Foliar applications of Lo-Biuret Urea and Potassium Phosphite to Navel Orange trees Glenn Wright and James Walworth 1 Abstract This experiment was established in January 2000 in a block of ‘Washington’ navel orange trees at Verde Growers, Stanfield, AZ. Treatments included: normal grower practice, winter low biuret (LB) urea application, summer LB urea application, winter LB urea application plus winter and spring potassium phosphite, winter LB urea application plus summer potassium phosphite, and normal grower practice plus spring potassium phosphite. Each treatment was applied to approximately four acres of trees. For 2000-01, yields ranged from 40 to 45 lbs. per tree, and there was no effect of treatments upon total yield. There was a slight effect upon fruit size and grade. Trees subject to summer LB urea application had significantly more fruit of size 56, compared to trees subject to winter LB urea, and untreated, and untreated trees had significantly more fruit of size 88 than did treated trees. Also, treated trees had slightly more fruit in the fancy grade than did untreated trees. Introduction Arizona citrus growers face increasing economic and political pressure to adapt best management practices for nitrogen fertilization. Foliar N fertilization offers an opportunity to apply a significant portion of the total tree N needs in a more efficient manner than traditional flood or ground applications. Navel oranges typically do not bear in great abundance in the desert. Water stress during fruit set, and/or high temperatures during bloom or fruit set can cause yield reductions. 2 Recent research by Lovatt suggests that pre-harvest applications of foliar urea and potassium phosphite can increase yield and fruit set on ‘Washington’ navel oranges in the San Joaquin Valley 3 1 The authors would like to than the Arizona Citrus Research Council for ongoing financial support necessary to complete this study. This is the final report for Project 2000-10 “Foliar applications of Lo-Biuret Urea and Potassium Phosphite to Navel Orange trees”. 2 Davies, F.S. 1986. The Navel orange. In: Janick, J. (ed.) Horticultural Reviews. AVI Publishing, Co., Westport, CT Pp. 79-99. 3 Lovatt, C.J. 1998. Managing yield with foliar fertilization. Calif. Citrograph 84:1. This is a part of publication az1275: "2001 Citrus and Deciduous Fruit and Nut Research Report," College of Agriculture and Life Sciences, the University of Arizona, Tucson, Arizona, 85721.