Subsurface Drain Tile Spacing Affects Soybean Grain Yield Variability in a Claypan Soil.

Claypan soils with less than one percent slope are poorly drained because of an argillic claypan layer 45 to 60 cm below the soil surface.  Field research was conducted near Bethel, Missouri to evaluate soybean (Glycine max [L.] Merr.) grain yields and plant populations above tile lines and 3.05 m distances from the tile lines for laterals installed at 6.1 and 12.2 m drain tile spacings.  Treatments included drainage only (DO) and drainage plus subirrigation (DSI) drain tile spacings (6.1 and 12.2 m) compared to non-drained (ND) and non-drained, delayed planting controls. The site was arranged as a split-plot design with four replications. In some years, sub-plots included multiple cultivars or fungicide/insecticide management systems. This resulted in 30 year-cultivar-management treatment comparisons from 2002 to 2015 that evaluated soybean response in low (< 3,360 kg/ha) and high (> 3,360 kg/ha) yield environments.  DSI at a 6.1 m spacing yielded 20% greater than the ND control.  Averaged over all of the 30 year-cultivar-management treatment comparisons, the highest yields (4,050 kg/ha) were observed above the 6.1 m DSI drainage tile line.  In drought years (2005, 2007, 2011, 2012, and 2013), grain yields above the tile lines were 43% greater than 3.05 m from the tile line, but yields were 89% greater than the non-drained control.  In high yielding environments, yields were greatest between the 6.1 or 12.2 m drain tile spacings. A narrower drain tile spacing may be needed to reduce yield variability in a claypan soil when yields are less than 3,360 kg/ha, but this was less evident in higher yielding environments. The cost-effectiveness of narrower subsurface drain tile spacings needs to be evaluated.