Olympians wear high compression swimsuits that are nothing short of technological marvels. They reduce drag and allow athletes to swim through the water like fish; is it cheating? No. It is an athlete’s attempt to maximize their performance. They train, they exercise, they eat a specialized diet… every opportunity to excel is exploited. This pursuit to maximize performance is no different from a gardener’s attempt to maximize a plant’s genetic potential. We provide an ideal temperature and humidity, we optimize light and CO2 levels, we even choose a specialized diet of fertilizers and additives to ensure our plants reach their genetic potential. Yet there is one thing that many growers do not exploit; that is the ability of a plant to absorb minerals though their leaves. The foliar application of minerals can quickly correct nutrient deficiencies, prevent diseases, and increase growth rates and sugar levels in your plants.
Foliar fertilization is the spray application of mineral nutrients to a plant in order to supplement traditional media based applications of fertilizers. Most often this is achieved by mixing a particular mineral or fertilizer formulation with water and an adjuvant (something used to improve a mixture’s ability to spread across and penetrate into a plant’s tissue). Then the solution is applied to a plant’s leaves and stem in the form of an atomized mist. Foliar solutions should be applied a fine mist and not to the point where droplets are streaming off of the leaves.
The benefits of foliar fertilization were first recognized in 1844 when Eusebe Gris demonstrated that certain chlorosis (yellowing of plant tissue) could be reversed with a foliar application of iron. To best understand how we can capitalize on the practice of foliar fertilization, we must first understand the movement of nutrients into a leaf. Once the mineral solution is atomized onto a leave’s surface it must first penetrate the outer cuticle (a thin waxy coating on a leaf which prevents unintended water loss due to fluctuations in the plant’s environment). Penetration of the minerals through the cuticle is the greatest obstacle of minerals moving from outside of a leaf into the plant’s cells. Using an adjuvant or wetting agent in this case significantly improves mineral absorption. After passing through the cuticle the mineral(s) must make their way through a wall of the epidermal cells which form a boundary between the plant and the external environment (think skin cells.) Once the minerals make it through the cuticle layer and epidermal cells they enter the cell cytoplasm and nutrient absorption is similar to minerals absorbed by the roots.
Some of the most effective elements to apply via foliar spray are nitrogen, phosphorous, and potassium. The so called “macro nutrients” (elements plants use in the greatest concentration) can only be absorbed through the leaf at relatively low levels compared to a plant’s need for them. Higher concentrations of these elements applied to a leaf will cause leaf tissue damage or even death. Therefore, the application of nitrogen, phosphorous, or potassium via foliar spray should be considered supplemental to media based fertilization or corrective to treat a known deficiency.
Other elements that are commonly applied via foliar spray include zinc, copper, manganese, and iron. Plants need lower levels of these “trace” minerals; making correcting trace mineral deficiencies easy with foliar fertilization. For example, at high media pH levels iron is not bioavailable even if it is present in the media. Under high media pH conditions iron chlorosis will occur as interveinal yellowing on new leaves. For immediate treatment of the symptoms and to minimize plant stress it is easy to correct iron deficiency with a foliar application of iron sulfate or a chelated iron. Multiple applications will be required until the underlying media pH issue has been corrected.
Aside from simply correcting nutrient deficiencies the foliar application of certain elements has proven to help prevent and control plant diseases. The foliar application of silicon has been shown to reduce the incidences of powdery mildew on grape leaves. Similar reductions in powdery mildew breakouts on cucumber leaves have been demonstrated with a prophylactic foliar treatment of micronutrient solutions containing manganese chloride, copper sulfate, and boric acid.
One of the most common and effective foliar sprays that can be used by gardeners is a mixture of seaweed extracts and amino acids. Spraying seaweed extracts causes increased levels of phosphorous and potassium content in leaves whereas, the foliar application of amino acids significantly increases nitrogen levels. Spraying seaweed extracts also produced increased root mass. Foliar spraying plants with seaweed extracts combined with amino acids significantly increased sugar content in plant tissue.
The biggest benefits of foliar fertilization are the ability to quickly correct nutrient deficiencies. Additionally, foliar feeding can reduce the incidence of powdery mildew outbreaks. So, if you are looking for every advantage to maximize the health and genetic potential of your crop consider utilizing foliar feeding in your grow plan.
by David Kessler