MICROMORPHOLOGIE DE SOLS BRUNS ACIDES SUR MATERIAUX GRANITIQUES

Abstract

The authors have studied brown acid soils on granites in Morvan (northeastern Massif Central). On granites, the fine material of A and B horizons results from: (1) the collapse of deeply weathered plagioclases altered in small secondary micas and in kaolinite; (2) the clay and silt clay coatings present in the B horizon and in the saprolith, (3) an intense comminution of micas, orthoclases and even quartz, and also plant remnants. On granites, vermiculite and hydroxy aluminous vermiculite increase considerably in the A and B horizons due to a rapid weathering of fine-textured micas, mostly biotites. Fine and silty material (mineral and organic) is randomly aggregated into small peds, 70–100 μm in size. These are called ocric aggregates. Often they are packed into secondary, blocky peds. The cement of ocric aggregates was assumed to be amorphous organic matter. All A and B horizons of brown acid soils have eluvial properties. On granites, such properties are weakly expressed, being limited to the edges of mica flakes. Clay and silt clay coatings can be found in a denser zone at the very base of the B horizon, but mostly as coatings in thinner voids of the saprolith. They can be found at great depth but the percentage of illuviated material in the saprolith is always low. In brown acid soils, comminution and weathering of mineral is more important than eluviation, thus preventing the development of an albic epipedon.