There has been little research with foliar application of micronutrients for soybean [Glycine max (L.) Merr.] in the Corn Belt, and results have been inconsistent. This study's objectives were to (1) evaluate the soybean grain yield response to foliar application of B, Cu, Mn, and Zn; and (2) study relationships between soil and plant-tissue test results for these micronutrients. Forty-two field trials were conducted in Iowa during 2012 and 2013 that involved 23 soil series. Six treatments replicated four times were a control, each nutrient applied separately, and their mixture sprayed twice (at the V6 and R2/R3 growth stages) (Fehr et al., 1971). Total applied rates of B, Cu, Mn, and Zn were 180, 87, 370, and 555 g ha-1, respectively. Micronutrient concentrations in the soil were analyzed in dried or undried samples. Results for the common dry sample handling procedure for B (15-cm depth, hot-water method) were 0.23 to 1.66 mg kg-1; for Cu, Mn, and Zn were 1.6 to 4.2, 31.5 to 128, and 1.2 to 11 mg kg-1 by Mehlich-3 (M3), and 0.28 to 1.83, 3.8 to 42.3, and 0.48 to 15.1 mg kg-1 by the diethylenetriamine-pentaacetic acid (DTPA) method. Total B, Cu, Mn, and Zn in plants at the V5-V6 stage were 24.3 to 41.5, 5.3 to 15.0, 29 to 113, and 24.5 to 47.8 mg kg-1; and in leaves at the R2-R3 stage were 27 to 62.3, 3.8 to 11, 26 to 88, and 17.8 to 47.3 mg kg-1. Fertilization did not increase grain yield at any site (P ≤ 0.05), the mixture decreased average yield across sites, and with the exception of Mn often increased grain concentration. Except for a good curvilinear relationship between soil Zn by Mehlich-3 and DTPA methods (R2 0.94), all other relationships among micronutrients in soil, plants, and leaves were poorer and often not significant. No yield response-based soil or plant-tissue concentrations could be identified given the lack of yield responses, but results suggest that the lowest observed soil and tissue test concentrations would be adequate for soybean in conditions similar to those in this study.