Foliar Fertilization of Soybeans. I. Effect of Fertilizer Sources, Rates, and Frequency of Application1

Abstract Foliar applications of fertilizer NPKS to soybeans [Glycine max (L.) Merr.] during the podfill stages of growth have been shown to increase yields. However, results have been inconsistent and in some cases severe yield depressions have occurred. The purpose of this study was to evaluate NPKS sources, rates of application, and frequency of application along with micronutrients, a fungicide, and a growth regulator on soybean seed yield, seed weight, and seed composition over several soybean genotypes and experimental locations during a 3‐year period. In 16 cultivar‐location‐year trials, soybean seed yields were increased significantly by the foliar fertilizer treatments over the control only once. Generally, yields were decreased when urea‐ammonium nitrate solution (UAN) and ammonium polyphosphate (APP) were substituted for urea and K‐polyphosphate in the basic NPKS formulation (24 + 2.4 + 7.2 + 1.2 kg/ha N + P + K + S). A positive relationship between leaf injury and yield depression by the various materials was noted; especially when the materials were applied during midday rather than in the early morning or late afternoon hours. The inclusion of micronutrients, a fungicide (Benlate), a growth regulator (Regime 8), or commercially available liquid fertilizers did not increase yields significantly. Seed weights were generally decreased with many of the foliar fertilizers and with the growth regulator. Seed N percentage and protein were increased and seed oil percentage decreased with four applications of the base NPKS formulation. These data indicate that further basic research is needed before the practice of foliar fertilization can be recommended.