Foliar Fertilization: Mechanisms and Magnitude of Nutrient Uptake

Foliar fertilization is a widely used method used to supplement soil applications to improve the yield and quality of field crops. However, questions and uncertainty surround this practice. Numerous field trials have clearly demonstrated the uptake of foliar-applied nutrients by leaves and subsequent translocation to the fruit. In cotton, foliar-applied N was rapidly absorbed by the leaf (30% within one hour) to which it was applied and translocated into the closest boll within 6 to 48 hours after application. The uptake of foliar-applied fertilizers highest in the early morning and late afternoon, and lowest at midday. Water deficit increases the amount of cuticular wax and changes the composition to longer-chain more hydrophobic waxes, and significantly reduces absorption of the foliar-applied nutrient. The cuticle represents the main barrier to absorption of foliar fertilizers. It is assumed that all liquid uptake of water and dissolved substances occurs exclusively through the leaf cuticle, and not through the stomates. There are two pathways by which exogenous chemicals may traverse the distance from the leaf surface into the symplast; a lipoidal route and an aqueous pathway. Compounds that penetrate the cuticle in the lipoidal-soluble form do so principally in the non-polar, undissociated form, whereas compounds that enter via the aqueous route move in slowly, and their penetration is greatly benefitted by a saturated atmosphere. Uptake of nutrient elements via the cuticle depends on whether the element it is in inorganic form or combined in an organic form, its ionic concentration, and on the existing environmental conditions which influence how long the nutrient remains in solution on the leaf. The cuticular layer can also function as a weak cation exchanger attributable to the negative charge of the pectic material and nonesterified cutin polymers. Foliar Fertilizer has the advantages of low cost and a quick plant response, and it is particularly important when soil problems occur and root growth is inadequate. On the other hand, it has disadvantages of possible foliar burn, solubility problems, and only a small amount of the nutrient can be applied at any one time. Variable yield responses to foliar fertilization have been reported. These are probably associated with incorrect timing of applications, the use of inappropriate fertilizer materials, and insufficient attention to soil available nutrients, the size of the fruit load, and environmental conditions. The efficiency of foliar fertilization can be influenced by the type of fertilizer, concentration and pH of the solution, the use of adjuvants, and compatibility with other agrochemicals. Attention also needs to be given to the ideal method and timing for incorporation of foliar fertilization into existing production practices.