 Adding foliar nutrition should be a supplement, and that's it. Second, don't spend very much. If you're going to invest $40 in a foliar fertilizer program in the Midwest on corn or soybeans, that's probably not going to pay. However, if you're already out spraying, so your application cost is zero, and you're using a low rate of nitrogen, potassium, or maybe a blended micronutrient package for $5 or $10, that could certainly work and provide a decent payoff if your crop is actively growing and needs a little boost of fertility.