Due to imbalanced groundwater withdrawal, Mg concentration has relatively increased in the majority of irrigation waters in arid and semiarid regions, which may negatively affect soil properties and productivity. Therefore, this study was aimed at understanding to what extent varying Ca to Mg ratios in the calcareous soil solution alter the ratio on the exchangeable sites. To do so, four calcareous soils were equilibrated with eight Ca to Mg ratios and then the relative adsorption of Ca and Mg ions on the exchangeable sites and the CEC of soil samples were measured using a modified silver‐thiourea method. To quantitatively explain our observations, we used the Gaines–Thomas equation, and its parameters were independently derived from the dissolution/precipitation reactions of carbonate minerals. Our results showed that among Ca and Mg, the one that had the lowest concentration in the soil solution was preferably adsorbed at exchangeable sites. The modeling results indicated that Ca and Mg ions are adsorbed with wide ranges of adsorption energies, which on the macroscopic scale are interpreted as ion preference. Ion pair formation of Ca and Mg with Cl in the soil solution had no significant effect on their surface reactions. In addition, modeling indicated that at high Ca or Mg concentration, the surface species stoichiometry may gradually change from bidentate to monodentate to accommodate more ions. Thus, it can be concluded that by application of Mg‐enriched irrigation water the risk for Mg accumulation in calcareous soils is highly increased which in turn increases the risk for soil degradation and indirectly for soil sodification.

中文翻译：

---

钙质土壤暴露于富镁灌溉水中的脆弱性

由于地下水取水量不平衡，干旱和半干旱地区大多数灌溉水中的镁含量相对增加，这可能对土壤特性和生产力产生负面影响。因此，本研究旨在了解钙质土壤溶液中钙镁比例的变化在多大程度上改变了可交换部位的比例。为此，用八种钙镁比值平衡了四块石灰质土壤，然后使用改良的银硫脲法测量了土壤中可交换位点上钙和镁离子的相对吸附以及CEC。为了定量解释我们的观察结果，我们使用了盖因斯-托马斯方程，其参数独立于碳酸盐矿物的溶解/沉淀反应。我们的结果表明，在钙和镁中，优选在土壤溶液中浓度最低的一种吸附在可交换位置。建模结果表明，Ca和Mg离子具有很宽的吸附能范围，在宏观上被解释为离子优先级。Ca和Mg与Cl在土壤溶液中的离子对形成对其表面反应没有明显影响。此外，建模表明，在高Ca或Mg浓度下，表面物种的化学计量可能会从双齿逐渐变为单齿，以容纳更多的离子。因此，可以得出结论，通过使用富含镁的灌溉水，钙质土壤中镁的积累风险大大增加，这反过来又增加了土壤退化的风险，并间接增加了土壤的成泥作用。