Sulfur Oxidation of Granular Fertilizers Containing Elemental Sulfur with/without Ammonium Sulfate in Soil.

Several newly developed granular fertilizers that contain elemental S (ES) with/without ammonium sulfate (AS) have been promoted to farmers in the market. In the case of granular Tiger XP which contains 85% ES +10% bentonite + 5% “activator”, the producer has claimed that ES oxidation in the soil can be enhanced by the added “activator”. In the case of granular MES-10S which is MAP-(5% ES + 5%AS-S), the producer has claimed that AS provides initial available S while oxidation of ES provides available S at later stages to plant growth before maturity. The purpose of this laboratory soil incubation study was to investigate the rate of ES oxidation of the granular (ES±AS) products in a loamy sand Norfolk soil collected from Alabama. The S sources used were (1) granular Tiger XP, (2) finely ground Tiger XP, (3) granular MES-10S and (4) finely ground MES-10S. The S rate applied was 200 mg S/kg plus check (no S). After different time intervals at 0, 1, 3, 6, and 10 weeks, the incubated soil samples were extracted with KH₂PO₄ solution containing 500 ppm P for extractable SO₄-S. The results show that there was no ES oxidation at all for granular Tiger XP and MES-10S up to 10 weeks whereas a significant ES oxidation occurred when the two products were finely ground. The implication of the results to the agronomic effectiveness of commercial-grade granular NP-(ES±AS) fertilizers will be discussed.