THE EFFECTS OF DIFFERENT MANUFACTURING TECHNIQUES ON THE AVAILABILITYOF 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 tripl e superphosphate and a single mixture of elemental sulfur with partial ly acidulated reactive phosphate rock were evaluated for the rate of r elease of sulfate-sulfur to pasture at two sites. The method of prepar ing the mixtures affected the size of sulfur particles released on dis persion of fertilizer granules. The rates of release of sulfate from o xidation of the elemental sulfur was controlled by the fineness of the particles of elemental sulfur. For rapid release of sulfate by oxidat ion a mixture prepared by sintering elemental sulfur with mature tripl e superphosphate was the most effective. It contained sulfur particles of diameter less than 75 mu m. The mixtures which comprised synthetic blends of elemental sulfur of particle size less than 150 mu m or 150 -250 mu m, each with mature triple superphosphate, released sulfate at a progressively slower rate than that with particles less than 75 mu m. However, both proved effective sulfur fertilizers releasing sulfate over a 1-2 year period. In contrast, mixtures prepared by adding molt en elemental sulfur during the process of manufacturing triple superph osphate resulted in a coarser distribution of sulfur particles and a s low release of sulfate to plants. The incorporation of sulfur into a m ixture with partially acidulated reactive phosphate rock was made via the dispersion of molten sulfur into phosphoric acid which was then us ed to partially acidulate phosphate rock. The resultant sulfur particl es ranged from finely divided (e.g. 38% < 150 mu m) to moderately fine (81% < 500 mu m), and the release of sulfate was predictable based on the particle size distribution. Differences between the two experimen tal sites in uptake of plant sulphate reflected the different pastures grown. At the cool temperate coastal site a ryegrass/clover pasture r apidly took up sulfate as it was released into the topsoil. Inland, in the summer-dry environment, the uptake of released sulfate by the dee p-rooted lucerne pasture was delayed for 12-18 months, indicating that the sulfate uptake increased after the nutrient had leached beyond th e surface soil layer.