The most common reason for failure of plant-growth regulators, micronutrients, and foliar spraysis that growers do not apply the material at the most critical time.
Plants absorb nutrients as well as other chemicals (eg herbicides) through their foliage.
Foliar fertilization is a particularly useful technique, when you understand the principles behind it.
Foliar fertilization enables you to correct deficiencies, strengthen weak or damaged crops, speed growth and grow better plants, which is of course, the bottom line.
The mere presence of a particular chemical element in the soil is not enough. Certain soil conditions, such as pH, excess moisture, or cool temperatures, may render a nutrient or nutrients unavailable to the plant root.
One advantage of foliar fertilization is that it often brings about immediate improvement (within hours) in plant health and growth.
Foliar fertilization can be designed to meet a plants specific needs for one or more micro and macro nutrients--especially trace minerals . More recently established, however, is the fact that foliar fertilization also stimulates the plant roots themselves to become more efficient in the uptake of all nutrient requirements.
Foliars are best applied at nutritional stress periods and tend to increase the uptake of major nutrients far in excess of the input levels.
Foliars should be applied when the plant is not in water stress, neither too wet nor too dry and are best applied when the plant is cool and filled with water (turgid).
Timing is crucial as applications that are misapplied or too late in the season may not be effective.
The most critical times to apply are during periods of plant stress, which are during periods of great growth activity or when the plant is changing from a vegetative to a reproductive state.
Most foliar sprays should contain nitrogen to act as an electrolyte to carry nutrients and a small amount of phosphorous is also recommended for internal circulation.
Often, the major means of foliar absorption is through the stomata (specialized
openings in the leaves).
When the stomata are open, foliar absorption is often easier.
Several techniques should be used when trying to maximize foliar absorption of nutrients.
Nutrients are generally only absorbed while still wet on the leaf.
This preferably means spraying during a cooler time of day, early in the morning, when humidity is up and leaves are wet with dew. Spraying in the middle of a hot day will give you reduced effectiveness in absorption.
The best time to foliar fertilize is between 7am and 10am, or after 5pm, when the stomata are open.
The optimum temperature is about 22° and if the temperature is 26° or above, the spray will be less effective.
Always mix the spray thoroughly and apply in as fine a mist as possible.
Try to coat both the upper and lower leaf surfaces where practical, as often the spray stays wet on the leaf longer and there are more stomata to facilitate absorption on the lower leaf surfaces of many plant varieties.
Take care to avoid leaf burning when spraying in direct sunlight.
The use of a quality wetting agent, will prevent formation of droplets on the leaves that act as prisms for the sunlight to focus on and burn. It will also maximize the amount that will stick to the leaves and aids absorbtion.
Low volume sprayers may not be as effective.