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Foliar feeding is a technique of feeding plants by applying liquid fertilizer directly to their leaves. Plants are able to absorb essential elements through their leaves. The absorption takes place through their stomata and also through 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.
Foliar feeding was earlier thought to damage tomatoes, but has become standard practice.
A popular version of the feeding is to use sea-based plant mixes, especially kelp, which contains many of the fifty "trace nutrients"; the more such nutrients are needed, the harder it is to balance them within the soil. Kelp also contains some hormones considered good for the development of the plants' leaves, flowers and fruit, of interest to organic gardeners who reject artificial hormone applications.
H.B. Tukey was head of Michigan State University (MSU) Department of Horticulture in the 1950s. Working with S. H. Wittwer, they proved conclusively that foliar feeding is effective. Radioactive phosphorus and potassium were applied to foliage. A Geiger counter was used to observe absorption, movement and nutrient utilization. The nutrients were transported at the rate of about one foot per hour to all parts of the plants.
Juice from plant leaves can be tested with a refractometer. If after feeding the amount of light refracted significantly rises, at least some nutrients have been absorbed. A spray enhancer can help nutrients stick to the leaf and then penetrate the leaves' cuticle.
Foliar feeding is generally done in evening, since heat causes the pores on some species' leaves to close.