The effect of urea foliar spray time on yield and fruit quality of ‘Thomson Navel’ sweet orange trees

The current study was aimed at determining the optimum time of urea foliar spray to improve fruit quantity and quality (fruit set%, yield, size, peel thickness of fruits, titrable acidity (TA), total soluble solid (TSS)) of ‘Thomson Navel’ orange trees (Citrus sinensis L. Osbeck.) for 2 consecutive years. Thus, a 0.5% solution of urea was sprayed on to 20 years old orange trees during January (T1), March (T2) and June (T3). There was a set of control trees that only sprayed with simple water (T0). The results showed that the trees receiving foliar applied urea, independent of application time, had significantly greater leaf nitrogen content and fruit set % than the control trees. In addition, the impact of the urea foliar application on the yield and quality properties of fruits was significant (P<0.05). The effect of urea spray time on TSS/TA (maturity index) and yield depended on studied year. In first year (low yield), the highest level of yield observed in T1 and T3 treatments and the highest level of TSS/TA observed in T0 treatment. In second year (high yield), the highest level of yield and TSS/TA observed in T2 and T3 treatments. Therefore, it is important to determine the optimal level and time of application of nutrients to stimulate a specific physiological process in citrus, depending on the studied year (high yield or low yield).