Charge Distribution and the Cation Exchange Capacity of an Iron‐rich Kaolinitic Soil

Summary The positive charge on an iron‐rich kaolinitic soil was found to increase from 11.1 to 14.2 me per 100 g as a result of lowering the pH from 8.5 to 3.0 in M NH 4 Cl solution and the negative charge decreased from 22.2 to 14.1. In 0.002M NH 4 C1 the positive charge increased from 0 to 0.3 and the negative charge decreased from 5.6 to 3.2. The variations in charge can be explained by the presence of 3 me per 100 g of permanent negative charge, pH dependent positive and negative charge influenced by the indifferent electrolyte concentration, and the overlap of diffuse layers causing a neutralization of charge. The ability of this soil to hold cations under field conditions is overestimated by m ammonium acetate at pH 7. Instead, a solution approximating to the pH, composition, and concentration of the soil solution should be used.