Foliar application is for a top up later in the season. If you think about the really efficient toxic chemical, glyphosate, only ~15% of it is absorbed by leaves.

Care must be taken in case of urea the Biuret must be less than 1.1%.otherwise the leaf and stem will burn. Some of the plant absorbed less quantity of Urea by stem and leaves. Foliar plant spray involves applying fertilizer directly to a plant’s leaves as opposed to putting it in the soil. More concentration of urea can burn the leaf and stem of cereals. Foliar feeding is similar to humans putting an aspirin under their tongue; the aspirin is more readily absorbed into the body than it would be if it were swallowed. A plant takes nutrients through the leaf much quicker than it does through the root and stem. Foliar spray of potassium 3% concentration was more efficient for increasing growth and yield of maize crop as compared to soil application and fertilization of potassium fertilizer. droplet size of spray should be small to medium so that it adheres to leaf surface/membrane, else coarse or bigger droplets will fall on ground away from leaf leading to poor efficiency.