The effects of different manufacturing techniques on the avalability of sulfate to pasture from mixtures of elemental sulfur with either triple superphosphate or partially acidulated reactive phosphate rock

Six different preparations of elemental sulfur incorporated into triple superphosphate and a single mixture of elemental sulfur with partially acidulated reactive phosphate rock were evaluated for the rate of release of sulfate-sulfur to pasture at two sites. The method of preparing the mixtures affected the size of sulfur particles released on dispersion of fertilizer granules. The rates of release of sulfate from oxidation of the elemental sulfur was controlled by the fineness of the particles of elemental sulfur. For rapid release of sulfate by oxidation a mixture prepared by sintering elemental sulfur with mature triple superphosphate was the most effective. It contained sulfur particles of diameter less than 75 micrometers. The mixtures which comprised synthetic blends of elemental sulfur of particle size less than 150 micrometers, or 150-250 micrometers, each with mature triple superphosphate, released sulfate at a progressively slower rate than that with particles less than 75 micrometers. However, both proved effective sulfur fertilizers releasing sulfate over a 1-2 year period. In contrast, mixtures prepared by adding molten elemental sulfur during the process of manufacturing triple superphosphate resulted in a coarser distribution of sulfur particles and a slow release of sulfate to plants. The incorporation of sulfur into a mixture with partially acidulated reactive phosphate rock was made via the dispersion of molten sulfur into phosphoric acid which was then used to partially acidulate phosphate rock. The resultant sulfur particles ranged from finely divided (e.g. 38% < 150 micrometers) to moderately fine (81% < 500 micrometers), and the release of sulfate was predictable based on the particle size distribution. Differences between the two experimental sites in uptake of plant sulphate reflected the different pastures grown. At the cool temperate coastal site a ryegrass/clover pasture rapidly took up sulfate as it was released into the topsoil. Inland, in the summer-dry environment, the uptake of released sulfate by the deep-rooted lucerne pasture was delayed for 12-18 months, indicating that the sulfate uptake increased after the nutrient had leached beyond the surface soil layer.