For soils with low plant-available phosphorous(P) content, the kinetics of P sorption can be described adequately using the kinetic Freundlich equation (Q = atm Cn), where a, m, and n are fitting parameters. When this equation is used for soils that already contain a large amount of plant-available P as a result of previous P additions, the problem arises as to how the initial P content, Q0,(Q at t = 0) can be taken into account. In this study, we tested the applicability of the equation to soils with a high level of plant-available P and several ways to include Q0, which was estimated using nonlinear regression. Adsorption isotherms for three Dutch soils (sandy soil, loamy sand, and sandy loam soil) were determined after 1, 3, 7, and 21 days. We have shown that for these soils, a higher value of Q0 must be used for a longer equilibration time. A description of the increase of Q0 with time of the form:Q0 = x tm appeared to be applicable for the three soils tested. This leads to an extended kinetic Freundlich equation of the form: QΔC = (a Cn - x)tm, where QΔC represents adsorbed P, determined from the amount that has disappeared from the shaking solution, and x is an additional fitting parameter.