Evaluation of the Effect of Combined Use of Organic and Inorganic Fertilizers on Bulb Onion (Allium Cepa L.) Yields and Forecast Potential Onion Yields Under Climate Change Scenarios in West Ugenya Sub-county, Kenya

Abstract

Reduced crop yields in West Ugenya sub-county are attributable to poor soil fertility exacerbated by climate change. The combined use of organic and inorganic fertilizers is a potential solution to reduced yields and was thus evaluated against singular use of either organic or inorganic fertilizers on bulb onion yields in this study area. A randomized complete block design field experiment was carried out for two seasons during the 2015 and 2016 short and long rains. Four soil fertility treatments consisting of, namely: T1 (5 Mg ha-1 cattle manure), T2 (46 kg P ha-1 x 26 kg N ha-1 inorganic fertilizers), T3 (unfertilized control), and T4 (half T1 x half T2) were evaluated. Highest bulb yield in Mg ha-1 and largest bulb diameter of grade 1 quality (i.e. ≥ 5 cm bulb diameter) were recorded in T4 compared to the other treatments in both seasons, indicating that onion yields could be significantly (P ≤ 0.05) increased with combined use of 2.5 Mgha-1 cattle manure and 23 kg P ha-1 x 13 kg N ha-1 inorganic fertilizers in the study area. Projected climate change data for study area in the 2020 - 2039 period, derived from a global circulation model indicated that temperature will rise by 1.5oC affecting predicted future onion yields modeled by AquaCrop model version 5.0. Predicted future yields would drop by between 4 and 10% in all treatments where the crop growth period will be reduced (P ≤ 0.05) by an average of 15 days in all treatments compared to the baseline 1986 - 2005 period. Hence, combined use or organic and inorganic fertilizers can mitigate the declining soil fertility and increase onion yields in West Ugenya sub-county; however climate change will negatively affect future yields unless adaptive measures such as supplemental irrigation and mulching are implemented.