The reason Urea is used to supply nitrogen via foliar spays, is because the time for 50% leaf absorption of the chemical is only 1/2 - 2 hours. Many chemicals take days or weeks. The best time to apply foliar fertilizers is early morning, when the plant's stomates are open.
If temperatures are projected to reach approximately 85-F or higher during the day, applications should be made just after daybreak or after the temperature drops in the evening and the relative humidity increases. Winter applications should be made at midday, when temperatures are suitable warm. Avoid spraying to runoff. Excessive liquid per tree causes pooling of the fertilizer at the tip of the leaf, which can result in tip burn. Nutrient uptake is best when the application solution is at a pH from 5.5 to 6.6 . Last note: For most cation nutrients (magnesium, potassium, zinc manganese and iron), application as a sulfate salt is equally effective as chelated products and less expensive.