Background: Little is known about the relationship between plant availability of P in different sludges and their derived ashes, its development over time and its interaction with soil pH. Aim: This study addresses this knowledge gap, by investigating the P availability and fractions of six sludges produced under different conditions and the incineration ashes derived from five of these, plus the effect of time (84 days) and soil pH on P availability. Methods: These materials were applied to two similar soils with contrasting pH (5.8 and 7.8), based on the assumption that lower pH would favor plant availability of P from these sources. Triple super phosphate and a control (no P addition) were included as references. Results: The relative P fertilizer availability (RFA) in soil of both sludges (18–77%) and ashes (0.1–25%), measured as water extractable P, was found to be highly dependent on their origin. While the P availability of sludges was strongly affected by soil pH, ash P availability showed either no or little soil pH effect, in contrast to our expectations. For the sludges, RFA was negatively correlated to the ratio (Fe+Al)/P in both pH soils and positively correlated to sludge water extractable P in the higher pH soil. For the ashes, the ratio Fe+Al to P and the pH reaction of ashes in water were strongly negatively correlated with RFA in the lower pH soil. Ca/P ratios did not predict RFA in either sludges or ashes. Nevertheless, we ascribe the comparably higher average RFA in sludges of 58% found in the low pH soil compared to 25% in the alkaline soil to the greater solubility of Ca-P species under lower pH. For ashes, the availability increased over time, while rather stable for sludges. Conclusion: In conclusion, the P availability of both sludges and ashes was highly dependent on their origin, and for sludges highly dependent on soil pH.