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The present investigation was conducted during two successive seasons to study the effect of replacing mineral nitrogen fertilization by organic nitrogen source with or without biofertilizer addition on Washington Navel orange trees, under sandy soil conditions. Percentages of 100, 50, and 25% of mineral nitrogen were replaced by 50, 75 and 100% of organic nitrogen with or with out Saccharomyces cerevisae (yeast) as a source of biofertilizer. Treatment of 50% mineral N + 50% organic N and treatment of 100% organic N as well as the control (100% mineral nitrogen) recorded the higher values of N in the leaves especially in the second season. Treatments included organic fertilization raised N% in the second season. Phosphorus percentage in the leaves did not differ by treatments in both seasons. All treatments enhanced K content in the leaves especially in the second season. On the other hand, adding biofertilizer treatments improved N, P and K content in the leaves especially in the presence of mineral N. As for yield per tree, all treatments increased number of fruits and yield weight/tree than the control especially in the second season. The effect of treatments on fruit quality was clearly shown in the first season, since most of the treatments improved fruit quality parameters especially treatment of 50% mineral N + 50% organic N fertilizers. On the other side, treatments containing organic or bio-fertilizer reduced nitrogen, nitrate and nitrite values in fruit juice comparing with the control (100% mineral N fertilizer). INTRODUCTION Citrus is the first fruit crop in Egypt. Washington Navel orange cultivar has a great importance either for the local market or export needs. Economically, it ranks the top among orange cvs., since it occupies 124271 feddan (one feddan = 4200 m) with fruiting area reached 110050 feddan, producing 1050462 tons according to Ministry of Agriculture and Land Reclamation statistics, 2004. This means that the yield is still low and attained about 9.55 tons per feddan. Increasing productivity and improving fruit quality are the main targets of many specialists. There is a general agreement that several factors affect productivity and fruit quality of orange trees. One of the important factors plays a vital role, in this concern, is nitrogen fertilization which considered as agent of accumulative harmful residues like nitrate and nitrite in fruit juice, Montasser et al (2003). Thus, a great attention is focused on minimizing the intensive amounts of mineral nitrogen fertilization especially under sandy soils conditions which are naturally poor either in nutrient elements or organic matter through using alternative organic N fertilization as well as using biofertilizer which had illustrated greater nutrient use efficiencies of crops and in particular fruit crops when such inoculates were added to either organic matter or soil, Sangakkora and Weerakera, 1999 (as quoted by Saleh et al 2006). Accordingly, the present investigation was planned and conducted to evaluate the effect of combined application of mineral N and Farmyard manure (FYM) as organic N fertilizer with or without addition of Saccharomyces cerevisae (yeast) as a source of biofertilizer on leaf mineral content, yield and fruit quality as well as nitrate and nitrite Eman El-Moneim; Abd-Allah and Sanaa Ebeed Arab Univ. J. Agric. Sci., 16(2), 2008 452 content in fruit juice of Washington Navel orange trees grown under sandy soil conditions. MATERIALS AND METHODS The present investigation was conducted during two successive seasons 2004 and 2005 in a private citrus orchard located at El-Sadat district, Menofiya Governorate, Egypt on 15 years old Washington Navel orange trees. The trees were budded on Volkamer lemon (C. volkameriana) rootstock and planted at 5X5 meters apart under drip irrigation system. The texture of the soil is sandy as the result of soil analysis (Table 1). Also, farmyard manure analysis according to Wilde et al (1985) are given in Table (2). The selected trees were nearly uniform in vigor as possible. Table 1. Analytical properties of the soil at the experimental location aMechanical analysis Sand (%) Silt (%) Clay (%) Texture

EFFECT OF SOME ORGANIC AND BIOFERTILIZER TREATMENTS ON MINIMIZING MINERAL NITROGEN FERTILIZATION OF WASHINGTON NAVEL ORANGE TREES