Various foliar nitrogen (N) fertilizer products are sometimes promoted as an option for spring fertilization of wheat. These products range in analysis and can include straight nitrogen products or mixtures of N plus other macro and micro nutrients. The straight nitrogen products will typically have an analysis similar to traditional liquid N fertilizers, such as 25 to 30 percent N.
One of the main differences between traditional UAN and the foliar products is that a certain percentage of the N in the foliar fertilizers is commonly in some type of slow-release form. As a result, these specialty products are generally safer for application directly to the foliage in later stages of growth and result in less leaf burn than traditional UAN products.
K-State has tested many different types of foliar N fertilizer products over the years. Foliar N fertilizer products are just as effective as traditional N fertilizers on a pound-for-pound basis, but they are not more effective than traditional N fertilizers. They can be applied in a broadcast spray application at later growth stages of wheat growth than traditional N fertilizer products without damaging the wheat.
One of the reasons the foliar products have not been found to be more effective than traditional soil application is that only a small portion of the N applied as a foliar application to wheat actually moves into the plant through the leaf tissue. An excellent study done in Canada a few years ago found that when care was taken to prevent foliar applied N from reaching the soil, only 8-12% of the applied N was recovered by the plant, compared to 35 to 70% of soil applied N being taken up by the plant. Thus it is very likely that many foliar applied fertilizers are actually taken up through the roots once they wash off the plant.
At the normal topdress time (prior to jointing), producers should simply compare a foliar product to a traditional N fertilizer product based on the cost of a pound of N per acre to determine which product gives the best value. Invariably, the foliar products will be higher in terms of cost-per-pound-of-N than the traditional N fertilizers. In unusual situations (well after jointing or when trying to increase protein levels), the foliar N products would have some premium value since traditional N products would have the potential to burn the foliage if applied in a broadcast spray application.
To reduce the potential for leaf burn, there are alternative ways to apply traditional liquid N sources other than the standard spray nozzle. Streamer bars, a 10- to 15-inch long plastic bar which can be used with traditional spray booms in place of the nozzle, provide a solid stream of liquid fertilizer spaced every 5-6 inches. These streams of liquid greatly reduce foliar burn as compared to complete foliage coverage with standard flat fan spray nozzle. Broadcast granular urea also produces limited leaf burn as compared to sprayed UAN.
The bottom line is, foliar N products can be used for later applications, but the limited amounts of N which can be applied based on the labels of many of these foliar products limits their use in situations where large amounts of N are needed.