PROBLEMS WITH UREA-N FOLIAR FERTILIZATION OF AVOCADO

Young, fully expanded leaves of 'Hass' avocado on clonal 'Duke 7' rootstock, 2 years from budding, were used to assess changes in leaf NH3-NH4 content. Foliar application of 1.5 g low biuret urea per tree failed to increase the leaf NH3-NH4 content: compare 23.6 ± 5.8 versus 20.1 ± 5.7 μg NH3-NH4 per g fr. wt. leaf tissue (x ± STD. DEV., N = 3 trees per treatment) from trees treated with and without urea, respectively (Nevin and Lovatt, 1989). Foliar application of 1.5 g of low biuret urea per tree to 5-yearold rooted cuttings of the 'Washington' navel orange of comparable size increased the leaf NH3-NH4 content of the trees subjected to 4, 6, or 8 weeks of low temperature 1.7-, 2.2-, and 1.2-fold, respectively.