The vulnerability of calcareous soils exposed to Mg‐enriched irrigation water

Abstract

AbstractDue 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.