Asbtract Foliar fertilization offers a possible means of increasing soybean (Glycine max (L.) Merr.] yields, but little is known of the utilization efficiency of foliar‐applied nutrients. Field experiments were conducted in 1976 and 1977 to determine percentage recovery of foliar applied 15N‐labeled urea by soybeans and translocation of 15N within the plant. Foliar treatments containing N, P, K, S or P, K, S were applied four times during growth stages R‐5 to R‐7. Urea, potassium polyphosphate and potassium sulfate were used to supply 84 + 9 + 28 + 5 kg/ha of N + P + K + S, respectively. The P, K, S treatment was similar except that urea was excluded. In 1976, the two foliar treatments tested on the cultivar ‘Williams’ did not result in a significant increase in seed yieId over the control value of 3,294 kg/ha. In 1977, using the cultivar ‘Kent,’ the N, P, K, S treatment resulted in a significant yield increase of 1,045 kg/ha over the control value of 3,203 kg/ha. Results from the P, K, S treatment, although consistently higher than control values, were not significantly different at the 5% level in either year. Of the total 15N applied, 44 and 67% were recovered in the plants in 1976 and 1977, respectively. Approximately 94% of the recovered 15N ultimately was found in the seed. A maximum of 0.7 kg/ ha of the fertilizer N was translocated to the roots. In a separate experiment, (15NH4)2SO4 was sidedressed at the rate of 112 kg N/ha at weekly intervals to different experimental units during late R‐4 to R‐7 stages. The uptake of 15N decreased from 39 kg/ha with the first application to 19 kg/ha with the last application, but the rate of 15N uptake did not change appreciably with time. Total N in the plants was not influenced by the (15NH4)2SO4 applications. The data indicated that changes in the rate of nutrient absorption by roots during pod‐fill were minimal and unlikely to be a major factor in determining the effectiveness of foliar fertilization.
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Vasilas et al. (1980) studied this question.