Effect of topography and land use on genesis, chemical forms of Fe and Mn, and clay mineralogy of soils of Yasouj western plain

This study was performed to evaluate the effect of topography position and land use on soil genesis, chemical forms of Fe and Mn and clay mineralogy of soils of Yasouj region, Kohgilouye province. Nine soil profiles were described and sampled along a toposequence in paddy and non-paddy soils. The soils were classified as Alfisols, Mollisols, Inceptisols and Entisols. The major contributing soil processes can be categorized as calcium carbonate leaching, clay eluviations, accumulation of organic matter at soil surface and reduced condition in paddy soils. According to the results, the amount of pedogenic Fe and Mn (Fed and Mnd) and Fe crystallinity index [(Fed-Feo)/Fet] showed positive correlation with soil development and higher amounts of these indices were observed in plateau and piedmont plain physigraphic units with non-paddy land use. Compared to non-paddy soils aquic condition in paddy soils increased Feo, Mno, Feo/Fed and Mno/Mnd, (2.9, 2.8. 5.0 and 5.8 times, respectively); but decreased Fed, Mnd and Fe crystallinity index (1.69, 1.75 and 1.96 times, respectively). Clay minerals investigation revealed the occurrence of smectite, illite, chlorite, vermiculite, quartz and kaolinite in the soils studied. Transformation of illite and chlorite to smectite increased along the down slope as well as from subsurface to surface horizons. Transformation of micaceous minerals to smectite in non-paddy soils and neoformation from soil solution seems to be the major pathways for the pedogenic occurrence of smectite in the soils studied.