Effect of phosphate on the sorption,desorption and plant-availability of selenium in soil

Phosphate, applied at 5µg P cm^–3, decreased selenite sorption by from 30–70% in three soils studied. Both maximum sorption (X_m) and the binding-energy of sorption as indicated by the binding-energy related constant (k) or the molar free energy (G) of the sorption reaction derived from the Langmuir equation were considerably decreased. On the other hand, phosphate sorption was decreased by increasing concentration of selenite from 0.2µg Se cm^–3 to 1.0µg Se cm^–3 in the initial solution. The competitive sorption of phosphate with selenite was likely the main mechanism involved in the P-Se interactions. The competitively sorbed selenite exhibited much larger desorption in 0.01M CaCl₂ solution, more readily extractable to 0.5M NaHCO₃ and significantly higher isotopic exchangeability compared to that sorbed without the competing anion. Results from pot trial using ryegrass indicated that phosphate application increased more efficiently the plant-availability of applied fertilizer Se than that of indegeneous Se in soil.