CLAY DISPERSION AND MACROAGGREGATE STABILITY AS AFFECTED BY EXCHANGEABLE POTASSIUM AND SODIUM

Domestic and industrial water-softening processes are among the major contributors to the Na content of waste water. Potassium salts can replace Na salts (mainly NaCl) successfully in many water softening treatments; such replacement leads to a decrease in exchangeable Na and to an increase to −12 in exchangeable potassium percentage (EPP). The objective of this study was to investigate the combined effect of K and Na on clay dispersion and macroaggregate stability in two high-charge smectitic soil types: a Typic Rhodoxeralf and a Typic Chromexerert. The soils were brought to equilibrium with solutions of differing SAR and PAR in the ranges of 0 to 6 and 0 to 4, respectively, such as would be expected in waste water obtained from sources where K salts replaced Na salts in water-softening treatments. The equilibrium studies showed that for a given sodium adsorption ratio (SAR) increasing the potassium adsorption ratio (PAR) resulted in a decrease in exchangeable Na. For SAR 6, increasing PAR from 0 to 2 led to decrease from 0.044 to 0.033 in exchangeable sodium ratio (ESR). In the clay dispersion studies, increasing exchangeable Na from 0.5 to 5.5 resulted in an increase in dispersed clay from 22.2% to 30.4% in the Typic Rhodoxeralf. A similar adverse effect of Na was noted in the macroaggregate studies. Conversely, increasing the EPP to 10 to 15 had no negative effect on either of the properties investigated in the two soils. The results indicate that (i) it is imperative to maintain as low a level of exchangeable Na as possible, in order to prevent sodicity hazards, and (ii) increasing soil EPP to 10 to 15 does not lead to a deterioration of those soil properties that affect soil structural stability and hydraulic properties.