Relationships Between Foliar Applied Micronutrients, Soil Tests, and Plant Tissue Analysis in Soybean.

Prior research with foliar application of micronutrients for soybean has shown inconsistent yield responses throughout the Midwest.  This study’s objectives were to further examine soybean plant-tissue and grain yield responses to foliar application of B, Cu, Mn, and Zn in Iowa. Treatments replicated four times applied at 22 field trials in 2012 and again in 2013 were a control, each nutrient applied separately, and their mixture (six treatments) sprayed twice (at the V6 and R2/R3 growth stages). Total applied rates for B, Cu, Mn, and Zn were 180, 87, 370, and 555 g ha^-1, respectively.  Initial soil samples were analyzed for B (hot H₂O method) and for Cu, Mn, and Zn using both Mehlich-3 (M3) and DTPA methods.  Zinc and Cu measured with M3 and DTPA were correlated (r² 0.93 and 0.31) but Mn was not (r² 0.07). Ranges for B (H₂O), Cu, Mn, and Zn (DTPA) were 0.23-1.23, 0.28-1.83, 3.84-42.30, and 0.48-5.32 mg kg^-1.  Nutrient ranges in small plant (V6) samples for B, Cu, Mn, and Zn were 24-420, 8.5-15, 30-113, and 25-42 mg kg^-1.  Nutrient ranges in top mature leaves (leaflets and petioles) in control plots at the R2/R3 for B, Cu, Mn, and Zn were 27-62, 3.8-11, 26-66, and 25-47 mg kg^-1. Foliar applications at the V6 stage resulted in small leaf concentration increases at the R2/R3 stage. There were no grain yield increases (P <0.05) at any site (mean yield ranged from 1900 to 4906 kg ha^-1 across sites), but fertilization often increased the grain micronutrient concentrations. Results suggest that sufficiency ranges for these micronutrients currently suggested for soil and plant-tissue in a few states are too high for Iowa.  If available, 2013 data also will be presented on the poster.