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Onion (Allium cepa L.) production in semi-arid area is primarily limited by fertilizer and water. In 2007/8 an experiment was conducted at Mekelle University, Northern Ethiopia to determine effect of N and P on yield and yield components of onion under different irrigation regimes. A split plot design was used. Two irrigation regimes (full and deficit) as main plot, factorial combination of four levels of N (0, 46, 92, 138 kg ha-1) and four levels of P (0, 20.1, 40.2, 60.3 kg ha-1) as a subplot with three replications were used. Statical Package Jmp 5.1 and M-STAT C was used as stastical tool. Climate, soil and crop characteristic data was used as input for BUdGET irrigation model and simulated with that. Agronomic data, phenological data, plant nutrional content was analysed. Most of the agromomic data were significantly affected by Nitrogen application, while Phosphorous was showed irregular reponse. The deficit irrigation resulted to high water use efficiency.

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