Soil Characteristics in Relation to Evergreen Forests of Brahmaputra Valley in Assam

Details of a preliminary soil study undertaken in some of the evergreen forests in Brahmaputra valley of Assam in 1960 are given. The soils are highly acidie deep alluvial and show certain differences in their chemical composition. However, the variations in the physical and morphological characteristics exhibited are of greater significance in influencing the growth and regeneration of important species. Thus, the coarse-textured soil with poor moisture regime and higher reaction carries mixed evergreen forest without Dipterocarpus macrocarpus, and Shorea assamica. On the other hand, the finer textured acidic soils having satisfactory drainage and good moisture retentive power support these evergreen species. Again, a soil with a sandy layer at the bottom restricting moisture retention or with impeded sub-soil drainage retards natural regeneration of D. macrocarpus. The highly clayey soil rich in organic matter, nitrogen and sesquioxides with higher moisture holding power appears to be more favourable for the development of Shorea assamica as compared to D. macrocarpus. In view of these striking observations a need for systematic studies on the soil-vegetation complex in this region bas been emphasized.