الفهرس 
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Abstract
This study was carried out during two consecutive experimental seasons 2018 /2019 and 2019/ 2020 on 21 Valencia orange trees, uniform in vigor, 27 years old at the starting of the experiment, budded on sour orange rootstock and spaced at of 6x6 meters apart.
The selected trees are grown in the orchard of Agricultural research station located at Mallway district, Minia Governorate –Egypt.
The texture of the soil orchard is clay and well drained with a water table not less than two meters deep. Surface irrigation system was used by Nile water.
The aim of this experiment was threshing the effect of different levels and frequencies of nitrogen as a soil fertilizer on fruit set, yield and some physical and chemical properties of Valencia orange fruits.
This experiment was done by adding different levels and frequencies of Nitrogen (Ammonium nitrate 33.5%) to the soil as follows:
1-Control application of N at 1000 g N / tree yearly via inorganic N source (ammonium nitrate) added at three equal doses at the start of spring growth cycle, just after fruit setting and at two months intervals.
2-Application of N at 600 g N / tree yearly via inorganic N source (ammonium nitrate) added at two equal doses in mid-November and first week of December.
3-Application of N at 900 g N / tree annually via inorganic N source (ammonium nitrate) added at two equal doses in mid –November and first week of December.
4-Application of N at 1200 g N / tree yearly via inorganic N source (ammonium nitrate) added at two equal doses in mid –November and first week of December.
5-Application of N at 600 g N / tree annually via inorganic N source (ammonium nitrate) added at three equal doses in mid –November, first week of December and the last week of December.
6-Application of N at 900 g N / tree annually via inorganic N source (ammonium nitrate) added at three equal doses in mid –November, first week of December and the last week of December.
7-Application of N at 1200 g N / tree annually via inorganic N source (ammonium nitrate) added at three equal doses in mid –November, first week of December and the last week of December.
Each treatment consisted three replicates one tree per each.
This experiment was designed in complete Randomized block design (CRBD).
The results obtained could be arranged as follows:
1-Percentages of Initial fruit setting, number of fruits and the yield/ tree (kg).
2-Physical and chemical characters of the fruits namely fruit weight (g.) fruit dimensions (height and diameter (cm), percentages of juice, fruit peel weight and fruit peel thickness (mm). Besides to T.S.S. %, total acidity % (as g citric acid / 100 ml / juice), T.S.S. / acid, total and
reducing sugars % and vitamin C content (mg/ 100 ml/ juice) were determined. The obtained data during the two years of the experiment could be summarized under the following main items.
1-Percentages of Initial fruit setting , Number of fruits and yield peer tree:
Fertilizing Valencia orange trees with nitrogen at a rate of 600 to 1200 g N / tree/ year adding to the soil at two or three equal doses had an obvious promotion on the percentages of initial fruit setting, yield and number of fruits peer tree. The promotion was considerably associated with increasing application of nitrogen from 600 to 1200 g / tree and applied two or three times yearly. The best results of yield were obtained by application of nitrogen at a rate of 1200 g N / tree annually added at three equal batches.
2- Fruit characteristics:
Subjecting Valencia orange trees with nitrogen at 1200 g / tree yearly applied at three equal doses enhancing fruit quality in terms of increasing weight, height and diameter of fruit, juice %, T.S.S. %, T.S.S / acid, total and reducing sugars and vitamin C content. Decreasing fruit peel weight and thickness and total acidity. The promotion on both physical and chemical characteristics of the fruits was increased by the application of nitrogen from 600 to 1200 g/ tree yearly and also by increasing the number of the application.