Leaf dressing

Foliar feeding of plants

In recent years, foliar feeding has been the subject of much debate. The practice includes applying water-based fertilizers to plant leaves to enhance their nutritional status. Claims to increase yields of up to 20 percent are based on the assumption that the leaves are more efficient at absorbing nutrients than the roots. This rationale goes back to research done at Michigan State University in the 1950s using radioisotopes of some of the main mineral elements.
Opponents of foliar feeding immediately point out that plant leaves are not designed to absorb nutrients. There are roots. It has been estimated that only 15-20 percent of the nutrients applied to the leaves are actually absorbed. In addition, transporting nutrients to other parts of the plant is much less efficient when they enter through the leaves as compared to when they are absorbed by the roots.
The way in which the main elements enter the leaves when applied as foliar fertilization is still under discussion. The traditional theory is that foliar feeding is effective because plants can receive the minerals they need in liquid form through pores in the cuticle of the leaf called stomata. The stomata serve as an entry point for air saturated with carbon dioxide used by the plant during photosynthesis.
However, studies have shown that nutrients are more likely to enter through the cuticle of the leaf. The latter contains a pathway of extremely small pores (<1 nm in diameter) with a density of about ten billion pores per cm3 of leaf surface area. These micropores are lined with negative charges that tend to attract (in ionic form) positively charged elements such as calcium (Ca ++), magnesium (Mg ++), potassium (K +), nitrogen in ammonia form (NH +), and D. through the cuticle depends on a number of factors, including nutrient concentration, molecule size, organic or inorganic substances, etc.
Conversely, negatively charged important elements in ionic form, such as phosphorus (HPO4-2), sulfur (SO4-2), and nitrogen in nitrate form (NO3-1), make it difficult for leaves to penetrate the cuticle. While opposite charges attract, similar charges repel each other.
As mentioned above, another consideration when foliar feeding is the fate of the nutrients after they enter the leaf. Smaller molecules or molecules with a less positive charge are more easily transported in the vascular system, from where they move 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 a higher positive charge tend to stay fairly close to their entry point as they stick to 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 a foliar fertilizer, elements with a strong positive charge, such as calcium, do not move much when hitting the foliage. Accordingly, negatively charged elements such as phosphorus slowly enter the leaf. Both are relatively motionless upon entry.
Additional research has shown that species vary greatly in their ability to absorb nutrients through their leaves. Differences in cuticle thickness, stomata, and resistance, as well as genetic and environmental factors, all affect the ability of a species to absorb nutrients applied to its leaves. If the spray concentration is increased to compensate for the plant’s limited ability to absorb nutrients applied to the leaves, leaf burn can become a serious problem. The latter can also occur with the introduction of macronutrients (for example, N, P, and K), which are necessary for the plant in large quantities, which makes the need for concentrated solutions.
If practiced, the foliar dressing should be seen as an addition to a well-designed soil fertility program, not a substitute for it. However, under certain circumstances, foliar feeding can be beneficial in maintaining the nutritional well-being of crops, especially when it comes to correcting micronutrient deficiencies.
According to Dr. Gordon Johnson, a vegetable and fruit specialist at the University of Delaware, in his publication, Foliar Nutrition of Vegetables, the following is a list of nutrients and norms that are effective in foliar nutrition.
Foliar application of nitrogen (N) can benefit most vegetables if the plant is low in N. The forms of urea nitrogen are most effective; methylene urea (urea-formaldehyde) and triazoles (C2H3N3) are effective with less potential for damage, and ammonium sulfate is also effective.
Foliar potassium (K) is used for fruiting vegetables such as tomatoes and melons. The best sources are potassium sulfate or potassium nitrate.
Foliar magnesium (Mg) is commonly used to process tomatoes, melons, and beans. The best source is magnesium sulfate.
Foliar calcium is often recommended, but because it moves very little, it must be applied at appropriate growth stages for it to be effective. For example, to reduce the rot of tomato or pepper fruit, it is necessary to apply calcium to the foliage when the fruit is very small. The best sources of calcium for foliage are calcium nitrate, calcium chloride, and some calcium chelated foods.
Iron (Fe), manganese (Mn), or zinc (Zn) are best applied as a foliar feed in the form of sulfates. Although these metallic trace elements are not mobile, foliar dressing is very effective in correcting local deficiencies in the leaves.
Another trace mineral that can be effective in foliar feeding is boron. Boron is often recommended for mustard crops such as cabbage for foliar application. Boron is very toxic to plants if applied in excess, so correct application is critical.

Additional points to consider when foliar feeding crops include:

Foliar feeding with micronutrients appears to be much more effective than macronutrients.
If practiced, foliar feeding should be done in relatively cool air. For most spring crops, this requires early morning or late evening application.
Do not apply foliar fertilizer until droplets form on the surface of the leaves – this will burn the leaves.
It is better to use weaker concentrations of foliar dressing more often than stronger concentrations less often.
As with any new cultural practice, first, initiate foliar feeding on a trial basis.
In short, foliar feeding is usually not the most cost-effective method of providing plants with nutrients. However, it has proven to be an effective method of treating certain nutrient deficiencies and (possibly) boosting plant growth during times of stress. Growers wishing to start a foliar feeding program should do a good job of research before proceeding.