A comparison between three methods for the determination of cation exchange capacity and exchangeable cations in soils

The object of this study is to compare the results obtained with three standardized methods of determination of cationic exchange capacity (CEC) and exchangeable cations (Ca, Mg, K) in soils. The three methods are based on different exchange reagents: cobalt hexamine (Cohex) trichloride, barium chloride and ammonium acetate. Exchange procedures are different as well; they are, respectively, single extraction, successive extractions and percolation. Values measured with barium and Cohex as index cations are very comparable and reveal the uniformity of the physicochemical processes at play. Concerning these processes, the two methods can be considered as equivalent. Ammonium acetate, buffering the pH of the extracts, causes significant variations in the proportion of negatively charged sites and particularly those bonded to organic matter. This produces large divergences from CEC values obtained with the two other methods. Nevertheless, it is possible to reduce these differences for a given soil sample, by considering its pH and organic carbon content. Quantities of calcium and potassium extracted may also change due to the dissolution of carbonated species and ammonium ability for exchanging fixed potassium.