Salinity-Induced Phosphate Binding to Soil Particles: Effects of Divalent Cations

Excess water and soil salinity are abiotic stresses for plants accessing soil nutrients and hamper crop production in coastal agricultural lands. The nutrient phosphorus in the form of dissolved orthophosphate is an essential element and often limiting to plant growth. Here, we evaluate the effects of water salinity on phosphate mobility at the soil–water interface by conducting batch phosphate adsorption isotherm (at pH 7 and 10) and Ph-dependent (4 to 10) experiments at varying salinities. Results from the adsorption isotherm experiments show that the phosphate binding constants to the soil particles increase with increasing background solution salinity, with higher values obtained at pH 10 than at pH 7. While the adsorption isotherm at pH 7 can be described by both Langmuir and Freundlich models, the isotherm at pH 10 is better explained by the Friedrich model. The salinity-induced enhanced phosphate adsorption is also evident from the pH dependent studies where phosphate adsorption increases from acidic to alkaline pH, different than the typical phosphate adsorption pattern on soil minerals. The enhanced adsorption of phosphate under elevated salinity and alkaline pH conditions suggests that the potential co-adsorption of phosphate with the divalent cations (e.g., Ca 2+ and Mg 2+ ) are the primary processes controlling phosphate binding with soil minerals. Therefore, the application of saline surface and groundwaters in agricultural lands may result in gradual phosphate buildup in the soil and may not be easily released to the soil porewater when needed for plant growth.