Phosphorus (P) availability often limits crop growth in acidic Ultisol because a large proportion of the applied P is retained by Al- and Fe-bearing soil constituents. This study examined whether rice husk biochar could improve soil chemical conditions and modify soil P availability and stevia responses to P fertilization. A pot experiment was arranged in a 4 × 4 factorial, completely randomized design with three replicates, resulting in 48 experimental units. Data were analyzed using two-way analysis of variance (ANOVA) followed by Tukey’s honestly significant difference (HSD) test at p < 0.05. Rice husk biochar was applied at 0, 10, 20, and 30 t ha⁻¹, while P was supplied at 0, 45, 90, and 135 kg P₂O₅ ha⁻¹. Biochar increased soil pH, organic C, cation exchange capacity (CEC), leaf area, and leaf greenness. Significant biochar × P interactions were observed for available P, leaf fresh and dry weights, and leaf P accumulation. The highest available P (250.90 mg kg⁻¹) was found with 30 t ha⁻¹ biochar and 135 kg P₂O₅ ha⁻¹, which was significantly higher than with 30 t ha⁻¹ biochar and 90 kg P₂O₅ ha⁻¹ (218.34 mg kg⁻¹). However, the highest numerical leaf fresh weight (29.96 g plant⁻¹), leaf dry weight (8.66 g plant⁻¹), and leaf P accumulation (2.981 mg P plant⁻¹) occurred with 30 t ha⁻¹ biochar combined with 90 kg P₂O₅ ha⁻¹. These findings show that rice husk biochar changed how soil P availability reacted to P fertilization. Raising P from 90 to 135 kg P₂O₅ ha⁻¹ at the highest biochar level significantly increased soil available P but did not further improve leaf biomass or leaf P accumulation.