Sewage sludge ashes provide interesting possibilities to recycle phosphorus (P) to soil, although the immediate plant availability is often low. The objective of this work was to increase the P-fertilising effect of sewage sludge ash through inoculation with the phosphate-solubilising fungus Penicillium bilaiae. In a soil incubation study, ash amendment enhanced the content of water-extractable P, but in contrast to previous in vitro experiments, inoculation with P. bilaiae did not further increase P availability, probably due to limited competitiveness of the fungus in soil. In a micro-plot field trial, a moderate P fertiliser effect of the ash was found without any additional effect of P. bilaiae ash inoculation on plant biomass and P-uptake. However, soil microbial functions determined by the MicroResp™ method were not negatively affected by the ash amendment. In conclusion, inoculation with P. bilaiae did not prove to be a suitable strategy to enhance P availability from sewage sludge ash. A better insight into the fungus’ ecology and colonisation ability in soil and rhizosphere as well as into its interactions with ash P is needed to further develop such alternative P-fertilising approaches.