Foliar fertilizer delivers nutrients by applying a liquid solution directly to the leaves, bypassing the slower and sometimes inefficient process of root absorption from the soil. This technique allows for rapid nutrient uptake. While not a replacement for traditional soil fertilization, foliar feeding supplements a plant’s nutritional needs. It is primarily used to correct sudden nutrient deficiencies or during critical growth periods requiring immediate nutrition. This method provides a quick boost, especially when soil conditions (such as high pH or drought) limit the roots’ ability to function.
How Plants Absorb Nutrients Through Leaves
Plants possess two primary pathways for absorbing dissolved nutrients directly from the leaf surface. The first pathway involves the stomata, which are tiny, pore-like openings usually found on the underside of the leaves. These pores primarily regulate gas exchange for photosynthesis and transpiration, but when open, they also serve as entry points for the aqueous nutrient solution.
The second absorption route is through the cuticle, the waxy, protective layer covering the leaf epidermis. Although the cuticle acts as a barrier against water loss, it contains microscopic polar pores. These pores allow water-soluble compounds to diffuse directly through the surface. Penetration through the cuticle is slower than through the stomata, but it is a significant entry point, especially for young leaves with a thinner cuticle.
Once nutrients penetrate the leaf surface, they move into the leaf cells and are distributed to other parts of the plant through translocation. This movement is rapid; some nutrients are transported at a rate of approximately one foot per hour to the stems and roots. Absorption efficiency is influenced by factors like the nutrient’s molecular size and charge. Smaller, neutral molecules are generally more successful at passing through both the stomata and the polar pores.
Essential Components of Foliar Formulations
Foliar sprays are most commonly used to deliver micronutrients because these elements are required in small quantities that are easily absorbed through the leaves. Elements like iron, zinc, manganese, and boron are frequently included in these formulations because they can become chemically “fixed” or unavailable to the plant when applied to certain soil types, especially those with a high pH. Macronutrients like Nitrogen, Phosphorus, and Potassium are typically not delivered exclusively via foliar application because the plant requires them in amounts too large for the leaves to absorb without risking damage.
Specialized ingredients are added to maximize nutrient delivery effectiveness. Chelates are chemical agents that surround and bond with nutrient ions, such as metals, forming a protective, neutral cage. This chelation process keeps the nutrients soluble in the spray solution, preventing reactions on the leaf surface and making them more accessible for plant uptake.
Surfactants, or wetting agents, are common additives that improve application success. These compounds reduce the surface tension of the liquid spray, allowing droplets to spread and wet the leaf surface uniformly. Better spreading ensures the solution maintains contact with the stomata and cuticle longer, increasing nutrient absorption before water evaporates.
Best Practices for Effective Application
Proper timing of foliar application is important for maximizing nutrient absorption and avoiding phytotoxicity (damage to plant tissue). The most effective time to spray is during the cooler parts of the day, such as early morning or late evening. Applying above 75°F (24°C) causes rapid water evaporation, which can lead to leaf burn if nutrients become too concentrated on the surface.
Optimal application occurs when stomata are open, correlating with high humidity and moderate temperature. Spray concentration must be carefully managed, as overly strong solutions will chemically scorch the leaves. Although general recommendations suggest 0.5% to 1.5% fertilizer solution, the specific concentration depends on the plant species and the nutrient applied.
For successful delivery, the spray mist must be fine enough to evenly cover the leaf surface without excessive runoff. Target both the upper and lower surfaces of the foliage, as many plant leaves have a high concentration of stomata on the underside. Avoid spraying during high wind, which prevents uniform coverage and causes spray drift. Finally, the pH of the mixed solution affects nutrient solubility and absorption; a range between 5.0 and 8.0 is generally optimal.