Methods: Two Iowa State on-farm trials (2012-2014) in central Iowa evaluated B, Cu, Mn, and Zn in corn-soybean rotations on fields with no recent manure or micronutrient history (Webster silty clay loam and Clarion loam). A soil-application trial (six treatments: control; B, Mn, or Zn banded; a banded mixture; a broadcast-incorporated mixture, 4 reps) used NuBor 10 (0.5 lb B/acre banded, 2 lb/acre broadcast), Broadman20 (5 lb Mn/acre), and EZ20 (5 lb Zn/acre). A separate foliar trial (control; B, Cu, Mn, or Zn alone; a four-nutrient mixture) sprayed Max-In Boron/Copper and MicroBolt Zinc/Manganese twice per season (V5/V6 and V8/V10 for corn, R2/R3 for soybean), totaling 0.16 lb B, 0.08 lb Cu, 0.33 lb Mn, and 0.495 lb Zn/acre. Soil B was tested by hot-water method; Cu, Mn, Zn by DTPA and Mehlich-3. Grain was sampled and analyzed for micronutrient concentration.

Findings: No statistically significant corn or soybean grain yield increase occurred from any micronutrient applied to soil or foliage in any trial-year (soybean 34.7-58.0 bu/acre; corn 162-234 bu/acre range across trials), though fertilization often raised grain micronutrient concentration. Soil Zn (DTPA) ranged 0.9-6.0 ppm; Iowa's only regional interpretation (deficient below 0.9 ppm for corn) correctly predicted the lack of response. Lack of response to Cu and Mn also matched other states' interpretations, since soil levels exceeded their sufficiency thresholds. For B, only the lowest end of other states' sufficiency range (0.5-2 ppm) could apply to Iowa; using the higher end would have incorrectly predicted a response in some site-years.

Conclusion: regional soil-Zn interpretations for corn remain reliable, but B, Cu, and Mn interpretations from outside Iowa should be applied cautiously.