Effect of using subirrigation and slit tillage to increase soybean (Glycine max) yield in clayey soils in rice paddies converted to uplands

Water stress reduces soybean (Glycine max) yield in fields converted from clayey-soil rice (Oryza sativa) paddies to uplands (upland converted paddies [UCPs]). Subirrigation and/or allowing roots to penetrate to a deeper soil layer were expected to effectively mitigate water stress. Therefore, we examined the effects of subirrigation and slit tillage on mitigating water stress in clayey soils (52% clay) in UCPs. Four treatment experiments (subirrigation, slit tillage, subirrigation control, and slit tillage control) were conducted in each of three growing seasons, with sampling for yield analysis conducted in triplicate. Following conventional tillage, a slit tillage blade was used to cut 15-mm-wide vertical slits in the surface of the soil (cutting depth = 15–20 cm) and seeds were sown 15 mm from the cut lines. Subirrigation then occurred for approximately 21–47 days, from the end of the rainy season, when water table elevation was at 35 cm below ground. We found that subirrigation only resulted in improved soybean yield in 2012, when precipitation after the flowering stage was low, whereas slit tillage significantly increased yield in all three growing seasons, including a particularly wet growing season. Overall, subirrigation and slit tillage increased soybean yield by 2% and 13%, respectively. Slit tillage also resulted in a significantly greater taproot length and root density. Stomatal conductance at midday in about the R5–6 stages was higher in slit-tilled plots than in control plots, but only in the subirrigated field.