Abstract
To show advantage and precaution of subsurface irrigation, we studied the effects of water table levels and their changes on shoot and root growth of onion. Onion plants were grown in light-colored andosol-filled stainless steel frame controlled water table at -30cm and -40cm with fewer amounts of surface watering (25% of normal precipitation). Control was irrigated with similar amount of normal precipitation. The -30cm water table treatment had higher water contents of surface soil (10cm) than those in the -40cm. In both water table treatments, water potential of each soil layer located from -15 to -35cm hovered higher than that in the control. Plants in higher water table had shallower root zone and smaller root length. More leaf fresh weight and maximum leaf length at harvest time was, -40cm > control > -30cm, and the difference in maximum leaf length seemed to arise during the bulb enlargement stage. On the other hand, no difference observed in those bulb yields among treatments. The -30cm treatment had higher bulb dry matter percentage than other treatments. At leaf development stage with elevated water table in pot slightly promoted leaf, bulb and root growth, while the treatment at the beginning of bulb formation stage decreased leaf and root growth. At bulb enlargement stage, the elevation of water level in root box began to necrotize root tip. These results showed that onion crops with lower water table than root zone could get bulb as much as weight of those with normal amount of precipitation. The elevation of water table especially after the beginning of bulb formation stage would inhibit root growth and result in smaller shoot.
