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 an d nutrients in granulated compound fertilizers fortified with finely d ivided elemental sulfur (S degrees) on the rate of S degrees oxidation . In one experiment, S degrees was banded together with or apart from triple superphosphate (TSP) while in two others, S degrees was granula ted with nutrient and inert carriers. A fourth experiment examined res ponse to S in an S degrees-fortified TSP from a range of granule sizes . Response and, in some cases, S degrees recovery (using S-35 labels) by test crops (maize, wheat, upland rice) was measured. In all experim ents, P mixed with S degrees increased plant growth and S recovery abo ve treatments in which P and S degrees were physically separated. Ther e was however, no effect of distance of separation on S recovery. In o ne experiment, N as urea and N and P as diammonium phosphate (DAP) wer e also found to enhance response to S degrees although to a lesser deg ree than P alone. These observations were attributed to a nutritional requirement of S degrees-oxidizing microorganisms for P and N. Granula tion of S degrees with carriers also influenced oxidation rate, as inf erred from the fertilizer S recovery. For a given S degrees concentrat ion, the effect was inversely proportional to the mean diameter of gra nules. It is shown that this relationship can be explained if one assu mes that S degrees particles in granules collapse into a fixed number of aggregates per granule irrespective of granule size when the solubl e nutrient carrier dissolves and diffuses away from the point of appli cation.