Structural stability of Chernozemic soils as affected by exchangeable sodium and electrolyte concentration

Abstract

The stability of soil structure in the presence of exchangeable Na is an important factor determining the success of irrigation developments using sodic waters. Our objective was to determine the effects of sodium adsorption ratio [SAR = Na/((Ca + Mg)/2)0.5, where concentrations are expressed in mmolc L−1] and electrolyte concentration on saturated hydraulic conductivity (K) and on macroscopic swelling in a range of Brown Chernozemic soils from southern Saskatchewan. All soils showed the same general response to sodicity (SAR) and electrolyte concentration of the leaching solution, i.e., K decreased as SAR increased and as salt concentration decreased. However, major differences existed between the soils in their susceptibility to Na-induced structural deterioration. For example, at SAR 20, the electrolyte concentration needed to maintain stable structure ranged from about 5 to 30 mmolc L−1. Our results indicated that use of a generalized threshold concentration curve to partition stable from unstable structure would not be satisfactory for all irrigated prairie soils. Texture was a major source of variation between soils; the limits on acceptable irrigation water SAR should generally be decreased as clay content increases. Swelling and dispersion of soil clays both contributed to sodicity-induced K decline. Soil clays swelled appreciably when soil exchangeable Na percentage exceeded about 10. In contrast to swelling, which was relatively insensitive to electrolyte concentration, clay dispersion was only observed at low salt concentrations (≤ 20 mmolc L−1. In implementing irrigation water:soil compatibility guidelines, it will be necessary to identify soils which deviate substantially from general or average behavior because of their propensity to disperse or swell. Key words: Sodicity hazard, threshold concentration curves, clay swelling