Yellowing, bleaching and ferritisation processes in soil mantle of the Western Ghats, South India

A sequence of red through yellow to grey soils from well drained hilltops to poorly drained valleys is common on the backslopes of the Western Ghats, Southern India. Detailed field observations along a representative toposequence developed from weathered schists, and local petrographic, mineralogical and chemical investigations are used to (i) link yellow, white and brown features to yellowing, bleaching and ferritisation processes, and (ii) relate the enlargement of these features and the development of these processes to aquic and drainage conditions. Precise demarcation of soil morphological features at toposequence scale permits to differentiate an upslope ferralitic domain from a dowslope hydromorphic domain. In each of these domains, geometrical relationships expressed in terms of either concordance or discordance between horizon and colour boundaries are taken as arguments to point out that aquic conditions have expanded upslope in the landscape at some time in the past. As a consequence partial and selective dissolution of iron oxides resulted in yellowing of reddish soil material, Dissolution which affected first hematite then goethite was controlled by Al substitution in the crystal structure of iron oxides. It has been reproduced in the laboratory using a chemical reducing agent. Yellowing is associated with the onset of aquic conditions: yellow colours occur predominantly at the periphery of the hydromorphic domain. In the hydromorphic domain, complete removal of iron oxides similarly resulted in bleaching of yellow materials and was linked to waterlogging. Iron mobilized by yellowing and bleaching processes moved along established flow paths, and locally crystallized and concentrated in saprolite by ferritisation. Bleaching and ferritisation occurred downslope and downward into the soil to form the inner white-brown zone of the hydromorphic domain. The amount of iron stored in low lying positions and the development of weakly weathering conditions suggest that the internal drainage of the ground watertables was limited. This study cannot conclude if yellowing, bleaching and ferritisation are still actively occurring, as no hydo-geochemical investigations were conducted along the toposequence.