Abstract
The objectives of this study were to obtain a reliable index for the evaluation of the S nutrition status in soybean [Glycine Max (L.) Merr.] and to identify the critical S level in relation to seed yield and quality. Two Oxisols were used: A‐horizon soil from Serra dos Gerais, and A‐ and B‐horizon soils from Sambaiba in Maranhão State, Brazil. Soybean plants in pots were grown in a greenhouse with the supply of 0 to 80 mg S kg−1 soil. The seed S concentration was a more reliable index of seed yield because of the higher correlation between S concentration and yield. In the plants with visible symptoms of S deficiency, the seeds contained 1.5 g S kg−1, and the seed yield was 60% of the control. Electrophoresis analysis indicated that the critical seed S concentration for deficiency of protein components was 2.0 g kg−1 when the yield was 80% of the control. The S concentration was 2.3 g kg−1 or higher for >90% yield when the composition of the protein components was identical with that in the original seeds obtained under sufficient S fertilization. We classified the S concentration in the seeds as: deficient (S < 1.5 g kg−1), very low (1.5 ≤ S < 2.0 g kg−1), low (2.0 ≤ S < 2.3 g kg−1), and normal (2.3 g kg−1 ≤ S). Because of stable S concentration, easy sampling, and sufficient time for planning of fertilizer application for the subsequent cropping, seed analysis is preferable to leaf analysis.
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