REACTIONS OF SOLUBLE PHOSPHATE WITH ACID SOILS: THE INTERPRETATION OF ADSORPTION-DESORPTION ISOTHERMS

Adsorption-desorption and isotopic-exchange isotherms for acids soils of known clay mineralogy indicated that when all the surfa? P accessible to the solution was considered, the bonding energy for phosphate decreased exponentially with site coverage. The soils differed markedly in the quantities of surface P held at sites of very high bonding energy, defined as those in equilibrium with a solution concentration < 1 μM; the values ranging from 9 μmol g^?1 in a soil with 40 per cent gibbsite in the clay fraction, to 1 μmol g^?1 in a soil with only 1 per cent gibbsite and no interlayered material. The reversibility of current P adsorption on soil surfaces depended on the immediate history of the soil sample. Complete reversibility occurred when soil, previously enriched with soluble P and stored for 4 to 5 months at constant temperature, was suspended in solutions of constant pH and ionic strength. It is suggested that irreversibility occurs due to incomplete attainment of equilibrium during the adsorption phase, a condition predisposed by high initial P concentrations (>1mM), and the use of soils that are naturally in dis-equilibrium because of recent fertilizer additions or severe depletion of P by plant uptake.