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 hillt ops to poorly drained valleys is common on the backslopes of the Weste rn Ghats, Southern India. Detailed field observations along a represen tative toposequence developed from weathered schists, and local petrog raphic, mineralogical and chemical investigations are used to (i) link yellow, white and brown features to yellowing, bleaching and ferritis ation 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 sca le permits to differentiate an upslope ferralitic domain from a dowslo pe hydromorphic domain. In each of these domains, geometrical relation ships 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 iro n oxides resulted in yellowing of reddish soil material, Dissolution w hich affected first hematite then goethite was controlled by Al substi tution in the crystal structure of iron oxides. It has been reproduced in the laboratory using a chemical reducing agent. Yellowing is assoc iated with the onset of aquic conditions: yellow colours occur predomi nantly at the periphery of the hydromorphic domain. In the hydromorphi c domain, complete removal of iron oxides similarly resulted in bleach ing of yellow materials and was linked to waterlogging. Iron mobilized by yellowing and bleaching processes moved along established flow pat hs, and locally crystallized and concentrated in saprolite by ferritis ation. Bleaching and ferritisation occurred downslope and downward int o the soil to form the inner white-brown zone of the hydromorphic doma in. The amount of iron stored in low lying positions and the developme nt of weakly weathering conditions suggest that the internal drainage of the ground watertables was limited. This study cannot conclude if y ellowing, bleaching and ferritisation are still actively occurring, as no hydo-geochemical investigations were conducted along the toposeque nce.