Effects of nitrogen, phosphorus and potassium fertilization on growth, yield, quality and storage life of onion (Allium cepa L.) At jimma, southwestern Ethiopia

Onion (Allium cepa L.) is one of the most important vegetable crops produced in Ethiopia. Yield and productivity of the crop has been far below the national standards owing to several factors; absence of location specific fertilizer recommendation being the major among others. Thus, field and laboratoty experiments were conducted at Jimma University College of Agriculture and Veterinary Medicine Research field and department of post-harvest management laboratory during 2010/2011 in dry season to study the effects of Nitrogen (N), Phosphorus (P) and Potassium (K) fertilization on growth, yield, quality and storage life of irrigated onion under Jimma conditions, Southwestern Ethiopia. The treatments consisted of factorial combinations of four levels of Nitrogen (0, 50, 100 and 150 kg ha-1) as Urea, three levels of Phosphorus (0, 46 and 92 kg ha-1) as TSP and four levels of Potassium (0, 40, 80, and 120 kg ha-1) as Potassium sulphate which were laid out in a Randomized Incomplete Block Design with three replications. Data on growth, yield, quality and storage life parameters were recorded and analyzed using GenStat 12.1 version computer software packages. Results of the study revealed that; N, P and K had shown a highly significant difference on growth, yield and quality parameters like: plant height, leaf diameter, leaf length, number leaves per plant, leaf sheath length, bolters percentage, days to physiological maturity, harvest index, mean bulb weight, bulb length, bulb diameter, TSS (o Brix), DMC(%) and bulb shape index. Similarly, keeping quality of the onion bulbs like bulb sprouts (%), weight loss (%), weeks to 50% bulb sprouts and storage rots (%) were highly influenced by application of N, P and K at different levels. The highest total bulb yield per hectare (18.78 t/ha) was recorded with the plot that received the maximum combined applications of N-P-K (150:92:120kg ha-1) and on par with the results obtained in the combined applications of NP-K at 150:46:120 and 150:46:80kg ha-1 which were significantly superior to the rest of other treatments. Maximum (150kg ha-1) application of Nitrogen caused higher bulb rots (%), bulb sprouts (%) and weight loss (%) over the others while, maximum (120kg ha-1) application of potassium significantly decreased bulb rots (%), bulb sprouts (%), weight loss (%) and prolonged weeks to 50% bulb sprout during the two month storage time at ambient storage temperature and humidity. However, according to the partial budget analysis; the highest economic benefit level was obtained in the combined applications of N-P-K (150:46:80kg ha-1) whereas the lowest net benefit was obtained from the control treatment. This could be recommended for the uses by potential onion investors or farmers with high initial capital in the study area. Nevertheless, more researches are needed in different locations and on different soils in various season to come up with specific soil test based fertilizer recommendation.