Behaviour of the clay fraction during desiccation in microaggregated Ferralsols from Brazil: role of microstructure and organic matter.

The physical properties of the microaggregates of Ferralsols are still unde r discussion. Two sites were selected for study: in each site, one soil und er a native savannah (Cerrado) and another one under pasture were studied. Samples were analysed in the laboratory by using scanning electron microsco py and mercury porosimetry and by measuring the water content at -0.1 and - 1.5 MPa potential. Results showed the presence of microaggregates 50 to 300 mum in size and bigger ones 500 to 1000 mum in size. The latter were often packed in dense volumes 2 to 20 mm in size, which were numerous in the top soil under pasture. These dense volumes are casts and would result from an imbalance in the faunal population as a consequence of deforestation. Resu lts showed also a shrinkage of the clayey phase for drying and this shrinka ge increased with the carbon content of the soil. These results showed that microaggregates are not rigid fabrics and the significant role of organic matter in physical properties of the soils studied. (C) 2001 Academic des s ciences / Editions scientifiques et medicales Elsevier SAS.