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Subsurface-tile drainage is designed to remove gravitational water and soluble salts from the soil-root zone. However, soil swelling, as influenced by soil Na and electrical conductivity (EC), will reduce saturated hydraulic conductivity. The objective of the experiments reported in this paper was to determine the influence of Na and EC on the amount of water retained at field capacity (− 33 kPa) in northern Great Plains Na-affected soils. Field capacity water (gravimetric water content) for all soils increased with increasing sodium adsorption ratio (SAR) and decreasing EC, respectively. For example, at an EC of 4 dS m − 1 , the amount of water retained at field capacity increased from 0.23 to 0.31 g g − 1 as SAR in the treatment solution increased from 7 to 28. For the Exline soil at 30–60 cm depth, field capacity water decreased from 0.31 to 0.18 g g − 1 when EC increased from 0.5 to 15 dS m − 1 at SAR 24. In general, across all SAR values, an EC greater than 4 dS m − 1 was required to prevent swelling. However, for soils with high natural salinity, no significant difference was observed for field capacity water using the above methods; high salt content and the presence of calcite in these soils may have reduced the potential for water retention and may have lower field capacity. Therefore, to maintain drainage performance in sodium-affected soils one should regularly monitor Na and EC within the soil profile so that EC values do not fall below critical threshold values. [ABSTRACT FROM AUTHOR]