Researchers at Iowa State summarized 112 field trials conducted in Iowa between 1994-2001 on the effect of fertilization on soybean protein and oil composition. Forty-two trials evaluated foliar fertilization of (N-P-K mixtures with or without S, B, Fe and Zn) applied at the V5-V8 growth stage. Seventy trials compared preplant application of P or K fertilizers. All trials were replicated with agronomic data and soil test data recorded. Results indicated that fertilization had infrequent, inconsistent and small (positive or negative) effects on soybean protein and oil levels. The researchers demonstrated that the total protein and oil levels per acre were related to yield; the higher yields resulted in more protein and oil harvested per land unit. The researchers concluded, based on these studies, farmers cannot consistently improve the levels of protein and oil in soybeans by fertility treatments; and increasing soybean yields will not result in significant changes in soybean composition. Comment: The practical aspects of this study were to show that fertility management practices used by soybean growers are not effective in increasing the protein and oil content of soybeans. The large year-to-year variation in growing conditions, soybean cultivars planted, and soil types in the field studies would indicate the results of the study would have application to large areas of the North Central Region.
Haq, M.U. and A.P. Mallarino. 2005. Response of soybean grain oil and protein concentrations to foliar and soil fertilization. Agronomy J. 97(3): 910-918.