Effect of pH on the development of acidic sites in clayey and sandy loam Oxisol from the Cerrado Region, Brazil

Titration curves were determined for soil from horizon samples of a clayey and a sandy loam Oxisol by (a) adding NaOH to soil suspensions and (b) incubating moist soils with Ca(OH)2. The organic fraction was primarily responsible for buffering in both soils. Humic acids were more important than fulvic acids in buffering against NaOH additions. With Ca(OH)2,greater buffer capacities were found due to carboxyl sites, primarily on fulvic acids, becoming complexed with Ca2+, so that in the clay soil humic and fulvic acids were equally important as buffering components while fulvic acids were more important in the sandy loam soil. The buffer capacity of organic matter against Ca(OH)2 additions was 1.1 molc kg− 1 pH− 1. In the incubated soils, exchangeable cations were also determined and changes in the amounts of exchangeable and non-exchangeable Ca2+, acidity and effective cation exchange capacity were calculated. Up to half the added Ca2+ became complexed and was non-exchangeable. Aluminum complexed by organic matter appears to be an important buffering component, together with non-exchangeable H+. With the increase of pH the dissociated sites from the carboxyl groups could complex Ca2+.