Field Measurement of Sulfur Uptake By Crops from Fertilizer Sulfate or Elemental Sulfur, Using a Stable Isotope Technique.

Sulfur deficiency has become more wide-spread, and hence interest in S-fertilizers has been growing. 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 contribution of ES and sulfate-S (SO₄-S) to crop uptake in a field trial, through the use of S-34 labeled fertilizer. The field site was located in Mato Grosso (Brazil), and the trial was conducted during two consecutive growing seasons with a soybean/corn rotation. Fertilizer (MAP with 7% ES and 2% SO₄-S) was applied at the start of the first growing season. In the first crop, 4.2% of plant S was derived from ES and 1.4% from SO₄-S. In the three following crops, 5-7% of plant S was derived from fertilizer ES and 0.6% from fertilizer SO₄-S. Over the four crops, 16% of fertilizer ES and 9% of fertilizer SO₄-S was recovered in the crop. A model was developed that takes into account the oxidation of ES and the turnover of organic S (with validation through independent lab experiments). The observed results could be explained assuming that 70% of SO₄-S had leached below the root zone and that ES oxidation occurred at a rate of 0.35% per day (corresponding to circa. 70% oxidation at the end of the second growth season). The relative low fertilizer recoveries could be attributed to dilution of fertilizer-derived S in the organic matter pool and (for SO₄-S) to leaching, because of the intensive rainfall shortly after fertilizer application. These results indicate the important role of organic S turnover and demonstrate that ES-enriched MAP fertilizer provides a useful slow release source of S in high-rainfall environments.