Soybean yield and nutrient composition as affected by early season foliar fertilization

The response of soybean [Glycine max (L.) Merr.] to foliar fertilization du ring early growth stages has received little attention. Recent Iowa researc h showed a 15% probability of positive yield response to a 3-15-8 (N-P-K) f ertilizer applied at the V5 growth stage. This study evaluated the effects of fertilizers varying in N-P-K ratio on soybean grain yield and tissue nut rient composition. Twenty-seven field trials were conducted in soils that t ested at or above optimum soil P and K levels for soybean. Six treatments i ncluded a control and nonfactorial combinations of rates and application fr equency of 28 to 56 L ha(-1) of 3-8-15, 10-4-8, and 8-0-7 fertilizers spray ed at the V5 stage. Differences between treatments were inconsistent across sites. Some or all treatments increased or decreased yields significantly at six sites. The mean yield increase or decrease for responsive sites was 400 kg ha(-1). The 3-8-15 fertilizer caused no leaf damage and other fertil izers caused little or no damage, although the damage was not clearly relat ed with yield decreases. Analyses by site showed that fertilization seldom increased tissue N-P-K composition, nutrient uptake, photosynthesis, or pla nt weight measured at the R2 growth stage. Multivariate analyses across sit es showed that 27% of the variation in yield response was explained by a co mbination of N, P, and K availability, vegetative growth, and rainfall. Pos itive yield responses tended to occur when soil or weather conditions reduc ed plant growth and nutrient availability. Foliar fertilization across all conditions will not offset the application costs.