Behaviors of Na-Affected Soils: Implications for Subsurface Drainage.

Recently, tile drainage has been installed in eastern North Dakota and western Minnesota to help lower the high water table and to remove soluble salts. However, the presence of excess Na in many of the soils within this region can negatively impact the performance of soil drainage. The objective of this research was to evaluate how Na, EC, and soil types, influence -33kPa soil water-holding capacity (field capacity water; metric used to determine soil swelling) before and after tile drainage. Soil series Exline (Fine, smectitic, frigid Leptic Natrudolls) was used for this study and sampling depths were 0-15, 15-30, 30-60, and 60-90 cm. Treatment solutions with sodium adsorption ratios (SAR) simulating the soil SAR_e and with successive six electrical conductivity (EC) solutions were prepared and applied to soils. Soils were saturated with each solution (SAR and EC combination) for 12 h and -33 kPa water content was determined using pressure plate after 48 h. Soil swelling was maximized when the concentration of Na, expressed as SAR, was high (SAR = 28) and the EC was low (EC = 0.5 dS/m), conditions that are typical of naturally occurring sodic soils in this region. The swelling was 12 to 42% greater at the lowest vs highest EC. This condition can also occur as salts are moved out of the soil profile in tile-drained systems. Although soil dispersion may also occur, soil swelling can greatly restrict water movement making the tiles to perform poorly. Understanding the effects of sodic or potentially sodic soils before installation of subsurface tile drains is highly recommended.