Determination of Irrigation Scheduling and Optimal Nitrogen Fertilizer Rate for Onion in Tiyo District, Arsi Zone, South Eastern Ethiopia

Abstract

The study was conducted at the Kulumsa Agricultural Research Center for three consecutive years. The objectives of this study were to determine optimum irrigation levels and nitrogen rates and investigate their interaction effect on the yield and quality of onion crops. The experiment was arranged in a Randomized Complete Block Design (RCBD) split plot with three replications. The experiment utilized three deficit irrigation levels (100%, 75%, and 50% ETc) were paired with five nitrogen rates ranging from 0 to 184 kg ha-1. The study result of onion bulb yield revealed significant differences at (p<0.05). The highest marketable bulb yield (28.66 tons ha-1) was found at 100% ETc and 138 kg ha-1 N fertilizer rate, whereas the lowest yield (16.84 tons ha-1) was found with 50% ETc and no N fertilizer. Water productivity (WP) varied significantly across treatments, with a maximum of 10.9 kg/m³ observed at 50% ETc and 138 kg ha-1 N fertilizer rate, while the minimum (4.42 kg/m³) was recorded at 100% ETc and no N fertilizer. This result highlights the significant interaction between N fertilizer and water levels for onion growth. Yield response factors (Ky) highlighted the sensitivity of onion crops to water deficit. The highest Ky (1.38) was observed at 75% ETc with no N fertilizer. Conversely, the lowest Ky values (0 and 0.26) were observed at 100% ETc with 138 kg ha-1 N and 75% ETc with 138 kg ha-1 N fertilizer rate, respectively. The economic analysis results showed that the treatments receiving nitrogen rates of 46, 92, and 138 kg ha-1 combined with all irrigation levels were significantly profitable. Consistently these treatments surpass a Marginal Rate of Return (MRR) of 100%. Therefore, a nitrogen rate of 138 kg ha-1 combined with irrigation levels of 100% and 75% ETc looks to be a good option for maximizing onion output; nevertheless, interactions of 46 and 92 kg ha-1 nitrogen with all irrigation levels are economically viable in the research area.