For the Brazilian site, the recovery from applied fertilizer was more from ES than SO4-S for all crops except soybean in the first year, where both forms of S had a similar recovery. The total recovery of S at the end of the second year was 9.3% for SO4-S compared to 15.9% for ES. Lower SO4-S recovery was due to faster immobilization and excess rainfall because nearly 600 mm rainfall occurred in the first 2 months after S application (Fig. 3). Higher recovery for ES was due to faster oxidation in a warmer climate. Lower total S recovery can also be due to leaching of SO4-S that was oxidized from ES during the 2-year period in the warmer climate. More contribution from ES in Brazilian conditions thus suggests the need for ES as an S source for better utilization of applied S fertilizers.
Recoveries of fertilizer S varied quite dramatically across different climatic and soil conditions. The SO4-S recovery in the year of the application was much smaller for the Brazilian site than for Argentinean and Canadian sites due to high leaching potential at the location in Brazil, indicating the importance of ES source in Brazil. Therefore, products like MicroEssentials®S9® with SO4 (2%) and ES (7%) are a great fit for Brazil’s climate and soil conditions. For colder climates like Canada, crop S needs for early growth stages can be met by including higher SO4-S amounts along with ES, like in MicroEssentials S15 (7.5% SO4 and 7.5% ES). Therefore, depending on climate and soil conditions, a fertilizer containing both forms of S in suitable amounts will help reduce leaching risks, provide readily available S, and supply season-long S. The study also showed a trend of decreased contribution from total fertilizer S in the second year, demonstrating that the fertilizer applied only once in the first year is not sufficient to meet crop demand. Therefore, the application of S every year, and to every crop, is very critical.
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Acknowledgement: Fertilizer Technology Research Centre, University of Adelaide.