Grain Yield, Soil-Test, and Tissue-Test Responses to Micronutrients Applied to the Soil in Corn-Soybean Rotations.

Micronutrients are essential for corn and soybean but prior research in Iowa and the north-central region has shown inconsistent grain yield responses to fertilization. The goal of this study was to evaluate these crops yield response to the micronutrients B, Mn, and Zn and assess the value of soil and plant tissue analysis to guide fertilization. Eight three-year trials with corn-soybean rotations were evaluated in Iowa during 2012-2014 (four began with corn and four with soybean). Soil pH, clay, and organic matter (15-cm depth) were 5.5-6.2, 87-344 g kg^-1, and 36-79 g kg^-1. Six annual treatments were a control, and single applications banded below and besides the seeds or broadcasted and incorporated into the soil. Solid granulated fertilizer rates applied were 0.56 kg B ha^-1 banded and 2.24 kg B ha^-1 broadcast, 5.60 kg Mn or Zn ha^-1 for both banded and broadcast applications. Initial micronutrient soil-test across sites ranged from 0.18 to 1.16 mg B kg^-1 (hot-water method), 2.1 to 39.7 mg Mn kg^-1 (DTPA), and 0.7 to 3.2 mg Zn kg^-1 (DTPA). Tissue-test levels of B, Mn, and Zn for the non-fertilized plots across sites and years in corn ear leaf blades at the R1 growth stage were 3-11, 13-98, 11-22 mg kg^-1; and for soybean leaves at the R2-R3 stage were 23-52, 24-79, and 20-48 mg kg^-1, respectively. There was no early plant growth (V5-V6 stage) or grain yield increases or decreases (P≤0.05) due to micronutrients application at any site or year. Fertilization sometimes increased the B and Zn concentrations in vegetative and grain tissues but Mn fertilization seldom did. In conclusion, corn and soybean yield increases from B, Mn, or Zn fertilization should not be expected in conditions similar to those in this study when soil or tissue test levels are within or above the observed ranges.