Forms of iron in clay fraction of paddy and associated non-paddy soils of Assam

Eight soil profiles collected from paddy and associated non-paddy soils of Jorhat (P1, NP1), Golaghat (P2, NP2), Sivsagar (P3, NP3) and Dibrugarh (P4, NP4) districts of Assam were studied for different forms of iron (Fe) viz., total (Fet), dithionite extractable (Fed), pyrophosphate extractable (Fep) and oxalate extractable (Feo) in the clay fraction using standard procedures. Total (Fet), dithionite extractable (Fed) and pyrophosphate extractable iron (Fep) in the clay fraction were slightly higher in non-paddy soils as compared to paddy soils whereas oxalate extractable iron (Feo) showed a reverse trend. Dithionite extractable iron (Fed) formed major portion of total iron content in the clay fraction followed by Feo and Fep in both paddy and non-paddy soils. Crystalline iron oxide (Fed-Feo) and silicate iron (Fet-Fed) was found to be higher in non-paddy soils while amorphous inorganic form of iron (Feo-Fep) showed the reverse trend. Different forms of Fe in the clay fraction were higher in illuvial B horizons due to presence of higher amount of clay. The Fed/Fet ratio was found to be higher in non-paddy soils than that of paddy soils. The active iron ratio (Feo/Fed) was higher in the surface horizon as compared to other horizons of a profile. Different fractions of soil organic carbon and the clay content had strong influence on the distribution of forms of Fe in the clay fraction of soil.