This Iowa State article addresses in-season (post-emergence) fertilization of soybean, primarily discussing phosphorus and potassium; explicit micronutrient content is limited to one set of field trials. About 100 replicated Iowa trials (1994-early 2000s) tested foliar sprays of low-salt fluid fertilizers (3-18-18 and 10-10-10 N-P2O5-K2O, with or without sulfur) applied with or without the micronutrients boron (B), iron (Fe), and zinc (Zn), at rates of 2-6 gal/acre from V5 to R3, sometimes tank-mixed with glyphosate. Foliar fertilization increased yield in only about 15 percent of fields on average (range 15-30 percent depending on trial set/year), with the best treatment averaging just 0.7 bu/acre gain across all fields. Critically, adding sulfur or micronutrients (B, Fe, Zn) to the fertilizer mix did not produce higher yields than the base fertilizer alone. Responses were more likely under conditions restricting early root growth or nutrient uptake (ridge-till/no-till, cool wet soils), not from micronutrient deficiency per se. A related five-trial (2005-2006) study of foliar fertilizer plus fungicide found no consistent yield benefit from the fertilizer component (fungicide alone drove the yield gains observed). Recommendation: in-season fertilization of soybean, including micronutrient-containing foliar blends, is seldom cost-effective in Iowa; economic response is unlikely unless deficiency symptoms are visually confirmed alongside a deficient soil test, or unless soil/climatic factors (other than drought) are clearly limiting nutrient uptake early in the season. The article does not identify B, Fe, or Zn deficiency as a driver of soybean response in these trials; their inclusion in blends showed no advantage over N-P-K-only mixtures.