Author(s):

Trishna Sarmah, Samiron Dutta, RM Karmakar and Danish Tamuly

Abstract: A study was carried out to investigate the profile distribution of phosphorus and its relation with pedogenesis of some alluvium derived soils of the Brahmaputra Valley of Assam. Three different locations of Jorhat district starting from Borholla at the extreme South of the District, at Koronga in midpoint and Alengmora located near the bank of Brahmaputra were selected for the purpose. From each area, two profiles were exposed, one from rice-ecosystem and another from adjacent non-rice ecosystem. Soil samples were collected from six profiles P1(Rice ecosystem, Borholla), P2((Non-rice ecosystem, Borholla), P3(Rice-ecosystem, Koronga), P4(Non-rice ecosystem Koronga,), P5(Rice-ecosystem, Alengmora) P6 (Non-rice ecosystem, Alengmora) with twenty seven numbers of horizons. The soils are dark brown to brownish yellow in colour with a dominant hue of 10YR. value ranged from 4 to 6,while chroma varied widely from 2 to 8 and varied widely in texture (sandy loam to clay), structure (single grain to moderately strong sub-angular blocky), clay content (7.5-44.5%), pH(4.3-6.9)was lower in the surface horizon and it tends to increase with soil depth and surface horizons of all the pedons recorded the highest organic carbon (OC) content(0.1-1.81%) within a profile and it decreased regularly with the soil depth except in P5 and P6 where an irregular depth distribution pattern was observed. Cation Exchange capacity (CEC) of the soils ranged from (4.7-12.3)and the bulk density ranged from(1.31Mg m^-3-1.81 Mg m^-3)which first increased with depth in P1, P2, P3 and P4 and thereafter it showed either a decreasing trend or irregular pattern. But it showed a regular increasing pattern with depth in P5 and P6. Different acidity components which includes the values of Exchange acidity that ranged from (0.50 to 3.25) cmol (p⁺) kg^-1. The values of exchange acidity were lowest in the surface horizon of P1 to P4 which increases with depth but in P5 and P6 it decreases with depth of the profile where total potential acidity was found higher in the surface of P1, P2 and P5 and thereafter it decreased with depth. The rest of the soils did not show any definite depth distribution pattern and similar kind of distribution trend was also observed in pH dependent acidity of the soils which varied from 4.00 to 11.05 cmol (p⁺)kg^-1. The soils were classified in the order Alfisols (P1 and P2), P2 (Non Rice) as Inceptisols, (P3 and P4) and as Entisols (P5 and P6) respectively.

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