Greenhouse Evaluation Of Sulfur Availability Of Granular Ammonium Sulfate and Sulfur-Enriched Monoammonium Phosphate.

Recently several conventional granular NP fertilizers (MAP/DAP/TSP) supplemented with micronized elemental S (ES) with or without ammonium sulfate (AS) to provide S nutrient have been widely marketed. The objective of this greenhouse study was to compare S availability of granular AS with a commercial-grade granular MAP containing 10% S (5% AS-S + 5% ES-S). In the first experiment, S sources were incorporated at 0, 10, 25 and 50 mg S/kg into an S-deficient sandy loam Greenville soil (pH 6.4) with ryegrass as testing crop after seven cuttings. In the second experiment, this application protocol (S source and rate) was repeated for canola grown to maturity in the same soil type. In the third experiment, S sources were applied at 0, 5, 10, 25, 50 and 75 mg S/kg to a limed sandy Lakeland soil (pH 6.5) with canola growing to maturity. In all three experiments, granular AS was more effective than granular MAP (5%AS-S+5%ES-S) in increasing cumulative ryegrass yield or canola straw and seed yields at the same rates of total S applied. When yield was plotted versus AS-S rate, both S sources followed the same S response trends suggesting that the micronized ES-S particles in granular MAP (5%AS-S+ES-S) did not contribute any available S to ryegrass or canola growth.  This was likely due to lack or inadequate oxidation of ES to sulfate due to the locality effect that limited contact between ES and soil particles.