated with bauxite residue.