Hysteresis in the phosphorus sorption and desorption processes of soils

Phosphorus sorption and desorption processes in selected soils were investigated to evaluate the usefulness of phosphorus sorption isotherms in the estimation of the phosphorus supplying capacity of soils. There was a distinct hysteresis in the phosphorus sorption and desorption isotherms, resulting in an overestimation of the replenishing ability of soils to supply phosphorus to the soil solution, when phosphorus sorption isotherms were used for the estimations. To attain a value of 0.2 ppm P in the soil solution, 115 µmol P/g soil are required in the Kuromatsunai Ando soil (Ochric Andosols) based on the estimation by the P desorption isotherms; this value is 29% higher than that indicated by the P sorption isotherm. Also 42 µmol P/g soil are required in the Mikatagahara Yellow soil (Helvic Acrisols)—which corresponds to a value 23% higher than that indicated by the P sorption isotherm. Two types of hysteresis subloops were observed: A reversible type and an irreversible type. The former was found typically in the highly weathered Mikatahagara Yellow soil while the latter was observed in the Kuromatsunai Ando soil.