A lyzimeter experiment was carried out to asses the most suitable form, rate and time of nitrogen fertilizer application and to follow up the potential pollution impact caused by excessive nitrogen fertilization. Corn (zeamaiz) (var. Giza 2) was cultivated and fertilized with ammonium sulfate (AS), urea (U), ammonium nitrate (AN) or ureaformaldehyde (UF)) at three rates of each (100,200,& 300 kg N/fed.), added either pre-planting or post-emergency. Higher application rate of N fertilizer induced an increase in NO3-N concentration in the drainage water during the first 5 weeks which exceeds the permissible limits (>10mgN/L), where it was more pronounced by using fast release N fertilizers particularly at preplanting addition. Increasing N fertilization rate for all nitrogen forms used and application time increased foliage dry matter yield. However, delaying fertilization was most effective in increasing grain yield. The efficiency of applied nitrogen was higher at the lowest application rate, however, efficiency of urea is less than that of other N sources. The efficiency of N was greater when it was applied post-emergency. Increasing N application rate raised protein content, and soluble N (nitrate) which was higher than the permissible limit for human and animal consumption (>2mg NO3-N/ g dry weight) as well as decreased carbohydrate in the grain. Nitrogen budget values were negative at post emergency application of the lowest N rate whatever its forms. Increasing N application rate raised N budget. The calculated N balance revealed that at the lowest rate of N fertilizer the highest proportion of N was taken up by corn crop, however, an increase in the application rate of N reduced its quantity utilized by the plants, and raised the amount remained in the soil to reach about 60% of N- inputs at the highest rate of application.  Finally, it could be concluded that the proper time of N fertilization was post-emergency and N rate must not exceed 200 kg N/fed for corn crop, where the higher rate can adversely reduce ground water quality and increase the risk of nitrate accumulation in plant tissue. The use of slow release N fertilizer could reduce the adverse effect of higher N application rates of other N- sources.