A slowly soluble, sulfur fertiliser from a by-product of mineral sands processing

Abstract

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.