BACKGROUND: Soybeans (Glycine max) are a major protein source both for humans and non-ruminant livestock; however, the usability of soybean protein is limited by the concentration of the essential sulfur (S)-containing amino acids methionine and cysteine (MET+CYS). Traditional efforts to improve protein quality in soybeans have largely been focused on plant breeding but soil S fertility may also influence seed MET+CYS concentration. Crop S deficiencies are increasingly common due to soil depletion by high yields and reduced atmospheric deposition. We report on a survey of commercial soybean fields and two replicated split-plot field experiments in the mid-Atlantic region, USA. The experimental treatments were two levels (0 or 100 kg S ha −1 ) of broadcast gypsum (CaSO 4 ) and two levels (0 or 11 kg-S ha −1 ) of foliar Epsom salt (MgSO 4 ) applied to two soybean cultivars. The objective was to assess the variability of, and effect of, S fertilization on S and MET+CYS concentrations in soybean seeds.RESULTS: Sulfur ranged from 2.35 to 3.54 mg g −1 and MET+CYS ranged from 5.5 to 9.2 mg g −1 protein in seeds from commercial fields surveyed. Sulfur application increased seed MET+CYS concentration 1.3 to twofold in two replicated field experiments. Overall, MET+CYS concentration in protein ranged from 3.9 to 12.8 mg g −1 and was linearly predicted (R 2 = 0.65) by seed S. CONCLUSIONS: Soybean seed S and MET+CYS concentrations vary widely. We show that field-scale S application can greatly enhance soybean MET+CYS content and therefore protein quality.

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