Foliar feeding is a technique of feeding plants by applying liquid fertilizer directly to the leaves. Plants are able to absorb essential elements through their leaves, especially trace minerals such as iron, zinc and calcium. Foliar feeding can be beneficial for plants that are stressed, deficient in a single nutrient, or growing in poor soil conditions. However, foliar feeding should not replace soil fertilization, as plants need most of their nutrients from the roots. Foliar feeding should be done in the early morning or late evening, when the stomata are open and the temperature is low. A surfactant can help the nutrients stick to the leaf and penetrate the cuticle.