A field experiment was conducted to investigate the effect of different levels of phosprouse (P) fertilizer on bulb yield and yield component of onion grown on clay soils at Wukro district, Tigray, Ethiopia under irrigation for two years. In this experiment six levels of P (i.e., 0, 46, 69, 92, 115, 138 kg P2O5 ha-1) were used. The experiment was arranged in a Randomized Complete Block Design with three replications. The result of this study revealed that application of P fertilizer significantly increased average bulb weight, marketable and total bulb yield of onion. However, other agronomic performances of onion were not significantly affected by P fertilizer application. The highest marketable (30.0 t ha-1) and total (31.2 t ha-1) bulb yield onion was obtained at P level applied at the rate of 138 Kg P2O5/ha1. The highest marketable and total bulb yield of onion increased by 29.0 and 30.2%, respectively compared to the control at this P rate. The highest net benefit (211833.5 birr ha-1) and marginal rate of return (1125.38 %) was obtained at P fertilizer rate of 138. It is concluded that application of P has significant and positive effect on bulb yield and yield component of onion . Hence, it is recommended to apply P: 138 kg P2O5 /ha as interim rate to attain optimum bulb yield, net income and return in the study area till further study on the P rate.

Keywords: Bulb, Bulb yield, Fertilizer, Marketable yield, Net income, Onion, Phosprouse