The Mechanism of Foliar Absorption of Nutrients

Plants absorb nutrients as well as other chemicals through their foliage to varying degrees. Growers in most all types of agriculture apply foliar nutritional sprays from time to time for various reasons. A basic philosophy many growers utilize is to apply what is believed to be required to the soil in the fertilization program, and use nutritional foliar supplements as a tool to give crops any nutrients they may still be lacking. Even though growers constantly use this technique as nutritional supplement, the mechanism of foliar absorption of nutrients is not well understood.
In order to understand foliar absorption, we must first take a look at the surface of a leaf. Moving from the outside. The leaf surface is composed of layers of cuticular wax, followed by the cuticle or “skin” of the leaf. The cuticle exudes the wax. Under the cuticle are the cell walls of various types of leaf cells. Inside the cell walls are the plasma membranes of the cells themselves.
A foliar applied nutrient must pass through the cuticular wax, the cuticle, the cell wall, and the membrane in that order. Sometimes the nutrient will pass through these various layers, while other times it may pass through the spaces between these layers. Such absorption involves both active and passive processes of the leaf. The second and most often the, major means of foliar absorption is through the stomates, which are microscopic pores in the epidermis of the leaf. When the stomates are open, foliar absorption is often easier. Plant species vary widely in the, number of stomates per leaf area, and in their relative distribution.
Some plants have more stomates on the lower leaf surface than on the upper and some vice versa.
In simpler terms, some plants are, good at absorbing nutrients through their leaves, while others are not. The variables tend to be how many stomates and how they are distributed, and how thick the waxy cuticle of the leaf is.
Plants with large, broad soft leaves such as Spathiphyllum or many bedding plant species are rather efficient at absorbing, foliar nutrients. Palms, Avocados, Cucurbits, some Citrus and Zamias for example are not as adept at this absorption, due to the thicker tougher nature of their foliage.
The speed of absorption of nutrients is quite variable according to the nutrient, and to some degree the plant type. Rates of foliar absorption have generally not been studied in ornamental varieties.
One Thing Not Widely Known is that Nutrients are Generally ONLY ABSORBED while the spray is wet on the Leaf!
Once the spray has dried, absorption generally ceases until the leaves are moistened again, either by the dew the next day or additional rainfall or overhead irrigation. The various types of chelating agents are also not equal in their ability to penetrate the leaf. Some chelating agents work better on some types of plants, but not necessarily as well on others. The best chelating agent will depend in part on what type of plant you are spraying.

Another Common Misconception Regards Rates of Foliar Nutritional Applications.
Generally, there is a great deal of difference between the amount of chemical it takes to maximize absorption and the amount it takes to burn. Absorption is the limiting factor, so don’t make your rates too high. You may be able to double or triple the spray rate, but it won’t necessary increase absorption. It will increase risk of spray injury, so be conservative in your foliar application rates.
There are a number of situations when foliar nutritional supplements are especially useful. One is during propagation of slow rooting plant material. Long term mist propagation can leach nutrients severely, and foliar nutritional sprays during that time are very helpful. Nutritional sprays can be used efficiently to overcome other problems.
Another useful foliar technique is during cold fronts. When a cold front comes down, frequently you get heavy rain followed by several cold days. During this period, the fertilizer is not releasing a great deal, and the plants are not feeding. That is a good time to come in and apply some foliar nutrition to keep the plants moving until things warm up.
Several Techniques should be Used When Trying to Maximize Foliar Absorption of Nutrients.
One is to try to maximize the time that the spray is wet on the foliage. This preferably means early in the morning, when humidity is up, leaves are wet with dew. Spraying in the middle of a hot day will give you reduced effectiveness in absorption. It also helps to add urea or potassium nitrate to nutritional sprays when applying trace elements.
The mechanism is not known, but there is substantial research that indicates applying these materials with trace, elements increases trace element absorption. Try to spray when the stomates are open, preferably during a cooler time of day. Some industries like to spray at night, and that can be useful in some situations.
Try also to coat both the upper and lower leaf surfaces where practical, as many times the spray stays wet on the leaf longer, and there are more stomates to facilitate absorption on the lower leaf surfaces of many plant varieties.
The use of wetting agents or surfactants also aids in absorption, by spreading out the spray from droplets into a broader shape, increasing contact with the foliage. Surfactants also reduce the angle at which the spray material enters the leaf, which can be useful. It is generally useful to thoroughly wet the foliage when applying nutritional sprays.
Low volume sprayers may not be as effective in some cases. You should spray to run off, and once again cover the lower leaf surfaces.
Finally, do not get too high on your rates. Going higher on the rates of chemicals applied can actually reduce absorption, as can mixing too many nutritionals in the tank at a time.
Foliar nutritional sprays can be a very useful technique, especially when you understand the principles behind it. Nutritional sprays enable you to correct deficiencies, strengthen weak or damaged crops, speed growth and overall grow better plants, which is of course, the bottom line.

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