INFLUENCE OF ORGANIC MATTER ON MEAN SIZE OF CLAY MINERALS IN BASALT SOILS IN SOUTHERN BRAZIL

The interaction of organic compounds with clay minerals can change the size of crystals. However, in soils this effect is still unclear because of limitations in separation of pure mineral phases for conducting dissolution experiments. In this study, the relationship between soil organic matter (SOM) and mean crystallite size (MCS) of clay minerals from surface and subsurface horizons of soils from a toposequence in Southern Brazil was evaluated. Soil C and N contents were determined and the nature of the SOM functional groups were evaluated by FTIR spectroscopy. The MCS of minerals was assessed by X ray diffraction performed on Fe-oxide free samples and on Fe-concentrated samples. The C and N content and the relative intensities of the FTIR spectra were considered as predictor variables, and the MCS of hematite, goethite, kaolinite, and gibbsite as predicated variables. The C and N content and carboxylic and C-O-alkyl groups showed significant effects on the variation of MCS. The crystallite dimension of the Fe-oxides and kaolinite was inversely affected by these SOM properties. In contrast, the MCS of gibbsite was not affected by the SOM properties. The SOM influence on the MCS of Fe-oxides was attributed to surface complexation reactions and to surface redox reactions that promote a continuous dissolution-precipitation process.