Characteristics of Some Selected Soils in the Caspian Sea Region of Iran

A physiographic sequence of soils was studied in the Caspian Sea region of Iran. This area is characterized by a warm, temperate, humid microclimate. Seven representative pedons, including 34 samples, were analyzed. Physico-chemical determinations revealed that the Mountain and Upper Alluvial Plain soils are acid with average pH values of 5.8 and 5.9, respectively. The Lower Alluvial and Coastal Plain soils are neutral with pH's of 7.0 and 7.3, respectively. The base saturation percentages are lower for highland soils (68% for Upper Alluvial Plain area) and higher for lowland soils (99% for Coastal Plain area). The exchange acidity also follows the reverse trend. The average organic matter content of the Lower Alluvial Plain soils is the highest (9%) among the soils studied, while that of the Upper Alluvial Plain soils is lowest (2.3%). The other soils have intermediate OM content, being variable throughout the pedons 1, 3 and 7. The Mountain soil exhibits the highest CEC value (41.6 meq/100 g), while the other soils have considerably lower values. However, the average vermiculite content of the Mountain soil is lower (20%) than those of the plains (ranging from 34 to 43.8%). Clay mica content of the soils also follows the same general trend. X-ray diffraction patterns of the clay fractions reveal that the dominant clay minerals consist of kaolinite, randomly interstratified montmorillonite-vermiculite, and some discrete vermiculite. The clay mica (illite) is more common in the soils of the plains, while discrete chlorite as well as clay mica-chlorite and/or vermiculite are observed in the clay fraction of the low-land soils. The soils are classified as follows: Mountain soil as Typic Hapludoll; Upper Alluvial Plain soils as Aquic Hapludalf, Fluventic Eutrochrept, and Dystric Fluventic Eutrochrept; Lower Alluvial Plain soils as Aeric Haplaquept; and the Coastal Plain soil as Fluventic Hapludoll.