Foliar feeding of plants involves the practice of applying water soluble fertilizer to their leaves. The assumption is the leaves of plants are more efficient at taking up nutrients than are the roots. This rationale dates back to research conducted at Michigan State University in the 1950’s using radioisotopes of certain essential mineral elements. It has been demonstrated that plants can take in minerals through pores in their leaf cuticle called stomata, through the leaf cuticle itself via pores (cracks) called ectodesmata and, in certain cases, through hairlike projections from the leaf epidermis called tricomes. One researcher advanced the argument that the essential mineral elements can be supplied to plants up to 20 % more efficiently via foliar feeding oppose to the soil application of fertilizer.
Not everyone in the scientific community “bought into” this assumption. It is well known that many things can happen to nutrients applied to the soil before roots have the chance to absorb them. Runoff, leaching, microbial activity, chemical precipitation, etc. all can reduce the supply of nutrients applied to the soil that are available to plants. However, after nutrients have been taken up by the roots, their translocation to other parts of the plant is much more efficient than when those same nutrients are taken in via the leaves. This is especially true for those elements (e.g. iron) which are considered to be “immobile” in the plant.
Additional research demonstrated that species vary greatly relative to their ability to take in nutrients through their leaves. Differences in cuticle thickness, stomata number and resistance as well as genetic and environmental factors all influence the ability of a species to taken in foliar-applied nutrients. If spray concentrations are increased to offset the restricted ability of a plant to take up foliar-applied nutrients, leaf burn can be a serious problem. The latter also can occur when applying the macronutrients (e.g. N, P, and K) which are needed in large amounts by the plant, making concentrated solutions a necessity.
Does this mean foliar feeding should be discouraged? The answer is “not necessarily”. Given the circumstances, foliar feeding can be helpful in managing the nutritional wellbeing of a crop, especially in correcting micronutrient deficiencies. The following are a few points to consider when contemplating initiating a foliar feeding program:
In short, foliar feeding is usually not the most cost effective method of supplying nutrients to plants. However, it has proven to be an effective method of treating certain nutrient deficiencies and (perhaps) boosting plant growth in times of stress. Growers wishing to initiate a foliar feeding program should research the subject well before proceeding.
REVISED: November 20, 2015