COMPARISON OF DIFFERENT FOLIAR-FERTILIZATION STRATEGIES ON YIELD, FRUIT SIZE AND QUALITY OF 'NULES' CLEMENTINE MANDARIN

Abstract

Foliar fertilizers reduce the potential for nutrients to accumulate in soil, move in run-off to surface waters (streams, lakes, ocean), and leach into the groundwater (drinking water supply), where they can contribute to salinity, eutrophication and nitrate contamination, leading to deleterious effects on the environment and human health. Thus, it is prudent to replace soil-applied fertilizer, at least in part, with foliar-applied fertilizer. Foliar fertilization is a rapid, efficient way to improve crop nutrient status during periods of high nutrient demand or when soil conditions (low temperature, salinity, pH) render soil nutrients and soil-applied fertilizers less available to the plant's roots. Foliar fertilization provides nutrients required for photosynthesis and other important metabolic functions essential to plant growth and productivity. However, not all nutrients move efficiently into leaves or other target organs. A priori knowledge (research) is necessary to develop a foliar fertilization program for a crop. Growers need this information to make cost-effective choices. Yield losses resulted when 'Nules' Clementine mandarin trees (Citrus reticulate) were sprayed with water containing no fertilizer. Thus, to attain yields equal to or greater than untreated control trees, foliar-fertilization strategies must compensate for the negative effect of application. For citrus, the standard time for applying foliar fertilizers is when leaves are 1/3 to 2/3 expanded (March-April) to take advantage of the thin cuticle, yet large surface area. Foliar fertilizers applied at this time to ‘Nules’ Clementine mandarin trees increased leaf concentrations of the applied nutrients through September (standard leaf analysis) (P < 0.0001); other application times had no effect on September leaf nutrient status, except boron applied at 10% anthesis. Winter prebloom foliar-applied low-biuret urea increased the 2-year cumulative yield (P = 0.0197) and 2-year average yield (P = 0.0273) of commercially valuable size fruit compared to all other treatments, except boron applied at 2/3-leaf expansion.