This experiment was carried out for two seasons in 2007/2008 and 2008/2009 at the Experimental Farm of Sakha Agricultural Research Station, ARC, Kafr El-Sheikh, Gevernorate, Egypt. The objective of this experiment was to study the effect of four times of potassium application i.e., 100% before transplanting (BT), 50% BT + 50% at 30 days after transplanting (DAT), 50% K at BT + 50% K at 60 DAT and 50% K at 30 DAT + 50% K at 60 DAT.  Three urea  rates as foliar spray i.e. 2, 4, and 6 g/l on vegetative growth at 90 and 120 DAT, yield and yield components, quality and storability of onion. The main important findings were as follows: 1- Plant height, bulbing ratio, plant fresh weight and plant dry weight recorded maximum mean values by adding 50% of potassium fertilizer at 30 DAT and 50% at 60 DAT at two growth stages in both seasons, except for plant height at 120 DAT in the first season and bulbing ratio at 90 DAT in both seasons. 2- Adding 50% of potassium at 30 DAT and 50% at 60 DAT attained the highest average of bulb weight, marketable yield/fed., culls yield/fed. and total yield/fed. While, adding 50% of potassium before transplanting and 50% at 60 DAT attained the lowest ones in both seasons. 3- Increasing urea rate as foliar spray from 2 to 4 or 6 g/l significantly increased mean values of plant height, plant fresh weight and plant dry weight at two growth stages, in both seasons. 4- Spraying onion plants with 6 g/l of urea attained of average bulb weight, marketable yield/fed., culls yield/fed. and total yield/fed. While spraying onion plants with 2 g/l gave lowest ones of all studied characters, except for culls yield/fed. in the second season. 5- Results showed that highest percentages of TSS% and dry matter% were obtained by adding 50% of potassium before transplanting and 50% at 60 DAT, while the smallest ones were obtained by adding 50% of potassium at 30 DAT and 50% at 60 DAT in both seasons. 6- Increasing urea as foliar spray  rate from 2 g/l to 4 or 6 g/l decreased mean values of TSS% and dry matter % in both seasons. 7- Results indicated that highest average of bulb weight, marketable yield/fed. and total yield/fed. were achieved by adding 50% of potassium at 30 DAT and 50% at 60 DAT when plants were sprayed with 6 g/l of urea in both seasons. It could be concluded that adding 50% of potassium at 30 DAT and 50% at 60 DAT and spraying onion plants with 6 g/l of urea.could be recommended for optimum onion yield per unit area.