Foliar Fertilization of Soybeans

Extensive research addressed foliar fertilization of soybeans at reproductive stages during the 70s and 80s. The soybean plant has been characterized by markedly reduced root activity during late seed development and increased translocation of nutrients and metabolites from other tissue into the seed. This depletion of nutrients from leaves could result in decreased photosynthesis, leaf senescence, and lower grain yields. Researchers theorized that if nutrients were applied directly to the foliage at this time, leaf senescence could be delayed and grain yields might be increased. Field research in Iowa conducted by Garcia and Hanway (1976) seemed to confirm this hypothesis. They evaluated various nutrient combinations for foliar application at the R2 to R7 growth stages and found that a 10-1-3-0.5 NPKS ratio increased yields by 7 to 8 bu/acre. They concluded that the four nutrients were needed and that the optimum time of application was between growth stages R5 and R6. Research conducted after Hanway published those results showed, however, that foliar fertilization of soybeans at late stages either produces insignificant yield increases, most often does not influence yield, or reduces yield. For example, in 1976 the Tennessee Valley Authority coordinated more than 200 research or demonstration field comparisons with foliar fertilization trials with soybeans at reproductive stages in 28 states. Summary reports of these studies (Gray, 1977; Peele, 1977) showed that yield responses varied from a maximum increase of 0.5 bu/acre to decreases of as much as 6 bu/acre. Later work in several states (Boote et al., 1978; Sesay and Shibles, 1980; Syverud et al., 1980; Vasilas et al., 1980; Poole et al, 1983) showed similar small and inconsistent increases but mostly no effects or negative responses that could seldom be explained by leaf damage, management practices, or several site characteristics. These results discouraged further research in foliar fertilization of soybeans and this practice has not been widely used by producers. In response to renewed interest in late foliar applications of urea, a research project was recently started in Minnesota. Data from two trials conducted in 1997 (G. Rehm, personal communication) showed no effect or yield decreases.