Saturday, 15 July 2006

Soil Evolution as a Function of Parent Material in Some Forest Areas of Northern Iran.

Hassan Ramezanpour, Raheleh Hesami, and Mohammad Ali Zanjanchi. Guilan Univ, Rasht , Gazvin-Rasht Highway, Faculty of Agriculture, Rasht, Iran

Parent material influences soil property such as clay mineral and chemical composition. Chemical weathering of soils and rock during pedogenesis is significant because it provides many essential elements .In this research , 6 representative soil pedons from 6 moutains in lahijan area comprising granite, andesite, basaltic-andesite , phyllite and limeston parent material were selected , sampled and analysed. The most outstanding pedological process and soil development was marked by a progressive increase in clay content and solum thickness, a decrease in silt/clay ratio, increase in free iron content (which was associated with maxima of clay content) and relatively high fine clay/total clay ratio in Bt. The net result of pedogenic weathering, i.e ., elemental loss and gain in < 2mm soil fraction (by XRF analysis) revealed that Al , Fe , Ti were gained whereas K , Ca and Si were lossed in saprolite (C horizon) compared to parent rock. Clay mineral transformation was marked by a decreasee or disappearance of mica and chlorite in clay fractions of surface horizon as well as increase in HIV (hydroxy interlayered vermiculite), smectite, vermiculite and randomly interstratified minerals. Moreover, smectite was present mostly in fine clay and chlorite in coarse clay. As a result, the presence of interstratified mica and chlorite or absence of these minerals confirmed relatively enhanced weathering under acidic leaching condition of the study area. We hypothesized that low pH influences the partitioning of Al into interlayers. Furthermore, alteration of clay mineral is often coincident with persistence of X-ray amorphous hydroxy –aluminosilicate products and irregular mixed layer formation or weakly crystalline clay minerals with broadened peak intensity. Soils derived from limestone ,basaltic–andesite and phyllite were classified as Hapludalfs whereas soils from granite and andesite were Udorthents. Key words: Evolution ,Saprolite, Transformation.

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