Methods: On-farm trial at Iowa State's Muscatine Island farm on a Toolesboro sandy loam (pH 6.2, 3.6% organic matter), with corn in 2012 and 2014 and soybean in 2013, no prior manure or micronutrient history. Six treatments (control; B, Mn, or Zn banded; a banded mixture; a broadcast-incorporated mixture) were replicated four times on the same plots each year. Rates: boron (NuBor 10) 0.5 lb B/acre banded or 2 lb/acre broadcast; manganese (Broadman20) 5 lb Mn/acre; zinc (EZ20) 5 lb Zn/acre, both banded and broadcast. Non-limiting P, K, and S were applied uniformly. Soil B was tested by hot-water extraction, Mn and Zn by DTPA and Mehlich-3. Corn ear leaves were sampled at silking (R1) and whole plants at V5-V6; soybean trifoliates were sampled at V5-V6 and R2-R3.

Findings: No statistically significant corn or soybean grain yield response occurred from B, Mn, or Zn in any year (corn 185-234 bu/acre; soybean 54-58 bu/acre). Fertilization increased corn tissue B and Zn concentrations at V5-V6 but rarely at R1, and rarely raised soybean tissue concentrations at either stage, attributed to dilution from growth. Initial soil Zn (DTPA) ranged 1.2-2.3 ppm, above Iowa State's 0.9 ppm deficiency threshold for corn (the only regional soil-test interpretation available), correctly predicting no Zn response. Applying other states' higher B, Mn, or Zn soil/tissue sufficiency thresholds to this site would have incorrectly predicted yield increases for several nutrient-crop combinations. The authors concluded there was no economic yield benefit to B, Mn, or Zn application on this sandy soil, reinforcing that Iowa-specific Zn interpretations were reliable while imported out-of-state thresholds were not.