Seed Number and Yield Determination in Sulfur Deficient Soybean Crops

In soybean, seed number and yield is largely determined by the environmental conditions between initial bloom and the beginning of seed filling. Four field experiments were conducted to determine the effects of two sources of sulfur (S) on crop growth during the critical period of seed number determination and yield. Ammonium sulfate and gypsum were applied at a rate of 15 kg S ha− 1. Seed yield was closely related to the number of seeds per m2 (R2 = 0.93), but seed number was not related to crop growth between bloom and the beginning of seed filling. There was no effect of fertilization on aboveground biomass accumulation until the seed filling period began. The results from this study suggest that a moderate S deficiency reduced seed yield by affecting crop growth during the seed filling period. This late effect of a moderate S deficiency could be a consequence of the known high sulfate mobility in soils and low S remobilization in plants.