الفهرس 
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Abstract
This study was conducted during the two consecutive seasons of 2006 and 2007 on thirty- six uniform in vigour Valencia orange trees budded onto sour orange rootstock in a private citrus orchard located at Abo Harb village, Bany Mazar district, Minia Governorate where the soil is silty clay and well drained and with a water table not less than two meters deep. Planting space was 5 x 5 meters. The trees were irrigated through surface irrigation system.
The goal of this study was examining the influence of different proportions of inorganic, organic and bioforms of N on the leaf area, leaf chemical composition, yield and fruit quality of Valencia orange trees (Citrus sinensis). The present study also focused on adjusting the best ratio of these N sources that responsible for maximizing the yield and at the same time obtaining fruits with fairly good quality.
This experiment included the following twelve treatments:
1-Application of the recommended rate of N (i.e. 1000g N/ tree)completely via ammonium sulphate (20.6% N ) (4855g/tree)
2-Application of 75% of the recommended rate of N ( 750g N/tree) via ammonium sulphate (3641 g/tree) plus 25% via compost El- Neel (2.15%N)(11.63 kg).
3-Application of 75% of the recommended rate of N (750g N /tree) via ammonium sulphate (3641 gN/tree) plus 25% via Minia Azotene (250g/tree).
4-Application of 75% of the recommended rate of N (750 gN/tree) via ammonium sulphate (3641g/tree) plus 12.5% compost El-Neel (5.81kg/tree) plus 12.5% Minia Azotene (125g/tree).
5-Application of 50% of the recommended rate of N (500g/tree) via ammonium sulphate (2427 g/tree) plus 50% compost El-Neel (23.26 kg/tree).
6-Application of 50 % of the recommended rate of N (500 g N/ tree) via ammonium sulphate (2427g /tree) plus 50% Minia Azotene (500g /tree)
7-Application of 50% of the recommended rate of N (500g N /tree) via ammonium sulphate (2427 g/tree), 25% compost El-Neel (11.63 kg/tree) plus 25 % Minia Azotene (250g/tree).
8-Applicatin of 25 % of the recommended rate of N (250g N/tree) via ammonium sulphate (1214 g/tree) plus 75% ( 750 g N/ tree) via Compost El- Neel ( 34.88 kg/ tree).
9- Applicatin of 25 % of the recommended rate of N (250g / N/tree) via ammonium sulphate (1214 g/tree) plus 75% Minia Azotene ( 750 g / tree).
10-Application of 25 % of the recommended rate of N (250g N/tree) via ammonium sulphate (1214 g/tree) plus 37.5% ( 375 g N) Compost El- Neel ( 17.44 kg/ tree) plus 37.5 % Minia Azotene ( 375 g / tree).
11- Application of 100% of the recommended rate of N ( 1000 g N/ tree) ) via Compost El- Neel ( 46.5 kg / tree).
12- Application of 100% of the recommended rate of N ( 1000 g N/ tree) via Minia Azotene ( 1000 g / tree).
Each treatment was replicated three times, one tree per each Ammonium sulphate ( 20.6 %N ) as a source of inorganic N was splitted into three equal batches and addded on the first week of Feb, April and June. Compost El- Neel ( 2.15 % N) as a source of organic N was added once at the first week of Jan. Minia Azotene biofertilizer was added once at the first week of Feb.
The chosen trees received a basal recommended P and K fertilizers and another normal horticultural practices that are followed in the orchard except N fertilization.
This experiment was set up in completely randomized block design.
In both seasons, the following measurements were recorded :
Leaf area ( cm2)
Percentages of N, P and K in the leaves from non fruiting shoots in the Spring growth cycle.
Yield/ tree expressed in weight (kg.) and number of fruits per tree.
Some physical characters of the fruits namely fruit weight (g.) and dimensions ( length and width in cm), as well as fruit peel weight and thickness .
Some chemical characters of the fruits namely percentages of total soluble solids, total acidity as well as total and reducing sugars and vitamin C content (mg/ 100 ml juice).
The nearly same results obtained in both seasons could be summarized under the following main items.
1- Leaf area:
It was greatly varied according to the different ratios between inorganic, organic and bioforms of N. Leaf area was remarkably reduced when the recommended rate of N was applied in inorganic form of N at percentages 25% with organic and bioforms of N as well as when the trees received N completely via organic or biofertilization comparted to using N completely via inorganic form. However, using N in the basis of 50 to 75% inorganic plus 12.5 to 50% organic or bioforms resulted in great stimulation on the leaf area compared to using N completelty via inorganic form. The promotion was associated with reducing inorganic N from 100 to 50 % and at the same time increasing percentages of organic and biofertilization from 25 to 50%. The minimum values were recorded on the trees received N completey via biofertilization. Supplying the trees with N as 50% inorganic , 25% organic and 25% bioforms of N gave the maximum values.
2- Percentages of N, P and K in the leaves
They were enhanced in response to application of N as 50 to 75% inorganic and both organic and bioforms of N at 25 to 50% compared to using N in each source alone. Reducing inorganic source of N from 50 to 25% and at the same time using 37.5 to 75% organic or bioforms of N resulted in great decline in these essential nutrients compared to using N completely via inorganic form. Biofertilization only gave the lowest values and the trees that received N via 50% inorganic, 25% organic and 25% bioform had the maximum values.
3- Yield / tree
Yield of Valencia orange trees expressed in weight ( kg.) and number of fruits per tree were negatively affected by supplying the trees with N as inorganic source at percentage 25 plus organic and biofertilization as well as using N completely via organic or bioforms of N compared to using N completely via inorganic form. Supplying the trees with N as 50 to 75% inorganic plus organic and biofertilization at 25 to 50% caused a considerable promotion on the yield compared to using N via inorganic form only. The promotion on the yield was in proportional to decreasing inorganic from 100 to 50 % and increasing organic and bioform of N from 25 to 50%. Using biofetilization only gave the minimum values. The maximum values were recorded on the trees received N as 50% inorganic, 25% bioform and 25% organic source.
4- Fruit quality
Physical and chemical quality of the fruits were greatly varied among the different N management treatments. Physical quality parameters were greatly enhanced when N was applied completely via biofertilization followed by using N completely via organic N source. Decreasing percentage of inorganic N from 100 to 50% and at the same time increasing the percentages from 12.5 to 100% of organic and biofertilization was accompained with decreasing fruit weight and dimensions and increasing fruit peel weight and thickness. Using N as 25% inorganic in addition to organic and biofertilization gave an acceptable promotion on physical quality of the fruits. Application of N via the three N sources was preferable in enhancing chemical quality of the fruits in terms of increasing total soluble solids, total and reducing sugars and vitamin C content and decreasing total acidity rather than using N completely via inorganic source.
The promotion on chemical quality of fruits was associated with decreasing inorganic source of N and at the same time increasing the proportion of organic and biofertilization. The best results with regard to chemical quality of the fruits were obtained when the trees received N completely via biofertilizer namely Minia Azotene followed by using N via compost El-Neel alone. Fertilizing the trees with N completely via inorganic N source gave unacceptable chemical quality of the fruits