Influence of co-granulated nutrients and granule size on plant responses to elemental sulfur in compound fertilizers

Experiments were undertaken to determine the effect of granule size and nutrients in granulated compound fertilizers fortified with finely divided elemental sulfur (S^o) on the rate of S^o oxidation. In one experiment, S^o was banded together with or apart from triple superphosphate (TSP) while in two others, S^o was granulated with nutrient and inert carriers. A fourth experiment examined response to S in an S^o-fortified TSP from a range of granule sizes. Response and, in some cases, S^o recovery (using ³⁵S labels) by test crops (maize, wheat, upland rice) was measured. In all experiments, P mixed with S^o increased plant growth and S recovery above treatments in which P and S^o were physically separated. There was however, no effect of distance of separation on S recovery. In one experiment, N as urea and N and P as diammonium phosphate (DAP) were also found to enhance response to S^o although to a lesser degree than P alone. These observations were attributed to a nutritional requirement of S^o-oxidizing microorganisms for P and N. Granulation of S^o with carriers also influenced oxidation rate, as inferred from the fertilizer S recovery. For a given S^o concentration, the effect was inversely proportional to the mean diameter of granules. It is shown that this relationship can be explained if one assumes that S^o particles in granules collapse into a fixed number of aggregates per granule irrespective of granule size when the soluble nutrient carrier dissolves and diffuses away from the point of application.