CLAY DISPERSION IN RELATION TO SODICITY, ELECTROLYTE CONCENTRATION, AND MECHANICAL EFFECTS

Clay dispersion is one of the major processes causing structural problems in lands irrigated with sodic waters. The dispersion characteristics of a group of prairie soils (Haploborolls) from southern Saskatchewan (Canada) were determined after reaction with solutions that had a range of sodium adsorption ratios [SAR = Na/(Ca/2)1/2, where concentrations are expressed in mmol(c) L-1], and electrolyte concentrations. When the input of mechanical energy was high (i.e., 16-h shaking), a well-defined threshold electrolyte concentration existed at each SAR, below which extensive clay dispersion occurred. The threshold concentration for dispersion ranged from 1 to 3 mmol(c) L-1 at SAR 0 and increased linearly as SAR increased to reach 21 to 50 mmol(c) L-1 at SAR 40. In contrast, when dispersion was measured after minimal shaking by hand, distinct thresholds were absent. Instead, dispersion increased gradually at each SAR as electrolyte concentration decreased. Threshold concentrations determined with low-energy inputs are likely to be most indicative of field dispersibility. Such threshold values can be substantially lower than those measured using vigorous agitation techniques, particularly when sodicity levels are low. Our results do not support recent reports that silt dispersion is a better indicator of structural stability in the presence of exchangeable Na than is clay dispersion. Such findings are probably the result of an experimental artifact caused by the inappropriate use of Stokes' law in estimating the particle size distribution of soil suspensions. Simple tests of clay dispersibility in water or of aggregate stability may be useful in screening soils for sensitivity to sodic irrigation waters.