Gagnon, B., Demers, I., Ziadi, N., Chantigny, M. H., Parent, L.-É., Forge, T. A., Larney, F. J. and Buckley, K. E. 2012. Forms of phosphorus in composts and in compost-amended soils following incubation. Can. J. Soil Sci. 92: 711-721. Information on the forms and bioavailability of P from composts applied to horticultural crops under organic production are required to promote soil quality and plant growth while protecting the environment. Seven composts were selected and characterized for their chemical composition. The composts were mostly mature and the total P contained 73 to 96% as inorganic P (Pi), although the total P concentration and P fractions varied amongst the materials selected for this study. These composts along with an inorganic P (KH₂PO₄) alone were added to three soils with contrasting chemical properties at a rate equivalent to 65 kg P ha^-1 and incubated in the dark at 25°C. An unfertilized control was included to estimate soil P contribution. The amount and relative solubility of P from these compost-soil mixtures were evaluated by a modified Hedley fractionation after 2 and 16 wk of incubation. Results indicated that P added with composts was preferentially found in the Pi portion of soil labile pools after 2 and 16 wk. Except for the dairy cattle manure compost, all composts resulted in the same amounts of labile P (51-58%) and total Pi (61-98%) in soils, which were significantly lower than the labile P (82%) and close to the total Pi (108%) from inorganic KH₂PO₄ alone. Unlike the other composts, the potato residue compost produced a significant increase in the organic P fraction (Po) extractable with NaHCO₃. This study indicated that Pi was the most important form of P in these composts and in the compost-amended soils. Nevertheless, it remains difficult to predict the availability of compost P following its application to soil.