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. soil analysis / cation exchange capacity / exchangeable cation / cobalt hexamine trichloride Rsum — Comparaison de trois mthodes de dtermination de la capacit d'change et des cations chan- geables des sols. L'objet de cet article est de comparer les rsultats obtenus au moyen de trois mthodes normalises de dtermination de la capacit d'change cationique des sols (CEC) et des cations changeables (Ca, Mg. K). Les mthodes examines utilisent des ractifs d'change diffrents : le chlorure de cobaltihexammine, le chlorure de baryum et l'actate d'ammonium. Les procdures d'change varient galement : ce sont, la simple agitation, les extrac- tions successives et la percolation. Les teneurs mesures avec le chlorure de baryum et le chlorure de cobaltihexam- mine sont troitement corrles et montrent l'uniformit des processus physicochimiques mis en jeu. De ce point de vue, les deux mthodes peuvent tre considres comme quivalentes. L'actate d'ammonium, en tamponnant le pH du milieu d'extraction, induit des variations importantes dans les proportions de sites lectrongatifs et en particulier, ceux lis la matire organique. La prise en compte du pH et de la teneur en carbone des chantillons nous a cependant permis de rduire les diffrences entre mthodes. Les quantits de calcium et de potassium extraites varient galement cause de la dissolution d'espces carbonates et l'aptitude particulire de l'ammonium s'changer avec le potassium.