Properly Timing Foliar-applied Fertilizers Increases Efficacy

Foliar fertilization efficiently meets the nutrient demand of tree fruit crops during periods when soil conditions (low temperature, low soil moisture, pH, salinity) render soil-applied fertilizers ineffective, when nutrients (e.g., phosphate, potassium, and trace elements) become fixed in the soil, and when tree nutrient demand is high. Applying nutrients directly to leaves ensures that the tree’s metabolic machinery is not compromised by low availability of an essential nutrient. It should be noted that phloem mobile nutrients applied to the foliage are translocated to all tree parts, even feeder roots. Because foliar application of fertilizers can reduce nutrient accumulation in soil, run-off water, surface water (streams, lakes, ocean), and groundwater (drinking water supply), where they contribute to salinity, eutrophication or nitrate contamination, with negative consequences to the environment and humans, it is highly desirable to replace soil-applied fertilizers at least in part with foliar-applied fertilizers. However, not all nutrients are taken up through leaves and, even if taken up, some nutrients are not phloem mobile. In addition, although foliar fertilizer rates are typically lower than soil fertilizer rates, application can be more costly. The approach of my research program has been to properly time the application of foliar fertilizers to key stages of citrus and avocado tree phenology when demand for the nutrient is likely to be high and especially when soil conditions are likely to compromise nutrient uptake by roots. This approach has proven successful for increasing yield, fruit size, and grower income even when the tree is NOT nutrient deficient by standard leaf analysis. For Citrus sinensis and C. reticulata, our research has identified two stages of tree phenology when foliar-applied low-biuret urea increases yield and yield of commercially valuable large size fruit, i.e., when the shoot apical meristem becomes determined and at maximum peel thickness, respectively. Due to the poor uptake of foliar-applied fertilizers by leaves of Persea americana under California-growing conditions, different stages of bloom were targeted. Low-biuret urea and boron applied at the cauliflower stage of inflorescence development increased ovule viability and the number of pollen tubes penetrating the ovule, respectively. Both increased yield. In this research, earlier or later foliar fertilizer applications did not increase citrus of avocado yield or fruit size. All properly timed foliar fertilizer applications resulted in a net increase in grower income.