Foliar Fertilization of Soybean at Early Vegetative Stages
The yield response of soybean [Glycine max (L.) Merr.] to foliar fertilization during reproductive stages has been inconsistent. This study evaluated soybean responses to foliar applications of a 3-8-15 (N-P-K) fertilizer at early vegetative stages in 48 trials conducted Iowa soils that tested mostly optimum or above in P and K. In 1994, the treatments were a control, single applications of 19, 28, or 38 L ha−1 at the V5 stage, and 38 or 56 L ha−1 split as one-half at the V5 stage and one-half 8 or 9 d later. In 1995 and 1996, only the single rate of 28 L ha−1 and the split rate of 38 L ha−1 were used. The lowest and highest volumes used encompassed N-P-K rates of 0.8-2.1-3.9 to 2.4-6.3-11.7 kg ha−1. All treatments were replicated four times. Some or all treatments increased yields in seven sites and decreased yields at two sites. Mean yield increases were 60 kg ha−1 in 1994 (not significant), 30 kg ha−1 in 1995 (not significant), 60 kg ha−1 in 1996, and 54 kg ha−1 across the 48 sites. Differences between treatments were small and inconsistent, and the single application of 28 L ha−1 (one of the lowest rates used) produced the highest mean yield increase across all responsive sites (375 kg ha−1). In 1994, the higher yield responses occurred on ridge-till and no-till fields and when the P concentration of young plants was low. No treatment caused leaf burning. Foliar fertilization seldom increased P and K concentrations of leaves at the R2 growth stage and did not affect plant maturity, grain moisture, or the weight of grains. No simple set of measurements explained the occurrence of yield responses. Results of factor and regression analyses suggested that responses tended to occur in soils with high cation exchange capacity, when plant-available P was low, and/or when rainfall in spring and midsummer was low. This group of variables explained only 14 to 23% of the responses in different years, however. Effective use of foliar fertilization of soybean at early growth stages in areas with predominantly high-testing soils requires further research to identify the conditions under which positive responses are more likely.
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