Methods: Two Iowa State on-farm trials (2012-2014) at the Armstrong/Neely-Kinyon farms in southwest Iowa evaluated B, Cu, Mn, and Zn on a Marshall silt loam with no recent manure or micronutrient use. A soil-application trial (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 foliar trial (control; B, Cu, Mn, or Zn alone; a four-nutrient mixture) sprayed Max-In B/Cu and MicroBolt Zn/Mn twice per season (V5/V6, plus V8/V10 for corn or 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. Crop sequence was soybean(2012)-corn(2013)-soybean(2014).

Findings: No statistically significant corn or soybean yield increase occurred from B, Cu, Mn, or Zn applied to soil or foliage in any trial-year, despite yields ranging normal to very high (soybean 38-67 bu/acre; corn 198-220 bu/acre); fertilization often increased grain micronutrient concentration without a yield benefit. Soil Zn (DTPA) for non-fertilized plots ranged 0.5-1.7 ppm (marginal to adequate by Iowa's corn-only interpretation of <0.9 ppm deficient), consistent with the lack of response. Lack of Cu and Mn response agreed with other states' higher sufficiency thresholds, since observed soil levels exceeded them. For B, other states' interpretations predicted a yield increase in some years/trials that was not observed.

Conclusion: Iowa's Zn interpretation for corn remains reliable in this soil, but B, Cu, and Mn recommendations imported from other states did not reliably predict yield response at this southwest Iowa site.