Onion is an important cultivated crop as a condiment as well as a source of income for many farmers in Ethiopia. However, the productivity of the crop is much lower due to different problems. A field experiment was conducted to determine the effect of seedling age and phosphorus rate on growth and yield performance of onion at Alage, Central Rift Valley of Ethiopia, during 2016/2017 season. The treatments comprised three seedling ages (6, 7 and 8 weeks of seedling age) and four phosphorus rates (0, 46, 92 and 138 kg P2O5ha-1). The experiment was laid out in randomized complete block design (RCBD) with four replications. The result showed that seedling age and phosphorus rate significantly affected plant height, leaf length, days to maturity, fresh bulb weight, bulb dry matter fraction, bulb length, marketable bulb yield, total bulb yield, and harvest index. Among these parameters, marketable bulb yield, total bulb yield and harvest index were also significantly affected by the interaction of seedling age and phosphorus rate. On the other hand, leaf number per plant, bulb diameter, total biomass yield and unmarketable bulb yield were only influenced by the effect of phosphorus rates. In this study, transplanting at 8 weeks of seedling age fertilized with 138 kg P2O5 ha-1 recorded the highest total bulb yield (50.6 t ha-1) and marketable bulb yield (48.33 t ha-1), but no significant difference was shown with that obtained at 92 kg P2O5 ha-1 with the same seedling age. Treatment combinations of seedling age at 6 weeks and no P (control) produced the lowest amounts of total bulb yield (24.27 t ha-1) and marketable bulb yield (21.63 t ha-1). The partial budget analysis revealed that the highest net benefit with low cost of production was obtained in response to the application of 92 kg P2O5 ha-1 and the transplanting age of 8 weeks. The marginal rate of return for this treatment was 5657% which is found to be economically feasible for producing onion in the study area.
Key words: Onion, seedling age, phosphorus, bulb yield, partial budget analysis.
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