Effect of extreme acid and alkali treatment on soil variable charge

This study concentrates on the effects of strong acid or alkaline treatments on subsequent titration curves of soil solid phase. Changes of soil mineral composition and organic matter content were estimated as a background. Samples of upper horizons of three brown, two chernozemic, one pseudopodzolic and three yellow-red soils were treated with hydrochloric acid or sodium hydroxide of concentrations ranging from 0.001 to 1 mol dm−3. Sodium homoionic forms of the original and the treated soils were back titrated to estimate variable charge vs. pH dependencies and surface acidity. The total amount of variable charge decreased after acid and alkali treatments of low concentrations and increased at elevated concentrations. The latter phenomenon was particularly evident for alkali-treated clay-rich soils. In general, surface acidity weakened in acidic treatments and for clay-poor soils in alkaline treatments. The alkali treatment produced stronger surface acidity in clay-rich soils. In yellow-red soils, surface groups of apparent surface dissociation constants around 10−4.5 were well distinguished. Amount of these groups decreased after acid treatment and sharply increased during alkali treatment. Marked increase of weakly acidic surface groups occurred after acid treatments. Long-term influence of pH on variable charge properties in natural soils can be similar to laboratory treatments, as evidenced by weaker surface acidity in soils of lower pH.