ZINC FERTILIZATION OF AVOCADO TREES

Methods for Zn fertilization of 'Hass' avocado (Persea americana Mill. ) trees were evaluated in a 2-year field experiment on a commercial or chard located on a calcareous soil (pH 7.8) in Ventura County, Calif, The fertilization methods included soil- or irrigation-applied ZnSO4; irrigation-applied Zn chelate (Zn-EDTA); trunk injection of Zn(NO3)(2) , and foliar applications of ZnSO4, ZnO, or Zn metalosate. Other exper iments evaluated the influence of various surfactants on the Zn conten ts of leaves treated with foliar-applied materials and on the retentio n and translocation of radiolabeled (ZnSO4)-Zn-65 and Zn-65 metalosate after application to the leaf surface. In the field experiment, tree responses to fertilization with soil-applied materials were affected s ignificantly by their initial status, such that only trees having <50 mu g . g(-1) had significant increases in foliar Zn contents after fer tilization. Among the three soil and irrigation treatments, ZnSO4 appl ied at 3.2 kg ZnSO4 per tree either as a quarterly irrigation or annua lly as a soil application was the most effective and increased leaf ti ssue Zn concentrations to 75 and 90 mu g . g(-1), respectively, Foliar -applied ZnSO4, ZnO, and Zn metalosate with Zn at 5.4, 0.8, and 0.9 g . liter(-1), respectively, also resulted in increased leaf Zn concentr ations. However, experiments with Zn-65 applied to leaves of greenhous e seedlings showed that <1% of Zn applied as ZnSO4 or Zn metalosate wa s actually taken up by the leaf tissue and that there was little trans location of Zn into leaf parenchyma tissue adjacent to the application spots or into the leaves above or below the treated leaves. Given the se problems with foliar Zn, fertilization using soil- or irrigation-ap plied ZnSO4 may provide the most reliable method for correction of Zn deficiency in avocado on calcareous soils.