الفهرس 
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Abstract
This study was achieved during 2018/2019 and 2019/2020 on 21 Valencia orange trees, uniform in vigour, 52 years old onto sour orange rootstock and planted at a spacing of 6x6 meters a part. The selected trees are grown in the orchard of agricultural research station located at Mallawy district, Minia governorate – Egypt.
The soil of the orchard is well drained clay in texture with a water table not less than two meters deep. Surface irrigation system was carried out using Nile water.
This study was carried out to examine the effect of different concentrations and frequencies of urea on the yield quantitatively and qualitatively of Valencia orange trees. The selected Valencia orange trees were subjected to the normal horticultural practices as recommended by the ministry of agriculture and land reclamation.
This experiment included the following seven treatments from concentrations and frequencies of urea applications.
1. Control - spraying only with water in winter (November).
2. Spraying urea at 2% once at the first of November.
3. Spraying urea at 3% once at the first of November.
4. Spraying urea at 2% twice at the first and mid of November.
5. Spraying urea at 3% twice at the first and mid of November.
6. Spraying urea at 2% thrice at the first , mid and last of November.
7. Spraying urea at 3% thrice at the first , mid and last of November.
Each treatment was of three replicates, one tree per each. Triton B was a wetting agent was added to all solutions at 0.05%. Spraying was done till run off Complete randomized block design (CRBD) was used in this experiment.
The following parameters had been studied in the two years of the experiment:
Fruit set%- yield per tree expressed in fruit number and weight in (kgs).
The physical and chemical characteristics estimated were fruit weight- fruit height and diameter- fruit peel weight and thickness- juice% as well as T.S.S%- total acidity%- T.S.S/acid%- total and reducing sugars% and vitamin C%.
The yield data could be summarised as follows:
Spraying valencia orange trees with urea at 2.0 or 3.0% during winter (November) once, twice and thrice was of positive effect in increasing fruit set%. The best treatment in this concern was that included spraying with urea at 3.0% thrice annually. The same treatment resulted in the highest number of fruits and the yield per tree.
Concerning the effect of the different treatments, resulted should that the highest fruit weight, fruit height and diameter as well as fruit juice percentage was enhanced by the different treatments. Moreover, spraying the trees with urea at 3.0% thrice annually was the more best treatment in this connection. The previous treatment had also improved T.S.S%, T.S.S/acid, total and reducing sugars and vitamin C (ascorbic acid). Simultaneously, reduced the juice total acidity compared with any other treatment including the control.
CONCLUSION:
Under the experimental conditions and resembling regions, it is suggested to spraying Valencia orange trees thrice annually at the first, mid and the last of November by urea at a concentration of 3.0% for increasing the yield per tree and improving fruit quality.