Foliar Fertilization of Soybeans During the Seed‐filling Period1

Abstract Foliar fertilization of soybeans ( Glycine max (L.) Merr.) with N, P, K and S during the seed‐filling period promises to increase soybean yields. Such foliar applications could be used to avoid the depletion of these nutrients in the leaves and the resulting reduction in photosynthetic rate during this period due to poor nutrient uptake from the soil and translocation of these elements from the leaves to the developing seeds. Field experiments were conducted to test this hypothesis by spraying soybeans with solutions of N, P, K, and/or S in different proportions and at different times and rates. We supplied N primarily as urea, P primarily as poly‐phosphate, K with the P as potassium polyphosphate or with the S as potassium sulfate. Very significant yield increases were obtained from two to four sprayings on different soybean cultivars at different experimental sites between developmental stages R5 and R7. The optimum proportion of N:P:K:S in the solution 10:1:3:0.5. a proportion similar to that of these elements in the seeds. The optimum rate of total nutrient application was about 80 + 8 + 24 + 4 kg/ha of N + P + K + S, respectively. Yield increases were due to increases in numbers of harvestable seeds, not seed size. The results indicate that foliar fertilization during the seed‐filling period can become a very practical method for increasing soybean yields.