Influences of salt adsorption ratio and salt concentration on the physical properties of typical sandy loam in Xinjiang

Abstract: Electrical Conductivity (EC) and Cation Ratio Of Soil Structural Stability (CROSS) are important indexes to evaluate the effects of brackish water on soil permeability. Although CROSS comprehensively considers the complex effects of Na+, K+, Ca2+ and Mg2+ on the stability of soil structure, the applicability of the ion concentration coefficient of CROSS varies in different regions. Therefore, it is necessary to determine the classification criteria based on the EC and CROSS assessment methods according to the local water quality conditions. Brackish water irrigation destroys the soil structure, changes the water and fertilizer retention capacity, and threatens crop growth and food security in Xinjiang Province. Based on this background, this study focused on the typical sandy loam soil in agricultural regions of Xinjiang and used common ions including Na+, Ca2+, Cl- to treat the soil with solutions under different Salt Adsorption Ratio (SAR) and salinity concentrations to explore the effects of SAR and concentration on physical properties of typical sandy loam in Xinjiang Province. Soil aggregates were measured according to Le Bissonnais (LB) method, which were quickly wetted by solutions at six SAR levels (40, 30, 20, 15, 10, 5 (mmol/L)1/2) and eight salinity concentration levels (200, 150, 100, 50, 20, 10, 5, 2 mmol/L). Soil Water Characteristic Curve (SWCC) was measured using a high-speed refrigerated centrifuge at four SAR levels (40, 30, 20, 5 (mmol/L)1/2) and three salinity concentration levels (200, 100, 20 mmol/L). Soil shrinkage during soil dehydration was recorded with vernier caliper at each centrifuge speed to investigate how SAR and salinity concentration influenced soil shrinkage. The results showed that The stability of soil aggregates showed a significant negative correlation with SAR and salinity concentration. The concentration of salinity didn't significantly affect five aggregates, namely, the Large Aggregates (LAs), the Small Aggregates (SAs), the Microaggregates (MAs) and Silt and Clay fractions (SC) and Mean Weight Diameter (MWD). SAR had a significant effect on LAs, SAs, SC and MWD (P<0.01), but had no significant effect on MAs. When the SAR value was lower than 30 (mmol/L)1/2, the stability of soil aggregates remained constant. The above result indicated that when the SAR in the irrigation water was at a low level and the salinity concentration was lower than 200 mmol/L (approximately 19.1 g/L), it could be considered that the soil structure was not damaged. Besides, SAR influenced SWCC of typical sandy loam in Xinjiang Province, which depended on the salinity concentration. SAR showed the negative effect on soil shrinkage, while the maximum soil shrinkage increased and then decreased with the increase of the salinity concentration, which was related to the SAR of the saturated solution. Those results can provide valuable information for the rational selection of brackish water for irrigation with different salt composition and concentration to protect the soil structure of farmland. What's more, this study discussed the short-term effects of salinity and SAR on the sandy loam soil aggregate structure and soil water retention curve in Xinjiang Province. Long-term effects of salinity and SAR on soil physical properties remain to be further explored in the future.