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 orchard located on a calcareous soil (pH 7.8) in Ventura County, Calif. The fertilization methods included soil- or irrigation-applied ZnSO 4 ; irrigation-applied Zn chelate (Zn-EDTA); trunk injection of Zn(NO 3 ) 2 , and foliar applications of ZnSO 4 , ZnO, or Zn metalosate. Other experiments evaluated the influence of various surfactants on the Zn contents of leaves treated with foliar-applied materials and on the retention and translocation of radiolabeled 65 ZnSO 4 and 65 Zn metalosate after application to the leaf surface. In the field experiment, tree responses to fertilization with soil-applied materials were affected significantly by their initial status, such that only trees having <50 μg·g –1 had significant increases in foliar Zn contents after fertilization. Among the three soil and irrigation treatments, ZnSO 4 applied at 3.2 kg ZnSO 4 per tree either as a quarterly irrigation or annually as a soil application was the most effective and increased leaf tissue Zn concentrations to 75 and 90 μg·g –1 , respectively. Foliar-applied ZnSO 4 , ZnO, and Zn metalosate with Zn at 5.4, 0.8, and 0.9 g·liter –1 , respectively, also resulted in increased leaf Zn concentrations. However, experiments with 65 Zn applied to leaves of greenhouse seedlings showed that <1% of Zn applied as ZnSO 4 or Zn metalosate was actually taken up by the leaf tissue and that there was little translocation of Zn into leaf parenchyma tissue adjacent to the application spots or into the leaves above or below the treated leaves. Given these problems with foliar Zn, fertilization using soil- or irrigation-applied ZnSO 4 may provide the most reliable method for correction of Zn deficiency in avocado on calcareous soils.