 Foliar fertilizer will not last long in the plant. Sure, you can increase nutrient levels initially, but that effect usually wears off in a week or two. In other words, foliar fertilizer can't replace a great soil fertility program. If you want a foliar feed, it's a good idea to look at soil tests and tissue tests first. If your crop starts running low on a nutrient, you can certainly foliar apply most nutrients like nitrogen, sulfur, or the micronutrients.