Two field experiments were performed aiming to assess the impact of irrigation with different types of water (Non-treated irrigation water and magnetically treated irrigation water) as main plots and exogenous application of some stimulants as sub plots at different rates ,i.e., 0.0,100,150 and 200 mg ascorbic acid  L^-1 and cobalt sulphate at rates of  20,40 and 60 mg Co L^-1  on the performance of onion plants during two successive seasons. The results indicated that onion plants irrigated with magnetically treated water possessed growth, yield and quality parameters better than that irrigated with non-treated water. Regarding foliar application, the control treatment gave the lowest performance compared to the other six treatments of both ascorbic acid and cobalt. On the other hand, the values of growth, yield and quality parameter under ascorbic acid treatments were better than that under cobalt treatments, where the performance significantly increased as the rate of ascorbic acid increased, while the performance under cobalt treatments significantly increased with increasing added cobalt from 0.0 mg L^-1 to 20 and 40 mg Co L^-1 and then significantly decreased at 60 mg Co L^-1, where the toxicity of cobalt  might appear on the plants grown at this rate. Also, the interaction between irrigation with magnetically treated water and foliar sprayed with ascorbic acid at rate of 200 mg L-1 gave highest significant  values of plant growth parameters, yield and quality parameters (Vit .C , TSS, dry matter , total sugar, fiber, Anthocyanin, pyruvic acid , carbohydrates and protein).