Technical Report No: ND14-01 SODIC SOIL SWELLING AND DISPERSION AND THEIR IMPLICATIONS FOR WATER MOVEMENT AND MANAGEMENT

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 this research was to determine the influence of Na and EC on the amount of water retained at field capacity (33 kPa), in northern Great Plains tile-drained Na-affected soils. The impact of six EC levels on the amount of water retained in the soil at field capacity was determined in subsurface soil collected from four sodium-affected soils. Field capacity water (gravimetric water content) for all soils increased with increasing and decreasing sodium absorption ratio (SAR) and EC, respectively. For example, at an EC of 4 dS m, the amount of water retained at field capacity increased from 0.23 to 0.31 g g as SAR increased from 7 to 28, respectively. For the same soil, field capacity water decreased from 0.31 to 0.18 g g when EC increased from 0.5 to 15 dS m at SAR 24. In general, across all SAR values, an EC greater than 4 dS m 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; the presence of calcite in these soils may have reduced the potential for water retention and may have reduced field capacity values. 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. Introduction Many sodium-affected soils have low to moderate plant production potentials, depending on the location of the sodium-rich horizon within the profile. In the Northern Great Plains of the