An experiment was conducted to investigate the effect of different levels of nitrogen (N) fertilizer on the bulb yield and yield components of onion grown on a clay soils of Wukro district, Tigray, Ethiopia under irrigation for two years. In this experiment six levels of N (i.e., 0, 23, 46, 69, 92, & 115 Kg/ha) were used. The experiment was arranged in a randomized complete block design with three replications. The recommended phosphorus (P) 92 P2O5 Kg/ha rate for onion was applied as TSP to all treatments equally. Application of N fertilizer significantly increased average bulb weight as well as, marketable, unmarketable, and total bulb yield of onion. However, days to physiological maturity, plant height, and bulb diameter and bulb length were not significantly affected by N fertilizer application. The highest marketable (22.79 t/ha) and total tuber yield (24.1 t/ha) was obtained at N level applied at the rate of 115 kg /ha, increasing the total bulb yield from 19.31 t/ ha in the control to 24.1 t /ha. The highest total and marketable bulb yield increased by 25 and 21 %, respectively compared to the control at this N rate The highest net benefit and marginal rate of return (2450%) was also obtained at N fertilizer application rate of 115kg/ ha N . It is concluded that application of N has significant and positive effect to bulb yield and yield components of onion in the study area. Hence, it is recommended to apply N: 115 kg/ha combined with 92 kg /ha P2O5 in the study area in order to attain optimum bulb yield, net income and return.

Keywords: Bulb, Fertilizer, Marketable yield, Nitrogen, Onion, bulb yield, net income