Investigation of the Soil-Landform Relationship in South of Herat, Western Afghanistan

The soils developed in the context of the landforms and they evolve under influence of similar factors. Therefore, the identification of soil-landform relationship is the first step for the soil survey, pedology and paleopedology and geomorphology researches. Afghanistan is located between arid environments of Iran and cold and humid highlands of Himalaya. Due to special social and political conditions, little information existed on the soils of Afghanistan. Alluvial fans are the common land forms in arid environments which usually are archive of soil formation and sedimentation periods in the past. Landform assemblage in southern Herat (alluvial fans to piedmont plan and flood plain), could provide suitable information on the soil landform relationship in arid areas of Afghanistan. This study was the first pedology research in Afganistan. The objectives of this research was to identify the soils along a transect from alluvial fan to flood plain of Hariroud River, determine the clay mineralogy and micromorphological features of the soils and finally determine the soil evolution in the study area.
Material and The study area located in southern Herat between longitudes of 34° 20’ 21” to 34° 25’ 33” E and latitudes of 43° 20’ 21” to 43° 25’ 33” N. The mean annual precipitation and temperature of the study area are 250 mm and 16. 1 ºC. A transect from an alluvial fan from Sefidkouh Mountain to flood plain of Hariroud River were selected. The geomorphic surfaces along the studied transect were alluvial fan, alluvial fan-piedmont plain transition, saline and non-saline piedmont plain and flood plain. Five representative soil profiles along a transect of Alluvial fan, intermediate Alluvial fan– piedmont plain (lowland), piedmont plain and flood plain were described and sampled. Undisturbed samples were taken for thin section analysis and recognizing pedofeatures. The mineralogy of the clay fraction was determined using X-ray diffraction method. Calcic-argillic (Btk) horizon was the most developed horizon which occurred in alluvial fan. Alternation of paleosol (Btk horizon) and undeveloped coarse-alluvial layers indicated alternation of soil formation and sedimentation periods in the past. Regarding the argillic horizon development conditions, the Btk is the relict features from a wetter climate in the past. Similar soils were observed in arid regions of Iran. The gleization and few secondary carbonates and salt accumulation was the pedogenic processes in other landforms. Bw horizon was common horizon in other studied soils. In Alluvial fan – piedmont plain transition geomorphic surface, due to depression in the landform, salt accumulated in the surface soil and electrical conductivity reached to 40 dS/m. However, salic horizon because of limited thickness was not identified. In non-saline piedmont plain and floodplain geomorphic surfaces, soils showed oxidizing and reducing conditions and consequently gleyic color pattern. Maximum EC (49 dS/m) was occurred in saline piedmont plain surface. The results showed that electrical conductivity values increased from alluvial fan toward lowland positions. Besides, fine soil textures were found in downward positions. The Soil Taxonomy classification of the soils were Typic Calciargids in alluvial fan, Aquic Haplocombids in alluvial fan-piedmont plain transition, non-saline piedmont plain and flood plain as well as Typic Haplocambids in saline piedmont plain. Chlorite, illite, palygorskite, smectite, and kaolinite were the main clay minerals. Chlorite and illite were inherited from parent materials. Micromorphological observations proved clay coating on carbonate nodules and coarse fractions in Btk horizon. The alternation of secondary carbonates on clay coating is the evidence of polygenetic soils and change of from wetter to drier climate. Also, iron oxides nodules in Btk horizon indicated suitable weathering conditions during clay accumulation. Lenticular gypsum was observed in the soil in saline piedmont plain geomorphic surface. Coating and hypocoating of iron oxides identified in flood plain soil. Results of this research showed the role of geomorphic processes and climate change in the soil evolution in the area. The results of this study indicated the progressive and regressive processes which are preserved in the soil of alluvial fan. The Btk horizon in alluvial fan represent wetter condition than today. The morphological, mineralogical and micromorphological features of the soil in the alluvial fan were similar to correspondence soil in arid environments of Iran. Continuous sedimentation in the piedmont plain and flood plain prevent the soil development in these landforms.