Soil incubation and pot experiments were conducted to follow the sorption processes of added phosphorus (P) fertiliser using the radioisotope tracer technique. Increasing doses of P fertiliser (40, 80, 160, 320 mg P/kg soil) were added to Chernozem and Arenosol and incubated for 1, 3, and 13 weeks. After incubation, perennial ryegrass (Lolium perenne L.) was sown in one group of pots, and the experiment had been continuing for another 9 weeks. The yield, grass P uptake, isotopically exchangeable (PIE), water-soluble (PW), and ammonium lactate soluble phosphorus (PAL) fractions of soils were measured. On Chernozem, plant P uptake, PIE, PW and PAL were significantly less in the case of the longest incubation period compared to shorter incubations. This suggests a transformation of P into tightly sorbed form. On Arenosol, there were only small changes in the parameters as the incubation period increased, suggesting less intense P transformation to tightly sorbed form. The PW/PIE ratio enhanced with increasing P-doses, and the ratios were higher on Arenosol. On Arenosol, the higher P doses caused a greater increase of PW than on Chernozem. The PIE + PW showed a good correlation with plant P uptake proving this value can be a good indicator of plant-available phosphorus.