Agronomic Effectiveness of Granular NP Fertilizers Containing Elemental Sulfur with/without Ammonium Sulfate: A Review.

Deficiency of sulfur (S) in soils has become a soil fertility issue worldwide because of a decrease in S deposition from air to soil due to legislation and increased crop removal. Continuous use of high-analysis nitrogen/phosphorus (NP) fertilizers lacking in S further exacerbates the S deficiency for crop production. Several developed granular NP fertilizers such as monoammonium phosphate (MAP), diammonium phosphate (DAP), and triple superphosphate (TSP) containing elemental S (ES) with/without ammonium sulfate (AS) have been marketed to farmers. It is claimed that these types of product can provide available SO₄-S through solubility of the AS and ES oxidation during the growing season. This review carefully examines the literature that addresses the topic. The review shows that oxidation of ES particles in granular NP fertilizers is generally nil or inadequate to provide available S to seasonal or first crops, except perhaps for pastures. This is due to the negative locality effect on granular ES oxidation.  In contrast, available S can be obtained from the associated AS component of the granular (ES+AS). In conclusion, the detailed evaluation of available data relevant to the use of these new products often cannot support the claims by the manufacturers of the fertilizers containing ES or (ES+AS) that can provide available S as compared with traditional SO₄-based S sources for season-long or first field crops.