WINTER APPLICATION OF LOW-BIURET UREA TO THE FOLIAGE OF WASHINGTON NAVEL ORANGE INCREASED YIELD

The objective of this study was to test whether a single winter prebloom foliar application of low-biuret urea would increase the yield of 30-year-old 'Washington' navel orange trees [Citrus sinensis (L.) Osbeck] on Troyer citrange rootstock [C. sinellsis 'Washington' x Poncirus trifoliata (L.) Raf.]. All trees received a winter (November to January) soil application of urea (0.5 kg N/tree). Trees were maintained under irrigation or irrigation was withheld from 1 Oct. to 1 Mar. To determine the optimal time for foliar urea application, trees in both irrigation main plots received one application of low-biuret urea in mid-November, mid-December, mid-January, or mid-February applied at a rate of 0.6 kg N/tree. There was a set of control trees that only received the soil application of urea. Trees receiving foliar-applied urea in mid-January or mid-February, independent of irrigation treatment, had significantly greater yield and fruit number per tree each year than the control trees for 3 consecutive years. The number of fruit with diameters of 6.1 to 8.0 cm increased significantly as yield increased (r(2)= 0.88). Withholding irrigation from 1 Oct. to 1 Mar. had a-negative impact on yield. Annual winter application of low-biuret urea to the foliage did not significantly increase leaf total N at the end of 3 years.