There is a recent tendency to use organic and slow release fertilizers in crops as a natural and safe substance. Two field experiments were carried out during winter seasons of 2008/09 and 2009/10 at Sakha Agricultural Research Station, Kafr El-Sheikh Governorate, Egypt, to compare the effect of different sources of nitrogen fertilizer: 6 tons compost/fed, 8 tons compost/fed, 10 tons compost/fed, 30 kg N/fed, as Enciabein (Enc., a commercial trade name of slow release N fertilizer), 60 kg N (Enc.)/fed, 90 kg N (Enc.)/fed and 90 kg N/fed as ammonium nitrate on growth, yield, quality, macroelements contents and storability of onion bulb cultivars (Giza 20 and Giza Red).The results showed that Giza Red onion cultivar had the highest means of vegetative growth i.e.(plant height, No. of leaves/plant, bulb diameter, neck diameter, bulb fresh weight and plant fresh and dry weight at 90 and 120 days after transplanting , marketable and total bulbs yield/fed, average bulb weight and bulb diameter compared with cv. Giza 20. ِ Application of 90 kg N/fed as mineral source produced the highest and significant values of plant height, number of leaves, neck diameter, bulb diameter, bulb fresh weight, plant fresh weight and plant dry weight. Application of 30 kg (Enc. /fed) or 6 t/fed compost resulted in the highest and significant dry matter content of bulbs.Giza Red cv. produced the highest and significant bulb fresh weight, plant fresh and dry weight, number of leaves, neck diameter, bulb diameter, bulb fresh weight, plant dry weight, marketable yield/fed, total yield/fed and average bulb weight under the application of 90 kg N/fed as mineral source, but it gave the lowest culls yield/fed. Meanwhile, application of 30 kg N (Enciabien) or application of 6 t/fed compost significantly increased bulbs dry matter content.Giza Red cv. with 90 kg N/fed (mineral) produced highest and significant marketable yield, total yield and average bulb weight, but it significantly decreased culls yield/fed. On the other hand, Giza 20 cv. with 30 kg N (Enciabien) produced highest and significant total soluble and dry matter content.Giza 20 cv. had the lower and significant P-content with the best of storability after 180 days storage period.
