Foliar feeding is a technique of feeding plants by applying liquid fertilizer directly to their leaves.
[1] 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.