Impact of Cassava and Rubber on Laterisation Process in a Selected Laterite Terrain from Kerala

Cassava (Manihot esculenta Crantz) and rubber (Hevea brasiliensis) form the major land uses along the midland region of Kerala state where laterite soils predominate. This study was conducted to find out the effect of cassava and rubber on laterisation process at Kannur in Kerala state which is considered as the ‘type locality’ of laterite soils. Profiles were dug to a depth of 2 m from three land uses viz., cassava, rubber and uncultivated barren and soil samples were collected for various studies. The silt/clay ratio values of all pedons were below 0.25 indicating a high rate of ferrallitisation. The Bv layer of soils under uncultivated barren had significantly lower value compared to those of other two land uses. The impact of rainfall in soils under cassava is so high that the water dispersible clay migration is more here and hence more induration. The LOI value of the Bv layer was significantly lower compared to the respective overlying Bt layer in soils under cassava and uncultivated barren, and not in soils under rubber. The Bv layer of soils under rubber and uncultivated barren had significantly higher crystalline iron oxide than that of cassava and a significantly lower amount of AI substitution in Fe oxides in Bv layer of soils under rubber compared to that of cassava. No contribution of AI from sources other than Fe oxides was also observed in any of the pedons. From the study of physicochemical characteristics and iron oxide mineralogy of the profiles, it can be concluded that laterisation process and thereby induration occurs faster in soils under rubber compared to cassava as indicated by the values of clay illuviation, loss on ignition, crystalline iron oxide, AI substitution in iron oxides, etc.