Foliar Application of Nutrients and Their Impacts on Growth, Yield, and Quality in Tree Fruits

Deciduous tree fruit yield and quality will suffer in response to insufficiency of any mineral nutrient; however, there are some elements, including nitrogen (N), calcium (Ca), potassium (K), and boron (B), that are closely tied to fruit quality.  Fruit growers have applied nutrients in foliar sprays for many years to influence tree and fruit nutritional status. Sprays of macronutrients and secondary nutrients typically are supplementary to soil applications. Such sprays may be made to target specific plant organs. For example, postharvest and prebloom foliar urea sprays are applied to enhance flower bud vigor, and Ca is applied directly to fruits during the growing season to reduce fruit susceptibility to physiological disorders. Macro- and secondary nutrient sprays also are applied to quickly alleviate nutrient deficiencies that appear suddenly, such as those induced by excessive vegetative growth or elemental imbalances caused by improper applications of fertilizers or soil amendments. Sprays of K, magnesium (Mg), sulfur, and phosphorus fall into this category; however, these elements are more appropriately managed using soil applications.  Over-zealous application of N, K, or Mg in foliar sprays can negate the beneficial effects of Ca by inducing elemental imbalances within fruit.  Micronutrients typically are applied in foliar sprays to uniformly distribute the small quantities of these required nutrients. Fruit quality is particularly sensitive to fruit B status, with both insufficient and excessive amounts of B causing severe deleterious responses.  Recent research focuses on developing foliar spray programs that enhance nutrient phytoavailability, thereby reducing nutrient requirements and minimizing release of potentially deleterious elements into the environment, and that allow safe tank-mixing of multiple nutrients, which would reduce the number of sprays required per season.