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Foliar feeding is a technique of feeding plants by applying liquid fertilizer directly to their leaves. It has been known for many years that plants are able to absorb essential elements through their leaves. The absorption takes place through the stomata of the leaves and also through the epidermis. Movement of elements is usually faster through the stomata, but the total absorption may be as great through the epidermis. Plants are also able to absorb nutrients through their bark.
A popular version of the feeding is to use sea-based nutrient mixes, especially kelp, because they contain many of the fifty "trace nutrients"; the more "trace" is needed, the harder it is to balance the element within the soil. Trace elements are considered most fit for delivery by foliar feeding. Kelp also contains some hormones considered good for the cellular development of the plants' leaves, flowers, and fruit, again making foliar feeding useful to organic gardeners who eschew artificial hormone applications.
"Dr. H.B. Tukey, renowned plant researcher and head of the Michigan State University (MSU) Department of Horticulture in the 1950s, working with research colleague S. H. Wittwer at MSU, first proved conclusively that foliar feeding of plant nutrients really works. Studies were conducted using radioactive phosphorus and potassium to verify its veracity. Plant foliage was sprayed and measured with a Geiger counter to observe absorption, movement, and the utilization of plant essential nutrients. Results from the experiments quantified plant nutrients moving at the rate of about one foot per hour to all parts of the plants."
Foliar feeding is considered especially useful for introducing trace elements, or for "emergency" feeding of plants which are found to have a specific shortage. But in some cases, with tomatoes, for example, it is believed that foliar feeding during flower set causes a dramatic increase in fruit production.
Generally speaking, it is recommended that foliar feeding be done in morning or evening, since hot days cause the pores on some plants' leaves to close.
One way to tell if a fertilizer has been effective is to squeeze juice from the leaves of plant and to test it with a refractometer. If after spraying with foliar fertilizer the amount of light refracted significantly rises, then foliar application has worked. To increase effectiveness it is recommended to use a spray enhancer to help the nutrients stick to the leaf and to penetrate the waxy cuticle of the leaf.
Use in organic gardening 
Foliar feeding is sometimes used in organic gardening by those who, in their effort to "feed the soil", keeping it naturally healthy, often find themselves unable to provide trace nutrients to plants in sufficient quantity. Feeding through the leaves may allow gardeners to add such nutrients directly, without disrupting the soil.
Conventional wisdom 
In some cases, a dramatic example being tomatoes, foliar feeding goes against long-standing strictures against ever allowing the leaves to get wet, as it greatly increases the chances of disease. While the conventional wisdom is, "Don't ever spray your tomato plants; water them only by soaking the ground beneath", modern gardening techniques strongly recommend spraying the leaves of a tomato plant with fertiliser, as part of the normal fertilisation routine.