Physicochemical properties and water holding characteristics of Tianjin coastal saline soil improved by foreign soil

Abstract: The method of soil replacement removes saline alkali soil and backfills with the soil, which has excellent physical and chemical properties and is suitable for plant growth, and this method can achieve the purpose of soil improvement in a short time compared with other technologies. Studying the physical and chemical properties of the soil, which has been amended with the method of soil replacement, has a very important meaning to the evaluation of soil improvement effect, the establishment of irrigation system in the improved area, and the improvement of soil replacement technologies. This study focuses on the physicochemical properties and water holding characteristics in the 5 sampled layers of the costal saline soil and the soil after foreign soil amelioration in Hangu District, Binhai New Area, Tianjin. Towards these 2 kinds of soil, the basic physicochemical properties of different layers within the depth of 0-60 cm are analyzed, as well as the soil water characteristic curve. In addition, The van Genuchten model (VG), the dual-porosity model (DP), the Brooks and Corey model (BC) and the lognormal distribution model (LND) were used to fit the soil water characteristic curve, respectively. The applicability of these models and the variety of soil moisture parameters were compared and evaluated. Upon the achievement of the optimal fitting model, more details were given, such as field water capacity, available water content, rapidly available water content, and permanent wilting point. The results showed that the conductivity of the soil after foreign soil amelioration was much lower than that of the costal saline soil, which was decreased by up to 97%. The average value of the conductivity in different soil layers was only 2.82% of that of the coastal saline soil. The density values of the 2 kinds of soil in all layers were estimated to exceed 1.4 g/cm3, only except 0-10 cm layer of the soil after foreign soil amelioration which was 1.30 g/cm3. The available water content in different layers of the soil after foreign soil amelioration was about 4% lower than that of the coast saline soil. Compared with the costal saline soil, the relative field water capacity value of the soil after foreign soil amelioration generally decreases, especially down to 67% in term of relative field water capacity in 0-10 cm layer. However, it still exceeded 70% at other layers. The rapidly available water contents of coastal saline soil and soil after foreign soil amelioration both vary from 0.07 to 0.14 cm3/cm3, and the average rapidly available water contents in different soil layers were both 0.10 cm3/cm3. The available water content in different layers of the soil after foreign soil amelioration was about 4% lower than that of the coast saline soil. The soil amendment measure in this study does not significantly improve the rapidly available water content and the available water content of coastal saline soil. The mean value of determination coefficient for fitting model follows the sequence as LND (0.949 6) > VG (0.934 9) > DP (0.934 7) > BC (0.884 5). Generally, the lognormal distribution model was the best one for fitting the water characteristic curve of both the costal saline soil and the soil after foreign soil amelioration. The salinization degree of the costal saline alkali soil after foreign soil amelioration decreased significantly compared to that before soil replacement, with the relative field water capacity of the costal saline soil slightly varying. Considering the relative field water capacity, the soil after foreign soil amelioration is still not suitable for plant growth, except the 0-10 cm layer soil. And there was no obvious improvement in water holding characteristics and physical properties, which had not reached the recommended value in the quality standard of garden planting soil. The heavy clay soil texture is an important factor to restrict the soil quality, which affects soil resources development and utilization in this area.