Plants require inorganic nutrients in addition to carbon dioxide and water for growth and production. Nutrients are present in soil, but get depleted unless supplied through fertilization. Soil feeding is the normal practice, but has limitations with respect to its availability to the plants. The elements such as phosphorus, potassium, and most of the micronutrients are fixed in the soil complex, while the more soluble nutrients such as nitrogen are easily leached down the soil. What is lost through leaching reaches the aquifer and pollutes the groundwater. For instance nitrates and phosphates can be harmful to humans. With increasing costs of fossil fuel, which provides the raw materials for fertilizer manufacture, there is a need to find innovations in fertilizer usage techniques. Foliar application is one such technique. Here I review the extensive work that has been carried out on the effectiveness of foliar-applied nutrients, the mechanisms of foliar absorption, and transport. The leaf components such as the cuticular membranes, the trichomes, the cuticular pores, ectoteichodes, their properties, and their role in the nutrient transport into the plant leaf are reviewed. Cuticles are permeable to nutrient ions present in aqueous forms and have distinct structures like pores. But it is not known if these pores facilitate easy entry into the leaf cells. The trichomes increase the amount transported into the leaf by providing more area for absorption. The cuticles have two types of lipophilic substances, the cutin and the cuticular wax, which influence the permeability of nutrient ions to varying degrees. It is clear that nutrients reach the leaf cells, after penetrating the cuticle, and are further transported to other parts through plasmadesmata. Some micronutrients are not as freely mobile as the major nutrient elements such as N, P, or K. The age of the leaf and the pH of the spray liquid are important for foliar absorption. The absence of plasmadesmatic connections between the guard cells and the epidermal cells is also important. One element Cl has been found to be transported from the applied leaf to other parts rapidly, showing it is freely mobile. This should be true for many anions. The concept of limiting factors and the law of the maximum proposed by Wallace group are useful in raising the yield plateau, and when soil supply poses the “limiting” factor, foliar feeding will help increase the crop yield. Modern technique of sprinkler irrigation system can be exploited to supply the nutrient elements in the irrigation water, which will be economical in foliar fertilization. Foliar nutrition is very practical to correct micronutrient deficiencies, which are very important for maximizing the yield. Crop breeders could also help evolve cultivars, which give good response to foliar feeding.
Critical growth stage cuticle eutrophication inorganic nutrients leaf uptake sprinkler irrigation