Few believed it was possible to bypass the plant root system and feed nutrients directly via the leaves. Unfortunately, many mistakes were made in the early years of foliar applications which fueled this widespread skepticism. For instance, overly high concentrations were applied as proper application rates were still unknown. The pH of the spray mixture was not considered. Leaf burn was common due to phytotoxicity from salts, chlorides and sulfates. Not much was understood about how a leaf might be safely coaxed into absorbing nutrients. It seemed unlikely that a few pounds of nutrients applied via a foliar application would ever successfully supplement the standard application of several hundred pounds/acre of N, P and K.
The eventual acceptance of foliar applications can be attributed to the success of pesticide companies in demonstrating that chemicals could indeed enter a plant via the leaves. Persistent innovators then allowed for another industry to be born. Foliar nutrition applications began to occur with mixed results. Early adopters manufactured products they touted as the best. It was not until later that the method of application was understood to be almost as important as the products themselves.
The following are guidelines to performing a foliar application that maximizes uptake of both nutrients and pesticides, and minimizes plant phytotoxicity:
- Buffer your water. Foliar nutrients are best absorbed, and most pesticides function best, at a pH between 5 – 6.5. Phosphoric acid or citric acid are two great choices to lower the pH. An exception to this is when applying a copper based fungicide. The pH should be closer to 7 then.
- Use a good surfactant. A good surfactant will function as a spreader, and reduce the surface tension of the spray solution, which will spread the product across more of the leaf. This will actually reduce the potential for phytotoxicity. If you are applying a systemic product, the surfactant should also be a penetrant.
- Late night or early morning are best times of day to apply. Stop applications when temperatures exceed 85 degrees, or wind speed is above 10 mph.
- DO NOT spray a hot mix when the plant is under stress. A hot mix, or one with a high potential for phytotoxicity, can form when the following are combined: high rates of fertilizer, summer spray oil, and emulsifiable concentrate forms of pesticides.
- DO NOT spray the leaves to the point of runoff. Doing so will concentrate the nutrient and/or pesticide on the tips and edges of the leaf. This will cause water to exit the leaf, resulting in necrosis at the spot where the solution resisted drying.
- Use as fine of a spray as possible. Smaller droplets will give you better coverage of the leaf. A spreader will generally help droplet size, particularly with hard water.
Following these simple instructions will greatly improve any foliar application. But concerning foliar nutrition, the method of application is only half the issue. The product choice is even more important. Nutrients need to be complexed to improve uptake into the stomata, and across the leaf cuticle surface. Modern nutrient technology links a specific nutrient with a carrier that will help transport the nutrient across the leaf surface. These forms of fertilizer, along with properly performed spray, will result in a highly effective foliar application.