Title : Effects of irrigation criteria on soybean grain yield and irrigation water use efficiencies under subsurface drip irrigation in a silt loam soil
Drought stress and yield instability have been increasing due to climate change. Subsurface drip irrigation (SDI) is a system to water crops through buried lines having embedded emitters with certain spacing. The SDI systems are usually installed below tillage depth, which can be intended for multiple-year use. The SDI is considered to be a very efficient irrigation system in terms of crop yield and water use efficiency as well as automated irrigation management. We evaluated the effects of irrigation criteria based on soil water content on soybean grain yield and irrigation water use efficiency under subsurface drip irrigation in a silt loam soil. Irrigation treatments consisted of 3 soil water content (SWC) with 25-26%, 20-21% and non-irrigated plot. SDI driplines were installed at a depth of 0.4 m below ground and 1.2 m spacing below every second ridge. The dripper delivery rate of 1 L h-1 was operated under a pressure of 62-76kPa. Water was provided for10 minutes every 30 minutes from 08:00 to 18:00, from July 1, 2022 to October 23, 2022. Time domain reflectometry was installed at 0.3 m deep below soil surface of ridge and measured for the determination of volumetric SWC. The gross irrigation amounts at 25-26% and 20-21% SWC as irrigation criteria were 529.1 mm and 221.9 mm, respectively. The volumetric SWC at 25-26% soil water content irrigation ranged from 20.0% to 34.2% (25.0% at average), while at the non-irrigated plot ranged from 16.6% to 30.5% (21.2% at average). The differences in the SWC among the irrigation criteria increased as the days without rain got longer. The number of pod per plant significantly increased at 25-26% irrigation criteria compared to 20-21% treatment or non-irrigated plot. However, Soybean grain yield and irrigation water use efficiency were higher at 20-21% SWC irrigation than at 25-26%. The excessive irrigation might enhance vegetative growth, but delay maturing and result in increased incomplete grain. The irrigation criteria at 20-21% volumetric soil water content should be optimal for soybean production in silt loam soil.