This Iowa State Extension presentation (J.E. Sawyer) reviews micronutrient issues for corn and soybean, focused on zinc (Zn) and iron (Fe), plus context on sulfur research methodology. For corn, Zn deficiency risk factors are low organic matter, high pH (>7.4), eroded/coarse-textured soils, high P with borderline Zn, and cool/wet soils. ISU Zn recommendations for corn (DTPA-extractable Zn): Low (0-0.4 ppm) = 10 lb Zn/acre broadcast or 2 lb/acre banded; Marginal (0.5-0.8 ppm) = 5 lb broadcast or 1 lb banded; Adequate (0.9+ ppm) = none. However, prior ISU trials (Webb 1970s-90s; Bickel & Killorn 2007) found inconsistent yield responses to Zn even at low soil-test levels. For soybean, iron deficiency chlorosis (IDC) is linked to high pH (>7.4), free calcium carbonates, high salts, poor aeration, high soil nitrate, and wet conditions. Management: choose IDC-tolerant varieties; soil-applied Fe, sulfur, or gypsum are not effective; seed-coated iron is expensive with limited response; in-furrow chelated FeEDDHA may help; reducing soil nitrate (e.g., interseeding soybean into a cover crop) may help. Foliar FeEDDHA (Sequestrene 138) can be applied at the two-trifoliate stage, within 7 days of chlorosis symptoms, at 0.15 lb Fe plus surfactant in 15-30 gal water/acre, possibly requiring multiple applications. A 2012-2014 ISU research program (46 soybean trials, 10 corn trials; foliar B/Cu/Mn/Zn; 26 additional strip trials; 8 soil-applied trials) found very unlikely yield response to micronutrients in Iowa: no yield increase in foliar plot trials (one soybean decrease from Cu/Zn/mixture), mixed results in strip trials (one soybean increase, one corn decrease), and no effect from soil application. Conclusions: published soil/tissue sufficiency levels are likely too high, soil and tissue tests show little agreement, and yield level is a poor predictor of micronutrient need. Overall recommendation: don't rely heavily on published test interpretations (use lowest suggested sufficiency values), lime acid soils, and watch sandy, eroded, or calcareous fields for Zn/Fe issues.