Growth, bulb yield and quality of onion (Allium cepa L.) as influenced by nitrogen and phosphorus fertilization on vertisol. II: Bulb quality and storability

An experiment was conducted to study the effect of different levels of nitrogen (N) and phosphorus (P) fertilizers on bulb quality and storability of onion (Allium cepa L.) grown on vertisol of Shewa Robit, North-East Ethiopia. Five rates of N (0, 69, 92, 115, 138 kg ha-1) and five rates of P (0, 10, 20, 30, 40 kg ha-1) were arranged in a randomized complete block design, replicated three times. Regardless of the rate, N fertilization decreased bulb dry matter content by about 4% over the control. Pungency measured as pyruvate concentration improved with increase in the rate of N application and reached the highest value of 2.72 µmol ml-1 at a rate of 138 kg N ha-1. Bulb storability study for eight weeks at ambient condition indicated that the highest level of N or P fertilizer caused the highest cumulative weight loss. It was also observed that bulb sprouting percentage of 63 and 53% were recorded from treatments that received 69 and 40 kg P ha-1,respectively. Nitrogen at the rate of 115 or 138 kg ha-1 resulted in about 2.9% rotting of the bulbs. N fertilization improved pungency, reduced bulb dry matter content and storability by enhancing sprouting and rotting percentage. Bulb dry matter content, pungency and rotting percentage were not significantly affected by P fertilization and the lack of response may be due to the availability of adequate amount of P (16.02 ppm) in the soil.   Key words: Bulb rotting, dry matter, pungency, sprouting, weight loss.