| The secondary soil salinization in Yellow River electric-pumped-irrigation areas, Gansu Province, has been becoming more and more serious since its development in 1960 because of dry climate, basin topography, high saline underwater and improper management The prevention of soil salinization and reclamation of saline soils have become a necessity to maintain a sustainable development However, salt composition, chemical and physical properties of salt-affected soils in the areas have not well been cleared about currently, this situation hinders countermeasures to be taken and salt-affected soils to be reclaimed. For this reason, this paper takes Jingdian irrigation section as an example to-y and investigat the following three aspects related: I .saluie characteristics and some other chemical properties; ii. stmctural properties and their correlations with saline characteristics; iii. the impact of salinity on some soil water properties. Focusing on the above three category problems, the author has obtained some important results as follows: 1. In the field of studies on chemical properties of salt-affected soils in China generally exchangeable cation composition and its impacts on soil physical properties have been often neglected. The relation between soluble cation composition and exchangeable cation composition is ambiguous. The results in this paper prove that salt composition is mainly chloride-sulphate type, sulphate-chloride type, chloride type and sulphate type in the area. Soluble cations are dominated by sodium ions, as a result, exchangeable sodium percentage (ESP) is high, which shows that soils bore sadic evidence or sodic properties. The values of pH of all soils are less than 8.5. SAR (sodium adsorption ratio) calculated according to cation concentrations in extracts with water: soil ratio of 5:1 is significantly related to ESP or ESR (exchangeable sodium ratio) with lineal correlation coefficients of 0.8566 and 0.861 S(n86), respectively. This means that SAR in those extracts can also reflects sodic properties. Soils in all profiles have calcic evidence or calcaric property. Meanwhile, a small amount of gypsum is distributed in the profile, which may mirror dry environment conditions and some desert characteristics. 2. Soil stnictural degradation is common in the above areas. Soil bulk density is high, with a range varied from 1.27 to 1.69 g/cm3 and an average value of 1.45 g/cm3 for top soils (0 0cm), a range of 1.22 .71 g/cm3 and an average value of 1.45 g/cm3 for sub-top soils (5S cm) and a range of 1.29 .70 g/cm3 and an average value of 1.47 g/cm3 for soil samples from profiles, respectively. Bulk density is significantly negatively correlated to organic matter content Dry aggregates are nearly all detached when immersed in water, this means their stability, which is positively correlated to organic matter and clay, is very low. Hydraulic conductivities of sub-top soils (5Scm) in 34 investigation sites average 3.17mm/h, ranging from 0.16 tolS.3 mm/h. This reflects that soil infiltration in the area varies greatly and is poor. Hydraulic conductivity is positively lineally correlated to organic matter (r = 0.7915, n 34) and negatively to dispersible clay with a power function (r = -0.5414). 3. Traditionally there is a consideration in China that there is no soil structural degradation problem in salt-affected soils (pH < 8.5). However the results in this paper clearly illustrate that the impact of secondary sali...
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