Purpose: This study aimed to improve the phosphorus (P) fertilizer value of sewage sludge ashes and biochars through different pre-treatments and application methods. Methods: In experiment 1, two sewage sludges, their respective ash, and biochar underwent three chemical pre-treatments (H₂SO₄, NaOH, and Ca(OH)₂. These materials were utilized as P fertilizers in a maize plant trial), using two application methods (mixed and placed). In experiment 2, biochars sourced from four sewage sludges were pre-treated with H₂SO₄ and NaOH and then employed as P fertilizers in a barley plant trial. Results: Experiment 1 revealed a significant impact of application method on shoot dry matter, with mixing outperforming placement. Acidification pre-treatment of the ash led to significantly higher shoot dry matter (2.7 ± 0.3 g) and P uptake (7.9 ± 0.9 mg per plant) compared to untreated ash (0.7 ± 0.1 g; 1.9 ± 0.2 mg per plant). Similarly, NaOH-treated biochar caused elevated shoot dry matter (2.6 ± 0.4 g) and P uptake (6.9 ± 0.4 mg per plant) compared to untreated biochar (0.7 ± 0.2 g; 1.9 ± 0.1 mg per plant). These outcomes highlight the dependence of pre-treatment efficacy on biomaterial composition. Experiment 2 demonstrated that despite differing elemental compositions of the biochars, NaOH treatment led to significantly greater shoot dry matter and P uptake compared to untreated and acidified treatments. Conclusions: Chemical pre-treatments have the potential to enhance the short-term P fertilizer value of ashes and biochar. However, further studies are essential to assess the economic feasibility and potential environmental risks, including increased heavy metal solubility. Graphical Abstract: [Figure not available: see fulltext.]