Determination of clay dispersion and flocculation in sodic soil as influenced by soil: Solution ratio

Abstract

Knowledge of the factors affecting clay dispersion and its counterpart, flocculation, is fundamental to understanding the behaviour of sodium (Na)‐affected soils. We evaluated the effect of soil: solution ratio on clay dispersion and flocculation with two Brown Chernozems (Haploborolls) that had been rendered sodic by pretreatment with sodium chloride‐calcium chloride (NaCl‐CaCl₂) solution with sodium adsorption ratio [SAR = Na/√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). The 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 flocculation value (electrolyte concentration needed to flocculate clay) decreased as soihsolution ratio increased, particularly when flocculation was evaluated after short settling periods. This is consistent with flocculation theory which predicts an increase in the rate of flocculation as the number of colloidal particles in suspension increases. There may be advantages to using narrow soiksolution 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 with low clay concentrations are used.