SOIL TYPE AND IRRIGATION AFFECT ONION YIELD AND STORABILITY

Although water stress during growth has been shown to reduce yield (Bekele and Tilahun, 2007; De Santa Olalla et al., 2004), there has been some work that suggests under certain conditions a preharvest water stress may increase the postharvest storage life or quality of onions (Leskovar and Agehara, 2012). However, deficit irrigation as an irrigation treatment has not yet been taken up by growers due to the lack of control, limited information and the risk of lower yields. The aim of this work was to investigate the sensitivity of onions to deficit irrigation, and to determine whether deficit irrigation can be used to increase onion marketable yield and storability. An onion crop was grown in a polytunnel at Harper Adams University; soil was amended with peat to give two defined uniform soil types; 25% (high OM soil) or 5% (low OM soil) in containers of 180 L volume. By alternating the timing and amount of irrigation, a range of different water stress treatments were imposed. This work has shown that the water status of the top 30 cm of the soil column is important for onion growth and that increasing irrigation can increase bulb yield. The effect of the organic matter (OM) amendments was not homogenous across all plant measurements taken. Overall, onions grown in the high OM soil had the least sprouting after 3 months in storage in all irrigation treatments, whilst the well watered irrigation treatment proved the best treatment overall to reduce postharvest sprout length. There was no effect observed for either soil organic matter content or irrigation treatment on the formation of multi growth centres in onion bulbs. This work suggests that deficit irrigation at the intensities used in this study is not suitable to increase storability of onions with regards to early sprouting.