The most commonly used method of hydroponic plant fertilization is through the nutrient solution applied to the root zone. While plant root systems are in the most part highly efficient at absorbing mineral nutrients, certain conditions can prevent optimal uptake rates of some of the elements plants require. When plants are stressed for some reason, have suffered root death or damage, are showing a nutrient deficiency or are being established from cuttings, then foliar feeding becomes a particularly useful method of supplementary nutrient application. Foliar feeding, provides nutrients through the foliage of the plant which has the ability to absorb and translocate certain minerals within plant tissues and this is a technique which is widely used. Foliar feeding however can not be used to replace root based nutrition.
In soil-less systems such as hydroponics, many nutrient interactions can occur within the root zone which makes it difficult for the plant to absorb certain minerals. Even well run hydroponic systems can become deficient in nutrients - either because of nutrient depletion, antagonism between certain elements, or due to elements becoming 'bound' and therefore unavailable for plant uptake. Imbalances in the combination of nutrients, pH levels which are too high or low for maximum plant uptake, and poor physical properties of the media surrounding the root zone, including oxygen starvation are more common in soil less systems than many growers realize. Furthermore elements such as iron, an essential trace element, can not only be prone to becoming unavailable for plant use at moderate to high pH levels, but uptake by the plant can also be severely limited under certain environmental conditions such as cool temperatures. Iron chlorosis in many crops which are overly stressed by low temperatures can be a common problem where root uptake is the only source of iron for the plant.
Any situation which damages the root system or restricts its growth, development or physical process such as respiration affects the uptake of minerals. Plant pathogens such as Fusarium Pythium and Phytophthora can not only rapidly destroy a crop, but low, less damaging levels can restrict function of the root zone to the point where mineral uptake is affected. While the crop may not show signs of severe infection, mineral and water uptake can be restricted and therefore crop yields and quality are affected. Other plant stress conditions such as anaerobic conditions in the root zone where oxygen is deficient, can limit nutrient uptake, with trace elements such as iron often affected to the greatest degree. Any other condition which stresses the plant - temperature stress, high or excessively low humidity levels, lack of light, high radiation levels, high plant densities, presence of pests or disease, will affect the ability of the root system to take up mineral elements. These conditions are common and occur in many growing systems from time to time without the grower even realizing that plant growth and mineral uptake is being limited in some way.
Foliar feeding is also beneficial where a grower wants to apply organically based nutrients or 'growth promotants' such as Veg Bloom plus and Bud Boom plus liquid add ons. Urea can also be used as part of a foliar spray, rather than as a nutrient solution addition. Adding large volumes of these types of organic products to the nutrient tank can result in nutrient imbalances, oxygen depletion, bacterial growth, equipment and filter blockages and other undesirable effects. However these can be safety diluted and applied as a foliar fertilizer on a regular basis.
Most leaves have stomata either only on the underside or on both sides of the leaf which enable gas to be exchanged for photosynthesis and respiration as well as releasing water vapour in stomata transpiration. The leaf with it's epidermis can also function as an organ that absorbs and excretes water and substances which may be dissolved in it. So foliar applied nutrients can enter the plant leaf via the stomata, provided the stomata and open at the time of application
One very important criterion of the effectiveness of nutrient sprays is the rate at which the foliar applied nutrients are absorbed by the leaves and translocated within the plant. The uptake of foliar nutrients is affected by a number of factors including environmental factors such as temperature, light, humidity, time of day and 'spray solution' factors such as concentration, application rate, application technique, wetting agent, pH, and sticking ability.
It would be difficult to ensure that all of these factors are optimal for foliar feeding at any one time, but some are more important than others. The use of a good quality, non ionic wetting or sticking agent is vital for foliar feeding. Wetting agents are necessary to ensure the adherence of droplets on difficult to wet leaves as well as assisting with the absorption of the fertilizer solution into the plant tissue.
The foliar fertilizer solution should then be applied as a fine mist until 'run off' so that the entire leaf surface (both upper and lower surface) is wetted. The time of day when the solution is applied is also important. Spraying early in the morning, when the lights have been on for an hour or so, but temperatures are still cool, or in the evening is best and conditions that allow the leaf to dry rather than stay wet for an extended length of time is also important to consider. Foliar solutions should not be applied during hot, bright conditions, if the plants are wilting or under water/osmotic stress as the plants stomata are likely to be closed making application ineffective.
Foliar feeding can by carried out on a regular, once-twice weekly basis, or can be limited to the times when the crop comes under high nutrient demand such as early fruit set and heavy fruit loading. Often the greatest response to foliar feeding will occur during the active growth phases of plants (period of exponential growth). During these active growth stages, leaves show a particularly high efficacy for absorbing nutrients.