Consider foliar fertilization carefully, and don't expect an economic yield increase in most fields. The probability of a yield response increases when preplant fertilization rates were lower than recommended; there are nutrient deficiency symptoms; or soil or climatic factors (other than drought) limit nutrient uptake in late spring or early summer.
Plant tissue testing together with soil testing for areas within a field that show deficiency symptoms or poor growth compared with unaffected areas may be useful to make decisions about foliar fertilization. You must be aware of possibly misleading results, however, because several growth factors can increase or reduce the nutrient concentration in tissue.
Although there are no clear rules, avoid high rates of N, K or S because may cause leaf burn and a yield decrease. Also avoid spraying early or during the day in days with prognosticated high temperature (temperatures below about 80-85° should be safe mainly for low-salt products), and do it in the evening instead.