Sulfur deficiency in soil has become more common since the 1980s due to reduced atmospheric deposition, increasing crop yields, and less use of sulfur-containing fertilizers. Sulfur fertilizers are based on either sulfate or elemental sulfur. While elemental sulfur does not leach and is 100% sulfur, it needs to be oxidized to sulfate to become available for plants.
Commercial products usually consist of elemental sulfur cogranulated with other macronutrient fertilizers. In this paper, a team from the Fertilizer Technology Research Centre at the University of Adelaide, Australia, assessed the fate of S-fortified monoammonium phosphate (MAP) fertilizers. Across four field sites in Argentina, Brazil, Canada, and the USA over two years, sulfur recovery by the crop ranged from 16 to 28% for MAP-elemental sulfur and from 9 to 86% for MAP-sulfate.
The data allowed constructing a model which takes into account organic sulfur cycling, sulfate leaching and elemental sulfur oxidation to explain the observed crop recoveries, and make long-term predictions. The model demonstrated that the total recovery of elemental sulfur will eventually reach those of sulfate or exceed them if there is sulfate leaching. Hence, long-term trials are needed to evaluate the true effectiveness of a slow-release fertilizer source such as elemental sulfur. The authors also point out that the dynamics of soil organic sulfur, which is a major contributor to sulfur uptake, are still poorly understood.
