Mineral and organic fractions of two oxisols and their influence on effective cation-exchange capacity

The mineral and organic fractions of a clayey Red Yellow Latosol (Typic Acrustox) and a sandy loam Dark Red Latosol (Typic Haplustox) sampled under the Cerrado Region of Brazil were characterized, and their influence on the effective cation-exchange capacity was evaluated, Kaolinite and sesquioxides dominate the clay fraction of both soils, but the clayey soil is at a more advanced stage of weathering, It also holds a larger amount of organic matter and has more fulvic acid than humic acid throughout the profile. In the sandy soil, the reverse applies to the surface horizon, with equal amounts of humic and fulvic acids in lower horizons. More humic and fulvic acids were extracted from both soils by NaOH than by Na4P2O5, with CaHPO4 and water extracting similar small amounts of C. The total acidity, the carboxyl groups, and the phenolic-OH groups were measured in humic and fulvic acids. The humic acid was about half as reactive as the fulvic acid in both soils. Fulvic acid is therefore the main source of charge in the clayey soil, but in the surface horizon of the sandy soil, humic acid contributes to more charge. The soils have low effective cation-exchange capacity values, which decrease with depth and depend primarily on the content of organic matter, as shown by regression and multiple regression analysis.