SOIL ALKALINITY. I. EQUILIBRIA AND ALKALINITY DEVELOPMENT

Abstract

Summary: Theoretical relationships between pH, CO₂ partial pressure and alkalinity (bicarbonate + carbonate concentrations) have been shown to apply to solutions and calcite and soil suspensions.The exchange of Na onto three Ca clays shows that Ca is preferred but with negative free energies of exchange. With decreasing total electrolyte concentration, the preference for Ca increases, so that only when the concentration is above 10^‐3M will significant amounts of exchangeable Na be found in soils. The preference for Ca is illite > montmorillonite > vermiculite.Dilution of the mixed Na‐Ca clay suspensions causes exchange, desorption of Na and. in some cases Ca. This desorbed or ‘alkaline’ Na (and Ca) is replaced by H some of which attacks the clays. The apparent hydrolysis coefficient, KG = H adsorbed (Na)/Na adsorbed (H), varies between 5 and 9 × 10⁵, increases with increase in electrolyte concentration and varies in the order vermiculite > illite > montmorillonite. The concentration of alkaline Na + Ca increases with increasing ESP, and with clay type in the same order as the KG values. The pH calculated from the alkaline Na + Ca, assumed equal to the alkalinity was equal to measured values except for montmorillonite where the calculated values were low. Magnesium release by acid attack of the montmorillonite may explain the differences.Vermiculite rich soils will be most likely to accept exchangeable Na and to hydrolyse and develop alkalinity.