Adsorption Isotherms of Boron by Soil under Different Sodium Absorption Ratios (SAR)

Boron (B) is an important plant essential micronutrient whose availability is influenced by many soil factors. Adsorption–desorption processes play a major role in governing the solubility of boron in soil solution and, consequently, its availability to growing plants. To assess the effect of sodium adsorption ratio (SAR) on availability of B, adsorption of B was investigated in a sampled calcareous soils that had low levels of electrical conductivity. Boron adsorption experiments were carried out on the soil in batch systems to determine adsorption isotherms (amount of B adsorbed as a function of equilibrium solution B concentrations). To determine the quantity of adsorbed B, 5 g of soil were shaken for 12 h, in propylene 50 mL tubes, with 20 mL of B solution (0, 2, 5, 8, 10, 15, 20 mg L^-1) (as boric acid) at ionic strengths of 0.12 M (prepared by NaC1, CaC1₂.2H₂O, MgCl₂.6H₂O) at five SAR levels (2, 5, 10, 15, 20). The reaction temperature was 25^◦C. The suspension was centrifuged, filtered, and a sample was removed and B was determined by Azomethine-H spectrophotometric method (at a wavelength of 420 nm). Soil B adsorption was calculated by subtracting B in the solution after filtration from the added boron. Langmuir, Freundlich, linear Freundlich isotherms fitted well to the adsorbed data based on R² and SEE. However, the fit of the Langmuir equation was closer to the experimental data. At different SAR values, the soil exhibited different adsorption behaviors. The effect of SAR on the boron adsorption was greater at high concentrations and the highest adsorption was at SAR 15, except for SAR 20 that had the lowest B adsorption. Effect of sodium adsorption ratio on adsorption of boron may be due to the dissociation of boric acid to borate anion and changes in the thickness of the diffuse double layer.