DETERMINATION OF CLAY DISPERSION AND FLOCCULATION IN SODIC SOIL AS INFLUENCED BY SOIL-SOLUTION RATIO

Knowledge of the factors affecting clay dispersion and its counterpart , flocculation, is fundamental to understanding the behaviour of sodiu m (Na)-affected soils. We evaluated the effect of soil: solution ratio on clay dispersion and flocculation with two Brown Chernozems (Haplob orolls) that had been rendered sodic by pretreatment with sodium chlor ide-calcium chloride (NaCl-CaCl2) solution with sodium adsorption rati o [SAR = Na/root Ca/2, with ionic concentrations in mmol(c)/L] of 20. The proportion of total clay dispersed by mechanical shaking more than doubled as soil:solution ratio increased from 0.5:30 to 4:30 (w/v). T he effect of soil:solution ratio on clay dispersion was attributed to greater shattering and abrasive action of soil aggregates against each other as their concentration in suspension increased. The flocculatio n value (electrolyte concentration needed to flocculate clay) decrease d as soil:solution ratio increased, particularly when flocculation was evaluated after short settling periods. This is consistent with flocc ulation theory which predicts an increase in the rate of flocculation as the number of colloidal particles in suspension increases. There ma y be advantages to using narrow soil:solution ratios (e.g., 1:10 w/v) to determine flocculation values because the choice of settling period is less critical than in the traditional approach where suspensions w ith low clay concentrations are used.