Effects of long-term nitrogen application on tree growth, yield, and fruitqualities in a 'Jonathan' apple orchard

A nitrogen application trial was conducted from 1973 (10-year-old trees) to 1995 (32-year-old trees) to analyze its effect on the growth, yield,and fr uit quality of 'Jonathan' apple trees in a sod orchard. The treatments incl uded no nitrogen application(-N), 5 kg N (1N), 10 kg N (2N), and 20 kg N . 10 a(-1) . year(-1) (4N). The leaf nitrogen content closely correlated with the amount of nitrogen ap plied. Differences in leaf nitrogen content appeared significant among all treatments after 1975. The increase of trunk girth in 4N was greater than i n the other treatments; the difference became significant after 1977. The fruit yield increased about two-fold from 1973 to 1990 in all the treat ments but no significant difference was found in the yield among the treatm ents except for 1990-1992. The percentage of highly colored fruit was lower in 2N and 4N than in the -N and 1N plots. The poorly colored fruit had low soluble solids and firmness, compared with the well colored one. These results indicate that excessive nitrogen application increases growth and leaf nitrogen content, but negatively affects both fruit coloring and quality. The optimum amount of nitrogen application in this orchard is esti mated to be less than 10 kg . 10 a(-1) . year(-1). Furthermore, apple trees grown with sufficient soil nitrogen fertility can grow vigorously and prod uce fruits for long term without any nitrogen fertilizer.