Influence of Nitrogen and Phosphorus Application on Bulb Yield and Yield Components of Onion (Allium Cepa L.)

Onion is an important cash crop for smallholder farmers in the Central Rift Valley of Ethiopia. However, the yield of the crop is low owing to a number of constraints out of which soil nutrient depletion and inappropriate soil fertility management practices are the most important ones.

A field experiment was carried out for two consecutive years of 2011 and 2012 using irrigation at Melkassa, Central Rift Valley Region of Ethiopia, to assess the response of onion to different levels of nitrogen and phosphorus fertilizers and to identify economical rates of the fertilizers for optimizing the yield and quality of the crop. The treatments consisted of five levels of nitrogen (0, 34.5, 69, 103.5 and 138 kg ha^-1) and four levels of phosphorus (0, 46, 92, and 138 kg P₂O₅ ha^-1). The experiment was carried out as a randomized complete block design in a factorial arrangement with three replications.

The results of the study revealed that the main effect of nitrogen significantly (P≤0.05) affected most of the growth parameters and bulb characters which all attained maximum values at 138 kg N ha^-1. The main effect of P significantly (P ≤ 0.05) influenced leaf number, leaf length, and bulb diameter. Nitrogen and phosphorus interacted to influence total and marketable bulb yields. Application of N at the rate of 103.5 kg N ha^-1 combined with 138 kg ha^-1 P₂O₅ led to the production of the highest total and marketable bulb yield of onion. However, results of the economic analysis revealed that application of N at the rate of 103.5 kg ha^-1 and P₂O₅ at 92 kg ha^-1 led to the highest net return.