Foliar Feeding Revisited

Published: April 16, 2019

Foliar feeding has been the subject of much debate in recent years. The practice involves applying water-based fertilizers to the leaves of plants to enhance their nutritional status. Claims of yield increases up to 20 percent have been made based on the assumption that leaves are more efficient at taking up nutrients than are roots. This rationale dates back to research conducted at Michigan State University in the 1950's using radio-isotopes of certain essential mineral elements.

Opponents of foliar feeding are quick to point out that the leaves of plants are not designed to take up nutrients. The roots are. It has been estimated that only 15 to 20 percent of the nutrients applied to leaves actually are taken in. Additionally, the translocation of nutrients to other parts of the plant is much less efficient when taken in through the leaves versus when absorbed by the roots.

The mode-of-entry of essential elements into the leaves when applied as a foliar fertilizer still is under debate. Conventional theory is that foliar feeding is effective because plants can take in essential minerals in liquid form through pores in their leaf cuticle called stomata. The latter serve as points-of-entry for air laden with carbon dioxide used by the plant during photosynthesis.

Research has revealed, however, nutrients are more likely to be taken in through the leaf cuticle. The latter contains a pathway of extremely minute pores (< 1 nm in diameter) with a density of about ten billion pores per cm³ of leaf surface area. These micro-pores are lined with negative charges which tend to attract (when in ionic form) positively charged elements such as calcium (Ca⁺⁺), magnesium (Mg⁺⁺), potassium (K⁺), ammonium-form nitrogen (NH⁺), etc. Movement through the cuticle is dependent upon a number of factors including, nutrient concentration, molecule size, organic vs. inorganic, etc.

Conversely, negatively charged essential elements in ionic form such as phosphorous (HPO₄^-2), sulfur (SO₄^-2) and nitrate-form nitrogen (NO₃^-1) find leaf entry through the cuticle more challenging. Whereas opposite charges attract, like charges repel one another.

As alluded to above, another consideration when foliar feeding is the fate of the nutrients after they enter the leaf. Smaller molecules or those with a lesser positive charge are more readily transported in the vascular system where they are translocated to other parts of the plant. Examples of the latter include ammonium (NH⁺), potassium (K⁺) and urea (NH₂CONH₂).

On the other hand, larger molecules and ions with greater positive charges tend to stay fairly close to their point-of-entry as they adhere to the negatively-charged cell walls. Examples of fairly tightly held (immobile) nutrients include calcium (Ca⁺⁺), iron (Fe⁺⁺), manganese (Mn⁺⁺), zinc (Zn⁺⁺), and copper (Cu⁺⁺).

Therefore, when applied as foliar fertilizers, elements with strong positive charges such as calcium do not move much upon entering the leaf. Accordingly, elements such as phosphorous which are negatively charged are slow to enter the leaf. Both are relatively immobile after gaining entry.

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 macro-nutrients (e.g. N, P, and K) which are needed in large amounts by the plant, making concentrated solutions a necessity.

If practiced, foliar feeding should be considered a supplement to a sound, well-designed soil fertility program, and not a substitute for the latter. However, given proper circumstances, foliar feeding can be helpful in managing the nutritional well-being of a crop, especially when it comes to correcting micro-nutrient deficiencies.

The following is a list of plant nutrients and rates that are effective as foliar applications, according to Dr. Gordon Johnson, Extension Vegetable & Fruit Specialist at the University of Delaware in his publication titled Foliar Fertilization of Vegetable Crops:

• Foliar applications of nitrogen (N) can benefit most vegetables if the plant is low in N. Urea forms of N are the most effective; methylene urea (urea formaldehyde) and triazoles (C₂H₃N₃) are effective with less injury potential; and ammonium sulfate is also effective. Recommended rates are 1 to 10 lbs./acre.
• Foliar potassium (K) is used on fruiting vegetables such as tomatoes and melons. Best sources are potassium sulfate or potassium nitrate. Recommended rate is 4 lbs./acre of K.
• Foliar magnesium (Mg) is used on tomatoes, melons, and beans commonly. The best source is magnesium sulfate and recommended rates are 0.5 to 2 lbs./acre of Mg.
• Foliar calcium is often recommended, but because it moves very little, it must be applied at proper growth stages to be effective. For example, for reducing blossom-end rot in tomato or pepper fruits, foliar calcium must be applied when fruits are very small. Best sources for foliar calcium are calcium nitrate (10 to 15 lbs./acre), calcium chloride (5 to 8 lbs./acre) and some chelated Ca products (manufacturers recommendations).
• Iron (Fe), manganese (Mn), or zinc (Zn) are best applied as foliar feeds in as sulfate forms. Rates are: Fe, Mn, 1 to 2 lbs./acre, and Zn ¼ lb./acre. While these metal micronutrients are not mobile, foliar applications are very effective at correcting local deficiencies in leaves.
• The other micronutrient that can be effective as a foliar application is boron. Boron in the Solubor® form is often recommended at 0.1 to 0.25 lbs./acre for mustard family crops such as cabbage as a foliar application. Boron is very toxic to plants if applied in excess so applying at correct rates is critical.

(Please note: an average greenhouse or high tunnel is about 3,000 sq ft which constitutes about 0.06 acres of cropped area. Conveniently one ounce is about 0.06 pounds, thus for each pound per acre, it would be one ounce for 3,000 sq ft)

Additional points to consider when foliar feeding crops include:

• Foliar feeding the micro-nutrients appears to be much more effective than the macro-nutrients.
• If practiced, foliar feeding should be done when the air is relatively cool (around 72°F). For most summer crops this dictates early morning or late evening application.
• Do not apply foliar fertilizer to the point that droplets form on leaf surfaces—this encourages leaf burn.
• It is better to apply weaker concentrations of foliar feed more frequently than stronger concentrations less frequently.
• As with any new cultural practice, initiate foliar feeding on a trial basis first.

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.