Pedological Significance of Lithological Discontinuity and its Effect on Various Land Use Systems in Upper Brahmaputra Valley Region of Assam.

Abstract

Lithological discontinuity (LD) is a common phenomenon in Brahmaputra alluvium derived soils of Assam under rice-fallow (rain-fed) as major land use system. The present study is aimed at investigating pedological significance of lithological discontinurty on various land use systems of Upper Brahmaputra Valleys, Assam. For this purpose, North West Jorhat Development Block, Jorhat district of Assam, was selected, which occupies afi area of 30,700 ha. The major landforms are older flood plains (OFP) followed by younger flood plains (YFP), active flood plains (AFP), marshes and swamps, channel fills and wet lands. Lithological discontinuity occurred between 25-50 cm in active flood plains, 50-75 cm in yonnger flood plains,75-100 cm in marshes and swamps and steadily disappeared below 100 cm in older flood plains. Lithological discontinuities within the soil horizons disappeared gradually from the North West to the South East direction, receding fi.lther away from Brahmaputra river bed. Soil-site suitability and yield gap analysis indicated that most of the field crops are marginally suitable except rice, tea and citrus (not suitable) in the soils of AFP. Lithological discontinuity in AFP begins below a depth of 25 cm from the surface soils and is continued with depth. These soils are of very recent origin with high sand fractions (70-90%) and very low silt (10-15%) and clay (5-10%) fractions in soils. They may have side slope> piedmont> nalrow valley due to loss of bases from upper reaches of the slope by gravity. Low soil pH has been observed in pine vegetation because it is known to create acidic conditions. High values of exchangeable aluminium percentage in most of the soils except paddy indicated poor base status of the soil. A significant positive correlation was observed between orgdnic carbon (OC) and total potential acidity (0.84 * * ) and significant negative correlation was found between pH and exchangeable acidity (- 0.90 * * ). This indicated that OC has strong influence on potential acidity while soil pH is more influenced by exchangeable acidity. Lime requirement on the basis of exchangeable Al was calculated for least Al tolerant crops for all the land uses except paddy. It ranged between 3.3 to 6.49 t/ha CaCO3with the least for tea and highest for vegetables. The study showed that due to less sub-soil acidity plantation crops can be recommended in the upper slope areas as compared to shallo-w rooted crop. Moreover, due to presence of high exchangeable Al, crops which are tolerant to the element should be adopted.