Bauxite residue from alumina refining was used to coat granules of single superphosphate to reduce the leaching of phosphorus in coarse, sandy soils for pastures in high rainfall areas of south-western Australia (>800 mm annual average). The impact of coating the superphosphate on the leaching of phosphorus was measured in a glasshouse experiment and the effectiveness of the fertiliser using dry herbage yield of subterranean clover (Trifolium subterraneum) was measured in a field experiment. The glasshouse experiment measured 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 of soil, where subterranean clover was grown under leaching conditions. A coating of 30%, by weight, reduced leaching of single superphosphate by about half. Increasing the coating of bauxite residue also increased the phosphorus uptake and increased the plant growth. In the field trial, the effectiveness of single superphosphate with a bauxite residue coating of 25% by weight was increased on average by 100% in Year 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. Limited soil phosphorus tests showed only minor increases in the residues of phosphorus where the superphosphate had been coated with bauxite residue.
The effectiveness of a pelletised by-product from mineral sands processing as a sulfur fertiliser was measured on high rainfall sandy soils of Western Australia. The by-product was a lime-neutralised, low-grade, sulfuric acid effluent which produces a precipitate of gypsum containing iron and manganese. The pelletised product has been given the name 'Ironman' gypsum, reflecting its major constituents. Ironman gypsum was compared with ordinary single superphosphate and naturally occurring gypsum from Wyalkatchem (fine) and Jurien Bay (coarse) as a sulfur fertiliser in 2 field experiments.When comparing the relative rates of sulfur release as indicated by plant yield and nitrogen:sulfur ratio, the order of effectiveness was: superphosphate = Ironman gypsum pellets >> coarse gypsum ≥� unpelletised by-product >>Wyalkatchem gypsum. Laboratory leaching studies showed the Ironman pellets to leach more slowly than superphosphate, which in turn leached more slowly than coarse rock gypsum. The soil sulfur tests were of limited value in predicting yield even though they were accurate in predicting the amount of sulfur previously applied.Compared with an unfertilised control, Ironman gypsum did not result in a significant increase in the uptake of heavy metals by plants. Most of the elements of concern, mercury, cadmium, and uranium, actually showed a significant reduction in concentration in the plant matter. This was probably due to the increased growth caused by the improved sulfur nutrition resulting in dilution of these elements. There was a slight but statistically significant increase in the nickel concentration that could be attributed to Ironman gypsum, but the level was still low.
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