Updated January 24, 2021:
Final Report
Sulfur (S) is a key component of amino acids, proteins, and peptides. Thus, S deficiency can significantly reduce soybean seed protein and amino acids and seed yield. Since there was little information available about S fertilization effect on soybean seed quality, the objective of this project was to determine the responses of soybean seed protein, amino acids, and yield to S fertilizer application rates and timings on soils with low to medium initial S fertility levels. The two-year results of this project showed that applying S fertilizer via soil application at 10 to 40 lbs S/acre at planting or fertigation at 20 lbs S/acre in early or late season resulted in significant increases in leaf S concentration during early season on both low S and medium S soils and in late season on low S soils. Significant increases in leaf chlorophyll, plant height, plant biomass, and numerical increases in seed yield were frequently observed on both low S and medium S soils. Averaged over the low and medium S soils, the effects of S applications on seed S, cysteine, and methionine concentrations were significant. All S applied treatments had higher seed S concentration than the zero S control. Numerical increases in seed protein were frequently observed with S application. In this study, the greatest increase in protein was from 40.55% under zero S application to 41.61% with application of 30 lb S/acre at planting. The largest increase in cysteine was from 0.625% under zero S to 0.665% with application of 30 lb S/acre. The maximum increase in methionine was from 0.567% under zero S to 0.595% with fertigation of 20 lb S/acre at V4. The concentrations of seed S and methionine were significantly related with S application rate under a quadratic model. The regression of cysteine concentration with S rate was very close to being significant (P=0.0597). In conclusion, applying S fertilizer via soil application at 10 to 40 lbs/acre at planting or fertigation at 20 lbs/acre in early or late season is beneficial for increasing seed S concentration, and thus enhancing cysteine and methionine concentrations in soybean seeds on low S and medium S soils. Sulfur applications may be beneficial for seed protein and seed yield under certain production and environmental conditions.