I’ve fielded several questions over the past few days concerning applications of foliar fertilizers as a method of spurring stressed cotton to allow it to catch cotton grown under more-optimum conditions.  A number of studies have been conducted in the mid-south on this topic.  Still, the quantity of data on the subject pales in comparison to the numbers (and strengths!) of opinions on foliar fertilization.  A few things have been brought to light over the years and should be mentioned here.

1. Applications are most efficient (i.e. greatest amounts of applied nutrients move into the plant) when cotton has not undergone periods of stress.  This includes, but is not limited to cool/flooded soils, damaged leaves, drought, etc.
2. Efficiency of the application will be greatest when leaf area of the plant is sufficiently large.  Surface area of the leaves and ability to cover these leaves with the product can be as important as rate or source.
3. Foliar feeding works best when a deficiency is present.  Water-logged cotton may be deficient in a given nutrient, but that nutrient may very well be present in sufficient quantities in the soil.  Therefore, if water content of soil drops more near field capacity cotton root growth will most likely resume and the deficiency may be naturally ameliorated.  One caveat here concerning N- water-logged soils and excessive rains can increase the amount of N loss.  If N deficiencies are present after normal conditions have resumed, a true N deficiency may be present.  Under these circumstances a soil-applied, side-dress N application will be preferred, followed by monitoring and possibly foliar N during boll-fill.
4. The best-timed foliar application will be when the demand for N is largest.  Peak N demand occurs during the boll-fill period, when root growth is slowing.  Demand for N earlier during the season (even near first week of flower) is not as large.
5. Foliar burn can occur if rates in excess of 5-7 lb N (~10-15 lb Urea).  Preferably, applications would be applied early in the morning or late evening (but not mid-day) under overcast conditions to reduce burn and increase uptake.

Additional information on this subject can be found in Physiology Today or a more recent review of this topic by Dr. Oosterhuis.