Surface charge of clay-humus fractions from Chernozems as a function of pH and Ca-concentration

The charge characteristics of clay-humus fractions from A-horizons of several Chernozems as a function of Ca concentration and pH of the soil solution was investigated. The surface charge was measured by titration with charge compensating polyelectrolytes; the endpoint was at zero potential. A logarithmic dependence of the surface charge [mol(c)/kg] on the Ca concentration of the soil solution in the range less than or equal to 2 mmol/L was observed. The influence of pH on the surface charge was almost linear over a wide range of pH-values, the change in pH-dependent charges being most pronounced for the horizons with low Ca saturation on the exchange sites. It was also found that the competition between protons and Ca ions for the exchange sites leads to a distinct decrease of the influence of Ca concentration on the surface charge in acid environment. The reason for slaking of the soil surface and of the displacement of clay-humus particles in Chernozems could be attributed to an increase in surface charge with decreasing Ca concentrations after leaching of lime. According to the present findings the drop in mobility of the clay-humus fractions during acidification could be explained by a charge compensating effect of protons and aluminium ions.