We expect plants to take in nutrients through the roots, but foliar uptake is a useful technique especially for correcting deficiencies or as a regular supplement of key elements to aid crop growth.
Foliar application means precise targeting. The main methods of entry are through the stomata, which are normally found on the underside of the leaves, or through the cuticle on the upper surface.
Environmental conditions will affect when the stomata are open or closed, so ideally application of foliar feeds should be made during periods of high humidity and not when sunlight levels and temperatures are high.
Penetration through the cuticle may also be difficult due to its waxy nature, although research has demonstrated that absorption is helped by the use of adjuvants, wetting the whole surface rather than depositing large droplets and by increasing the drying time.
Most soluble nutrients can be absorbed through the leaves but macro nutrients are generally required in substantial quantities and uptake through the leaves is likely to be insufficient. However, foliar feeding with macro nutrients can prove to be particularly beneficial in situations where the root system is under-functioning.
Note that soluble NPK fertilisers applied to foliage may result in residues on the leaves, especially in hard-water areas.