The concentration (Ct) of phosphate ions (Pi) in soil solution is the primary variable on which plant nutrition with phosphorus and eutrophication depend. The description of soil mobile Pi (Pm) is a necessary step in the modeling ofCtkinetics. The description of the change in the content of soil mobile Pi (ΔPm) must take into account the initial amount of mobile Pi (Pm,0). The change in the content of soil mobile Pi has been described by ΔPm=a(Cbt−x)tc,wherea, b, x,andcare fitted parameters. We tested the suitability of a derived simplified equation where the native sorbed Pi, i.e.,x,is calculated from the measured initial Pi concentration in solution (C0): ΔPm=a(Cbt−Cb0)tc.Both equations were compared for their suitability for describing the kinetics ofCt.Two sorption and one sorption-desorption experiments were performed on soil samples taken from plots having different P fertilizer histories in two long-term field trials located on Luvisols. The differences in conditions between experiments included (i) number of Pi addition rates (up to 120 mg P kg−1), (ii) soil-to-solution ratio, and (iii) period of contact between soil and solution (up to 7 days).In the adsorption domain, both equations fitted experimentalCtvalues (r2>0.99) closely. In the desorption domain, however, some slight discrepancies were observed between measuredCtvalues and those calculated by the simplified equation. They were probably the result of overestimation ofC0. Thus, the proposed simplified equation accounts for the participation in the soil solution Pi dynamics of the initial mobile Pi content of the soil that determines the initial Pi concentration in solution (C0).