Potentially significant amounts of sulfate-S found in phosphorus fertilizers

Abstract

The occurrence of S deficiency in Midwest crops in the past 20 years is likely a result of the consistent decline of atmospheric S deposition during this time period. In the absence of intentional S fertilization, crops utilize SO₄-S mineralized from soil organic matter and potentially the incidental application of S in non-S fertilizers. Based on the analysis of hundreds of P fertilizer samples in 2021 and 2022, we found monoammonium phosphate (MAP), diammonium phosphate (DAP), triple superphosphate (TSP), and ammonium polyphosphate (APP) had SO₄-S concentrations of 1.88 ± 0.35, 1.80 ± 0.30, 1.66 ± 0.27, and 0.61 ± 0.18% SO₄-S (mean ± standard deviation), respectively. If MAP, DAP, and TSP are applied to replace P removal of average yielding corn (Zea mays L.) and soybean (Glycine max L.) crops grown in rotation, SO₄-S applied by MAP, DAP, and TSP at median and 3rd quartile values would be 4.0–4.6 lb SO₄-S acre⁻¹, approximately equivalent to ∼42–52% of the S removed in the grain of a single crop. If used as a starter fertilizer (5 gal acre⁻¹) APP would apply <0.4 lb acre⁻¹, <4% of grain S removal. The crop availability of SO₄-S in P fertilizers is conditional on the timing of their application relative to crop need, soil properties, and rainfall in addition to the amount of S applied. The contribution of P fertilizers to S cycling in environmental studies should also be considered.