Studying Clay Minerals in Soils Formed on Loess Parent Materials in a Climatic Gradient in Golestan Province

This study was conducted in order to evaluate both temperature and rainfall (climatic factors) effects on soil properties and transformation of clay minerals. A climatic gradient was selected about 80 km in length, located between 55° 16 ′56 ′′to 54° 55′ 46′′ longitude and 37° 03′ 26′′ to 37° 30 ′26′′ latitude in Golestan Province, from mountainous area in the south to the coastal plain of Caspian Sea in the north. Soil moisture and temperature regime varies across this climatic gradient as, respectively, udic, xeric, aridic, mesic and thermic. Seven soil profiles were studied on loess parent materials. Soil horizons were sampled for laboratory analysis. Soil characteristics including texture, pH, EC, organic carbon, CaCO3 and CEC were measured in the collected samples. Clay minerals were separated using sedimentation method. Results showed that there was a good positive relationship between soil development stages and rainfall increase and reverse with temperature decrease. In the areas dominated by aridic soil moisture regime, Entisols and weakly developed Aridisols prevailed, while in the areas with udic soil moisture regime, highly developed Mollisols with an argillic and calcic subsurface horizons were formed. Mineral studies showed that Illite, Chlorite, and some Kaolinite were the dominant clay minerals in the soils. Mineralogical studies showed that mineral composition of the clay fraction in the soils were, respectively, Illite, Chlorite, Smectite, Vermiculite, Kaolinite, and Interstratified clay minerals. Good reverse relationship occurred between Illite and Chlorite content of the soils with climatic index (P/ET°), however, Smectite had a positive relation. In arid region, clay minerals were mostly inherited from parent material. Vermiculite was merely found in the soils distributed in the region with udic soil moisture regime (profile 1). It seems that Vermicullite is unstable in other soil moisture regime condition because of high soil reaction, presence of Si4+ and Mg2+ and low solubility of Al3+, and is not found.