Effects of nitrogen nutrition on the acid and soluble solids of Satsuma orange fruits

Abstract

1. In order to clarify the effects of N nutrition on the fruit qualities of Satsuma oranges, N rate experiment was carried out for 4 years on 16-year-old trees growing in concrete frame on diluvial loamy land of Ehime Fruit Tree Exp. Station. Ammonium sulphate and ammonium nitrate were respectively applied at the annual rates of 0 (non N), 200 (low N), 400 (medium N) and 800g (high N) of N per tree. The rates were split equally into March, April, July and November application. Other fertilizer elements were applied uniformly to all plots.
2. No difference in leaf N among N rates was found in the 1st year. Though the N concentration in relation to age of leaf varied substantially from year to year, there was the significant increase in leaf N of high N fertilized trees after the 2nd year, and the decrease in the non N. However, leaf N was not greatly different between the trees under low and medium N plot. Means of N concentration in 4- to 6-month-old leaves during the 3 years, the 2 nd to the 4th year, were as follows: 2.72, 2.94, 3.05 and 3.21 per cent in dry basis in the order of non, low, medium and high N plot. In October of the 4th year, the leaf N in non N plot decreased to about 2.60 per cent that seems to be the upper limit of low range for the nutrient status of Satsuma orange trees.
3. Yield during 4 years was not affected by N rates, and also tree vigor was not appreciably different among the low, medium and high N plot. Fruit qualities were not influenced by N rates in the 1st year. After the 2nd year, fruits showed that with increased amounts of applied N, soluble solids and acid in juice were increased, and ratios of soluble solids to acid except the 4th year were lowered. Especially in the 4th year, there was the significant difference in juice qualities among N rates respectively. The degree of increase in acid of fruits grown on the high N fertilized trees was greater than that in soluble solids. High N fertilization tended to produce a high proportion of late and poorly colored fruits with slightly thick and coarse-textured rind.
4. When the status of bearing was differential, N fertilization influenced greatly more on the juice quality in abundant bearing (small fruits) than in less bearing (large fruits). Conspicuous variation of juice quality from year to year was dominantly caused by the difference of meteorological condition in autumn. In this experiment, the fruit qualities of Satsuma oranges appeared to be affected greatly more by meteorological condition than by N fertilization. The difference of juice acid due to N fertilization occurred in young fruits in July or August. The same trend was found also in soluble solids 3 months before harvest.
5. The application of ammonium nitrate produced somewhat lighter green foliage and colored fruits than that of ammonium sulphate. Soluble solids and acid in juice were both slightly decreased by the application of ammonium nitrate, but ratio of soluble solids to acid was somewhat higher in ammonium nitrate than in ammonium sulphate. When calcium nitrate was compared with ammonium sulphate in pot experiment with young Satsuma orange trees, the fruit qualities showed remarkable difference with the same tendency.