The study of sodic soils properties and related formation factors in Abyek plain

Generally, formation and development of sodic soils often appear as almost large in flat plains capable of cultivation, especially in arid and semi-arid regions. Due to their unsuitable characteristics, Slickspots leave bad effects on plants growth and finally on human health. High levels of soluble and exchangeable sodium ions and colloidal material are the main marks of sodic soils. Different surface areas of Slickspot are spread over the flat and arable plains in Iran. The aim of this study was to evaluate the different properties of sodic soils and related soil formation factors in the semi-arid soils of Abyek plain.
The study area, with the coordinates 35° 47′ - 35° 53′ N and 50° 31′ - 50° 33′ E, was located in the southeastern of Abyek city, Ghazvin providence. Piedmont plain was the main physiography of the area and altitudes were divided in three topographic zones: 1190-1180, 1170-1160, 1150-1140 meters above sea level that the zones were classified into upper, middle and flat parts, respectively. Based on topography and site properties, 13 soil profiles were excavated in the topographic zones and all profiles were described based on USDA Standard Soil Description Manual
The results showed that soil acidity measured in saturated extraction ranged from 8.6 to 9.1, 9.8 to 9.7 and 9.1 to 10.1 for upper, middle and flat areas, respectively. Field observation studies of upper parts revealed that gravelly and subangular blocky soil structures were found in surface and subsurface horizons, respectively, while the subangular blocky and massive structures were found in subsurface horizons of middle parts profiles. The subangular blocky and columnar structures were demonstrated structures in profiles of the flat areas. Despite the low topography difference, 5 to 10 m in upper lands, exchangeable sodium content and electrical conductivity were low, and saline or sodic soils were not observed. These soils were classified as Xeric Haplocambids. In the middle part with 2 to 5 m difference in elevation, soils were classified as Sodic Xeric Haplocambids and Sodic Xeric Haplocalcids. The white spots observed in the sodic soils were classified as Xeric and Vertic Natrargids. Compare with the adjacent areas, the concentration of carbonate and bicarbonate anions were relatively high in soils of the flat areas that led to considerable increase in soil acidity. This can shows the accumulation of sodium carbonate salts in the soils. The presence of carbonate and bicarbonate anions in middle areas, probably was due to the development of Sodicization in the soils. The XRD diffractometers showed illite, montmorilonite, chlorite and palygorskite as the clay minerals in soil heorizons. Illite was found in all soil horizons of flat areas with deep decline. This decline was along with increasing of smectite clay minerals in Natric horizons that had poor drainage conditions. The clay coatings in the natric horizons were confirmed by micromorphology and scanning electron microscopy techniques. The cumulative clays on external surfaces of soil aggregates and wall pores, in flat areas, revealed the clay eluviation process. Because of the high soluble and exchangeable sodium cations, the conditions were favorable for transfer of clay in the soils, even in the presence of lime.
Consequently, the main soil formation factors in sodic soils can be presented as different in soil positions on piedmont physiography, the local relief, lateral and vertical movement of water and soluble salts from neighboring areas into the downstream lands and also salt and sodium containing minerals deposited by wind. The Slickspots and related soils were one of the major terrestrial phenomena in the plain Abyek. The Sodic soils in the plain were formed in the absence of high ground water table. Other environmental factors such as micro reliefs, position on the Landform, lateral movement of water and soluble salts and windborne sediments, played and essential role in the formation of sodic soils. The results of the experiments indicated that Sodicization process is developing towards the adjacent land and the absence of gypsum accelerated this development in these areas. Also, mineralogical studies indicate the presence of smectite mineral clay in Natric horizon where the drainage condition was poor and gave the possibility of neoformation of smectite, and that clay movement evidence from upper parts of profile was confirmed by micromorphological studies.