Uptake of elemental or sulfate-S from fall- or spring-applied co-granulated fertilizer by corn-A stable isotope and modeling study

Sulfur deficiency has become more common in the last decades and the demand for S fertilizers has increased. Commercial fertilizers containing elemental S (S0 or ES) are usually in granular form, but their efficiency under field conditions has rarely been studied. A field trial with stable isotope (34S) as tracer was carried out to assess the uptake of ES and SO4-S applied as S-fortified ammonium phosphate fertilizer. The fertilizer, which contained 5% ES and 5% SO4-S, was broadcast applied in spring or in fall and the contribution of fertilizer S was assessed over two years, by analyzing the corn plants at early stage and at maturity. In the first year, near equal amounts (12-14%) of S in the plant were derived from fertilizer ES and SO4-S for the spring applied fertilizer, while more S was derived from fertilizer ES (12%) than from fertilizer SO4-S (5%) with fall-applied fertilizer. In the second year, the contribution of fertilizer S decreased and was greater for ES than for SO4-S in all cases. As demonstrated through modeling, the results could be explained based on leaching of applied SO4-S, particularly when fall-applied, cycling in organic matter, and oxidation of ES with an estimated rate of 0.005 d−1 at 20 °C. This study demonstrates the benefit of ES as a slow release S fertilizer in high-rainfall environments.