Influence of Water-Soluble Salts in Saline-Alkali Soil in Xinjiang on the Determination of Exchangeable Bases

Soil exchangeable bases are a key indicator for assessing soil salinization degree, and their accurate determination is crucial for scientifically evaluating soil quality, guiding agricultural production, and formulating reclamation measures. The saline-alkali land area in Xinjiang accounts for about 30% of China’s total. Due to its unique geographical and climatic conditions, the soils exhibit high total water-soluble salt content, complex salt composition, and significant regional differences. Classified by salt composition, the common saline-alkali soils in Xinjiang are typically sulfate-type and chloride-type. Traditional methods for determining exchangeable bases are mostly suitable for low-salt soils, and there is still a lack of systematic solutions for the effective separation and accurate determination of exchangeable base ions under high-salinity conditions where water-soluble ions and exchangeable base ions coexist in large quantities. Existing methods under high-salinity conditions suffer from problems such as incomplete salt removal or additional dissolution of exchangeable ions during washing, leading to deviations in the determination results. This study used the matching relationship between soil cation exchange capacity (CEC) and the total amount of exchangeable bases to judge the accuracy of the determination. Saline-alkali soils with different salt types from Xinjiang were selected to investigate the effects of the total water-soluble salt content and salt ion composition on the determination of exchangeable bases. Simulation experiments involving the addition of different amounts of NaCl, MgCl₂, Na₂SO₄, and CaCl₂ to the soil were conducted to verify the reliability of the research findings. The results showed that the interference of water-soluble ions with the determination of exchangeable bases increases with higher salt content and exhibits a distinct threshold effect. When the total water-soluble salt content is less than 10 g/kg, washing twice with a 70% ethanol solution can sufficiently remove the salts. When the total water-soluble salt content is greater than 10 g/kg and the saline-alkali soil is primarily chloride-type, washing 2–4 times with a 70% ethanol solution can thoroughly remove chloride ions. When the total water-soluble salt content is greater than 10 g/kg and the saline-alkali soil is primarily sulfate-type, water washing can appropriately improve salt removal efficiency, although complete removal is difficult to achieve. The simulation experiments were consistent with the research conclusions, and it was found that chloride ions hinder the exchange of calcium ions in the soil due to ionic effects, while sulfate ions strongly inhibit the exchange of calcium ions by forming slightly soluble calcium sulfate.