Department of Soils, Punjab Agricultural University, Ludhiana, Punjab
*Present address: Assam Agricultural University, Regional Research Station (ICAR Research Complex), Diphu, Assam
The distribution, size and shape of light and heavy minerals (in sand fraction of 50–100 microns) from eight pedons developed under different climatic and topographic positions in the southern bank of the Brahmaputra river in Assam indicate the origin of the alluvium from a variety of rocks, viz, igneous, metamorphic and sedimentary occurring in Assam Plateau and Assam Himalayas. The mixed mineralogy of the Brahmaputra alluvium, dominated by the resistant minerals, like quartz, zircon and weatherable minerals, like mica, feldspar, chlorite, etc., is an expression of weathering from parent rocks. The decrease of resistant and a corresponding increase of weatherable minerals with depth in the pedons in upland is attributed to higher rate of surface weathering.
The weathering mineral ratios (wrl=Q/F,wrh=Z+T/H+A) are observed to be the lowest in soils developed on the recent alluvium (near the Brahmaputra river) and the highest in soils on the older alluvium (towards the Assam Plateau), establishing thereby the influence of time on the degree of weathering. The weathering intensity of the upland soils, under different climatic environments follows the sequence: west > east > middle region.
The use of sand/silt ratio, in conjunction with the weathering ratio in both the light and heavy mineral fractions, has been found to be imperative for establishing the lithologic discontinuity in the alluvium-derived soils of the Brahmaputra valley.
Sand mineralogy, alluvium-derived soils, weathering ratio, weathering intensity, sand/silt ratio, lithologic discontinuity
