STRUCTURAL STABILITY OF CHERNOZEMIC SOILS AS AFFECTED BY EXCHANGEABLESODIUM AND ELECTROLYTE CONCENTRATION

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 sod ium adsorption ratio [SAR = Na/((Ca + Mg)/2)0.5, where concentrations are expressed in mmol(c) L-1] and electrolyte concentration on saturat ed hydraulic conductivity (K) and on macroscopic swelling in a range o f Brown Chemozemic soils from southern Saskatchewan. All soils showed the same general response to sodicity (SAR) and electrolyte concentrat ion of the leaching solution, i.e., K decreased as SAR increased and a s salt concentration decreased. However, major differences existed bet ween the soils in their susceptibility to Na-induced structural deteri oration. For example, at SAR 20, the electrolyte concentration needed to maintain stable structure ranged from about 5 to 30 mmol(c) L-1. Ou r results indicated that use of a generalized threshold concentration curve to partition stable from unstable structure would not be satisfa ctory for all irrigated prairie soils. Texture was a major source of v ariation 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 declin e. Soil clays swelled appreciably when soil exchangeable Na percentage exceeded about 10. In contrast to swelling, which was relatively inse nsitive to electrolyte concentration, clay dispersion was only observe d at low salt concentrations (less-than-or-equal-to 20 mmol(c) L-1. In implementing irrigation water: soil compatibility guidelines, it will be necessary to identify soils which deviate substantially from gener al or average behavior because of their propensity to disperse or swel l.