STUDY OF THE EVOLUTION OF CLAY MINERALS IN PADDY (Pa) AND NON-PADDY (NPa) SOILS ON A TOPOSEQUENCE IN GHAEMSHAHR CITY, MAZANDARAN PROVINCE

This research was carried out to investigate the effect of rice cultivation and topography on the development of soil clay minerals in Ghaemshahr City in Mazandaran province. A total of 10 profiles were selected on a toposequence–five profiles with rice fields and five with rangeland land uses. The soils studied were classified as Inceptisols and Mollisols. The results of clay mineralogy show that the amount of smectite is higher in paddy (Pa) soils, while the amount of vermiculite is higher in non-paddy (NPa) soils. The higher presence of smectite in rice fields is probably due to the neoformation process. Also, smectite was found to be higher in lower regions with shallow groundwater. The cation exchange capacity, clay content, and bulk density are higher in the Pa soils. Rice cultivation increases the amount of iron extracted in oxalate (Feo) form and decreases the amount of iron extracted in dithionate (Fed) form. Also, during the toposequence from lower areas to areas with higher altitudes, the amount of Fe dithionate increased while Fe oxalate decreased respectively. The amount of pedogenic iron (Fed) has a direct correlation with the degree of evolution of the soil; this amount increased during the toposequence from low to higher regions.