PROPERLY TIMED FOLIAR FERTILIZATION CAN AND SHOULD RESULT IN A YIELD BENEFIT AND NET INCREASE IN GROWER INCOME

Foliar fertilization reduces nutrient accumulation in soil, run-off and groundwater, where they contribute to salinity, eutrophication or nitrate contamination, with negative consequences to humans and the environment. Soil-applied fertilizers should be replaced in part with foliar-applied fertilizers. Foliar fertilizers can meet the crop's nutrient demand when soil temperature, moisture, pH, or salinity renders soil-applied fertilizers ineffective. Applying nutrients directly to leaves ensures that the plant’s photosynthetic machinery is not compromised by low availability of an essential nutrient. Foliar-applied phloem-mobile nutrients are translocated to all plant parts, even feeder roots. However, not all nutrients are taken up through leaves and, even if taken up, some nutrients are not phloem mobile. Foliar fertilizer rates are typically lower than soil fertilizer rates, but application can be more costly. The goal of our research is to properly time the application of foliar fertilizers at specific stages of tree phenology, when nutrient demand is likely high, to increase yield, fruit size or quality, such that foliar fertilization results in a net increase in grower income even when the tree is not nutrient deficient by standard leaf analysis. A single foliar application of zinc sulphate to Citrus sinensis, ‘Washington’ navel orange, at 10% anthesis significantly increased fruit set through harvest in January, without impact on fruit size or quality. Later applications reduced fruit set. Winter (December) prebloom foliar application of low-biuret urea to C. reticulate, ‘Nules’ Clementine mandarin, increased 2-year cumulative yield of commercially valuable fruit (transverse diameter 63.51-69.85 mm) as both kg and number of fruit per tree with no effect on total yield or fruit quality. Earlier or later applications were without effect. Foliar application of potassium phosphite to Persea Americana, ‘Hass’, at the cauliflower stage of inflorescence development significantly increased 3-year cumulative yield (kg and number of fruit per tree) of commercially valuable fruit (178-325 g per fruit) compared to trees receiving foliar- or soil-applied potassium phosphate. All cases resulted in a net increase in grower income.