Sulfur Uptake By Corn from Fall- or Spring-Applied S-34 Labelled Fertilizer.

Sulfur (S) deficiency has become more wide-spread, increasing the need for S fertilization. Sulfur in fertilizers is usually present as sulfate or as elemental S (ES). Sulfate is immediately available to the plant, but leaches easily. Elemental S does not leach and has lower transportation cost, but only becomes available to plants after oxidation to sulfate.

We assessed the effect of fertilizer application time on the contribution of ES and sulfate-S (SO₄-S) to crop uptake in a field trial in Champaign (Illinois), through the use of S-34 labeled fertilizer. Labelled fertilizer (MAP with 5% ES and 5% SO₄-S) was applied at 28 kg S/ha either in the fall of 2013 or spring of 2014 and S uptake by corn was determined. For the treatment with spring-applied fertilizer, a similar percentage (ca. 11%) of plant S was derived from fertilizer ES and SO₄-S at maturity, and for both sources, ca. 16% of the added S was recovered in the crop. When the fertilizer was fall-applied, the recovery of fertilizer ES was similar to that in the spring-applied treatment, but only 6% of the fertilizer SO₄-S was recovered in the crop. The much lower recovery of fertilizer SO₄-S with the fall-applied fertilizer could be attributed to more leaching of sulfate below the root zone. Sampling of soil in a plot amended with high rates of sulfate indicated that about 80% of the applied sulfate had leached below 91 cm (36 in) of depth when fall applied compared to 40% when spring applied. These results demonstrate the benefit of ES as a slow release S fertilizer in high-rainfall environments, especially with fall applications.