Phosphorus (P) can be a limiting nutrient in terrestrial ecosystems and adding biochar or wood ash can increase plant-available P. We added wood ash and biochar to microcosms containing three acidic Ontario soils planted with red pine or sugar maple seedlings and observed seedling growth responses, as well as amendment-induced changes in soil P pools, microbial P, and enzyme activity. Neither ash nor biochar consistently increased seedling growth; instead sugar maple and red pine seedlings often had opposing responses to the same amendment–soil combination. Overall, these results indicate that it is important to carefully consider both the chemical and physical characteristics of the soil and the ash or biochar, as well as the nutrient requirements of the target tree species, to effectively use these amendments to reduce P limitation.