Methods: Iowa State University researchers (Mallarino et al., 2011 ICM Conference) measured micronutrient (boron, manganese, zinc) concentrations in corn and soybean grain across long-term Iowa research-farm trials (multiple counties, no-till and chisel-plow/disk tillage, several P/K fertilizer treatments, spanning years/sites) as part of a broader P/K removal and residue-recycling study. Grain samples were analyzed for nutrient concentration, and micronutrient removal was calculated from concentration × yield across a wide range of corn and soybean grain yields.

Findings: Micronutrient uptake by both crops is very small, typically under 1% of P or K uptake, so grain concentrations and removal amounts are correspondingly minor. Across all fields, years, and treatments, there was no relationship between grain yield level and B, Mn, or Zn concentration in either crop, though boron showed unusually wide variation among high-yielding corn fields (removal ranging from near zero to about 0.08 lb/acre). In corn, removal of B, Mn, and Zn increased with yield in a slight exponential pattern (steeper increase at higher yields) rather than the linear pattern seen for P and K. In soybean, micronutrient concentrations in grain were several times higher than in corn, but removal-yield relationships were linear (not exponential) and more variable than in corn; zinc removal by soybean was roughly double that of corn, while boron and manganese removal were only slightly higher than corn. The authors concluded that because removed amounts are insignificant relative to soil reserves and to P/K removal, micronutrient removal should not be used as a fertilization-decision criterion the way it is for P and K, and flagged a need for further research on how removal affects soil/tissue micronutrient levels and fertilizer requirements over time.