THE RELATIONSHIP BETWEEN pH AND EXCHANGEABLE SODIUM IN A SODIC SOIL

Sodic soils formed under the influence of sodium carbonate have a distinct geographic distribution and physicochemical properties. The pH of calcareous sodic soils can be defined by the equilibrium partial pressure of carbon dioxide and alkalinity. The high pH and exchangeable sodium ratio (ESR) of calcareous sodic soils are primarily due to high alkalinity in low pCO2-salt environments. A theoretical expression based on Na-(Ca + Mg) exchange equilibrium in sodic soils is developed to quantitatively relate exchangeable sodium ratio with pCO2, pH, sodium concentration, and ionic strength. For calcareous sodic soils containing sodium carbonate, the pH of the system adequately describes the ESR and, therefore, the exchangeable sodium percentage (ESP) of the soil.