Using Plant Tissue and Soil Testing to Predict Corn and Soybean Yield Response to Foliar Applied Micronutrients in Iowa.

Well documented micronutrients deficiencies for corn and soybean are rare in the western Corn Belt, but several crop consultants are using published soil or plant-tissue sufficiency levels that may, or may not, apply to this region. A study was conducted during 2012 and 2013 in Iowa to evaluate the corn and soybean grain yield response to foliar-applied micronutrients and study relationships among soil-test results, plant-tissue test results, and grain yield response. Forty-two trials with soybean and 14 trials with corn assessed foliar applications of B, Cu, Mn, and Zn alone or in mixture sprayed twice at the V5-V6 and R2-R3 growth stages. The trials encompassed 23 soil series, some also found in areas of neighboring states. Initial soil-test values (15-cm depth) were for B 0.23 to 1.7 mg kg^-1 (hot-water method); for Cu, Mn, and Zn were 2-4, 32-128, and 1-11 mg kg^-1 by Mehlich-3 (M3) and 0.3-1.8, 4-42, and 0.5-15 mg kg^-1 by the DTPA method. In soybean, total B, Cu, Mn, and Zn in plants at the V5-V6 stage were 24-42, 5-15, 29-113, and 25-48 mg kg^-1; and in leaves at the R2-R3 stage were 27-62, 4-11, 26-88, and 18-47 mg kg^-1. In corn, total B, Cu, Mn, and Zn in plants at the V5-V6 stage were 5-12, 7-21, 67-102, and 32-48 mg kg^-1; and in ear leaves at the R1 stage were 3-14, 7-11, 23-65, and 14-33 mg kg^-1. Fertilization did not increase grain yield at any site (P ≤ 0.05), and the mixture decreased average soybean yield across all sites. However, fertilization often increased grain micronutrient concentrations. Except for a good curvilinear relationship between soil Zn by M3 and DTPA methods (R² 0.94), all other relationships among micronutrients in soil and plant-tissue were not significant or very poor. Critical concentrations could not be identified given the lack of yield response, but results suggest that the lowest range of published soil and tissue test sufficiency levels would be adequate in conditions similar to those in this study.