Effect of Nitrogen Application Management in Different Phonological Stages on Yield and Alternate Bearing of Satsuma Mandarin (Citrus unshiu)

An important issue in some citrus varieties including Satsuma mandarin is alternate bearing. Therefore, to reduce alternate bearing in Satsuma mandarin trees, an experiment was conducted as randomized complete block design with 4 treatments and 4 replications. The experiment was done during 7 years with 32 Satsuma mandarin trees which were almost the same age and size. The experimental treatments of nitrogen use management included T1= Control (three splits, the first split late in March, the second late in April after fruit formation, and the third one month after the second split); T2= Summer application (15% before flowering, 30% after fruit formation, 20% at the time of physiological drop, and 35% after the beginning of fruit development); T3= Autumn application (15% before flowering, 30% after fruit formation, 30% at the time of physiological drop and 25% after fruit harvest); T4= Summer and autumn application (15% before flowering, 30% after fruit formation, 20% at the time of physiological drop, 20% after starting development, and 15% after fruit harvest). Results showed that fruit yield had the greatest difference in the first and second years of experiment and then the yield difference gradually decreased. The lowest cumulative yield (379  kg/tree) was obtained from the control and the highest cumulative yield (446 kg/tree) was obtained from summer and autumn N treatment. Results showed that the control had the highest alternate bearing index (0.40) and autumn N application treatment had the lowest treatment index (0.18). Also, year had a significant effect on fruit average weight as the highest fruit average weight was obtained in the fifth year. In all N application treatments, fruit average weight was significantly more (at 5% level) than the control. Based on the results of this study, to minimize alternate bearing, the time and amount of nitrogen fertilizers are recommended in the following four stages: 15% at the beginning of flowering, 30% between fruit set and the beginning of June drop; 30% at the beginning of June drop to early phase of the second growth of fruits; and 25% after fruit harvest.