A foliar spray delivers nutrients or beneficial substances directly to a plant’s leaves, bypassing the soil. This technique involves dissolving supplements in a liquid solution and spraying it onto the foliage for rapid absorption. The primary advantage of foliar feeding is the speed at which plants access these supplements, offering a quick response to nutrient deficiencies or providing a boost during high growth periods.
Understanding How Leaves Absorb Nutrients
Plant leaves are covered by a waxy layer called the cuticle, which is the primary barrier to nutrient uptake. This layer is hydrophobic, meaning it naturally repels water and dissolved substances, but water-soluble nutrients can still pass through it via microscopic aqueous pores. The efficiency of nutrient passage through the cuticle is influenced by the concentration gradient between the leaf surface and the interior cells.
Leaves also possess tiny openings called stomata, primarily responsible for gas exchange and water transpiration. While nutrients can pass through open stomata, this is generally not the main route for foliar absorption, as stomata often close during the heat of the day. High humidity is beneficial because it keeps the spray solution on the leaf surface longer, preventing rapid evaporation and allowing more time for diffusion. Absorption is improved when the solution forms small droplets and includes a surfactant to help the liquid adhere to the waxy leaf surface.
Formulating Common Nutrient Sprays
Formulating an effective foliar spray requires attention to both the ingredients and the water used in the mixture. Use clean water, such as distilled or reverse osmosis water, to minimize contaminants that could interfere with nutrient uptake. Solids must be completely dissolved before application to prevent clogging the sprayer nozzle. A surfactant, such as non-ionic liquid soap or a commercial wetting agent, should be added to help the spray droplets spread and stick to the leaf surface.
For a common micronutrient boost, such as addressing a magnesium deficiency, prepare a simple Epsom salt solution. Mix one tablespoon of Epsom salt (magnesium sulfate) into one gallon of water, ensuring the crystals are fully dissolved. Liquid seaweed, which contains a broad spectrum of trace elements and growth hormones, serves as an excellent general health booster. A typical dilution is about two tablespoons of concentrated liquid seaweed per gallon of water.
Compost tea provides beneficial microbes and enzymes, made by steeping high-quality compost in water for 24 hours. For foliar use, the liquid must be strained thoroughly to remove solid particles that could clog the sprayer. This tea should be used immediately after brewing to maintain the viability of the microorganisms. Always use lower concentrations for foliar sprays than for soil applications to avoid the risk of leaf burn.
Proper Application Timing and Technique
The timing of a foliar application is paramount to maximize nutrient absorption and prevent damage to the plant tissue. Applying the spray during the cooler parts of the day, specifically in the early morning or late evening, is highly recommended. High temperatures cause the stomata to close and increase the risk of leaf burn as the water evaporates quickly, leaving concentrated salts on the leaf surface. It is generally best to apply the spray when the air temperature is below 75°F (24°C).
Proper technique involves aiming for a fine mist that completely coats the foliage without excessive runoff. Target the underside of the leaves, as this area often has a thinner cuticle and may contain more stomata, facilitating better uptake. The solution’s pH also affects absorption; a slightly acidic range of 5.8 to 7.0 is optimal for nutrient solubility and penetration.
Before treating an entire plant, test the spray on a small section of a single leaf and wait 24 to 48 hours to check for adverse reactions, such as discoloration or burn. Leftover spray solutions, particularly compost teas containing living microbes, should not be stored for long periods. Dispose of remaining liquid thoughtfully by pouring it around the base of plants. Monitoring the water pH and adjusting it with a buffer before mixing nutrients improves application performance.