Citrus readily responds to nitrogen nutrition. Nitrogen applied via foliar application proves to be beneficial. A study was done by UC Professor of Plant Physiology, Dr. Carol J. Lovatt, where a single winter pre-bloom foliar application of nitrogen as low-biuret urea was made to 30-year-old Washington navel orange trees during flower initiation. The result was a significant increase in yield and fruit number per tree for each of three consecutive years.
Late winter and early spring fertilizer applications via foliage can increase fruit set and yield because nutrients essential for flowering and fruit set must be present in trees at optimal levels before the trees begin using them to stimulate these physiological processes. Pairing your low-biuret urea sprays with a good foliar product with other essential nutrients will give trees a nice boost in nutrition.