Abstract
Effects of application time in spring on nitrogen absorption by satsuma mandarin (Citrus unshiu Marc. cv. ‘Nankan No. 20’) were investigated. Field and pot tests were performed using an 15N-tracer method in which the nitrogen was applied on March 1, April 1, and May 1. In field experiments, the absorption rate by trees was greater in April- and May-plots than in March-plots. The 15N content of one-year-old leaves showed similar changes in the March- and April-plots, which was consistently higher than that in May-plots. The 15N content in newly developed organs, such as new leaves, flowers and young fruit, was apparently greater in March- and April-plots than in May-plots between early May and early June. After late June, however, the 15N content was consistently greatest in May-plots. In pot experiments, the utilization rate of trees receiving nitrogen application in the spring was 31.5%, 34.6% and 37.1% in March-, April-, May-plots, respectively. Thus, the efficient application timing of nitrogen for spring fertilizing seems to be immediately before the bud burst in early April.
