Limited dosage – Nutrients applied in foliar application cannot meet the entire nutrient requirements of the crop.
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Author: Mr. Guy Sela, agronomist, expert in plant nutrition and irrigation.
The efficiency of nutrient uptake is considered to be 8-9 folds higher when nutrients are applied to the leaves, when compared with nutrients applied to soil.
In specific growth stages – Plants require different amounts of nutrients in different growth stages. It is sometimes difficult to control the nutrient balance in soil. Foliar applications of essential nutrients during key stages can improve yield and quality.
- pH affects the charge of the cuticle (a waxy layer covering the leaves) and therefore its selectivity to ions;
- The ionic form of nutrients is pH dependent, and therefore pH can affect the penetration rate;
- pH might affect the phytotoxicity of the sprayed compounds.
We can conclude that pH of the spray solution must be adjusted according to the applied nutrient.
- Use of surfactants – Surfactants contribute to a more uniform coverage of the foliage. They increase the retention of the spray solution by reducing the surface tension of the droplets.
- Time of the day – the best time to foliar feed is early morning or late evening, when the stomata are open. Foliar feeding is not recommended when temperature exceeds 80°F ( 27° C).
- Droplet size – Smaller droplets cover a larger area and increase efficiency of foliar applications. However, when droplets are too small (less than 100 microns), a drift might occur.
Under certain conditions, foliar feeding has an advantage over soil applications.
- Spray volume – Spray volume has a significant effect on the nutrient absorption efficiency. Spray volume must be such that it is sufficient to fully cover the plant canopy, but not too high so it does not run off the leaves.
In addition, pH affects foliar absorption of nutrients in three other ways:
11 facts about foliar-sprays you must know before your next foliar application.
Foliar feeding is generally done in the early morning or late evening, preferably at temperatures below 75 degrees Fahrenheit (24 degrees Celsius), since heat causes the pores on some species’ leaves to close.
The absorption takes place through their stomata and their epidermis. Transport is usually faster through the stomata, but total absorption may be as great through the epidermis. Plants are also able to absorb nutrients through their bark.
A foliar application can refer to either foliar feeding or foliar pesticide application. If a pesticide or fertilizer says that you need to make a foliar application, you are being advised to put it directly on the foliage. Leaves can absorb some nutrients and chemicals through the pores on their surface.
Fruit trees can be treated by foliar application for an infestation of spider mites during the summer. Consider horticultural oil, which is an organic acaricide. It is always a good idea to aim to use chemicals as little as possible unless necessary. Look for biological and mechanical controls when available.
The purpose of foliar feeding is not to replace soil fertilization. Supplying a plant’s major nutrient needs (nitrogen, phosphorus, and potassium) is the most effective and economic via soil application. However, a foliar application has proven to be an excellent method of supplying plant requirements for secondary nutrients (calcium, magnesium, and sulfur) and micronutrients (zinc, manganese, iron, copper, boron, and molybdenum), while supplementing N-P-K needs for short or critical growth three-stage periods. However, a foliar application has been shown to avoid the problem of leaching-out in soils and prompts a quick reaction in the plant.
For all landscape and garden plants, the major pathway for nutrient uptake is by way of the roots. Leaves have a waxy cuticle, which restricts the entry of water, nutrients, and other substances into the plant. To a limited extent, nutrients applied to leaves can be absorbed and used by the plant, but for the major nutrients (nitrogen, phosphorus, and potassium) the quantity absorbed at any one time is small relative to plant needs. That means that foliar application of these three nutrients can only supply a tiny fraction of the total needed by the plant, so a foliar application should be considered a supplement to regular soil application of these nutrients.
Foliar feeding has been widely used and accepted as an essential part of crop production, especially on horticultural crops. Although not as widespread on agronomic crops, the benefits of foliar feeding have been well documented and increasing efforts, have been made to achieve consistent responses. For instance, foliar feeding was earlier thought to damage tomatoes but has become standard practice.
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Foliar feeding has been documented as early as 1844 when an iron sulfate solution was sprayed as a possible remedy for “chlorosis sickness.”
If a pesticide or fertilizer says that you need to make a foliar application, you are being advised to put it directly on the foliage.