Shallow surface and subsurface drains alleviate waterlogging and salinity in a clay-textured soil and improve the yield of sunflower in the Ganges Delta

Waterlogging and salinity can occur together in salinised landscapes and restrict crop production. Drainage can alleviate waterlogging and salinity, but previous research suggested the need for deep drains, which may not be acceptable to smallholder farmers. Consequently, for the first time to our knowledge, we tested the usefulness of shallow drains in sunflower cultivation for smallholder farmers in a salt-affected, waterlogged coastal clay soil in the Ganges Delta. Experimental treatments were as follows: undrained, open surface drains (SD; 0.1 m deep, 1.8 m apart), slotted-pipe subsoil drains (SSD; 0.5 m deep, 4.5 m apart) and SSD+SD. All plots were inundated (2–3 cm above the soil surface) for 24 h before opening drains, at vegetative emergence and then at the V8 stage of plants. Relative to the most-drained (SSD+SD) treatment, the SD and SSD treatments gave 15–29% less yield, while the undrained treatment depressed yield by 48%. Soil water content (SWC) at 0–60 cm depth early in the season was 6–21, 4–10 and 3–5% less in SSD+SD treatment than in the undrained, SD and SSD treatments, respectively, while from flowering to harvest, SWC in SSD+SD was 2–4, 4–8 and 4–10% higher than in the undrained, SD and SSD treatments, respectively. In addition, soil electrical conductivity EC1:5 at 0–60 cm depth in SSD+SD treatment was 31–52, 16–38 and 11–32% lower than in the undrained, SD and SSD treatments, respectively. Across all treatments, the increases in yield due to drainage were associated with decreases in waterlogging (in the 0–30 cm layer) early in the growing season, increases in SWC late in the growing season and decreases in EC1:5 throughout the cropping season. While the shallow surface drains alone increased the yield, additional shallow subsoil drains further increased crop yield on coastal saline soils