Purpose <p>Mitigating phosphorus (P) loss from soil to water bodies is crucial for controlling non-point source pollution and protecting aquatic ecosystems. This study investigated the effects of a composite soil conditioner (comprising biochar, bentonite, and humic acid) on P dynamics in high-P soils from the western shore of Erhai Lake. In order to prevent and control phosphorus non-point source pollution, protect the water environment and improve crop quality.</p> Materials and methods <p>A pot experiment with cabbage was conducted to analyze P fractions in soil, P concentrations in leachate, and plant P uptake. Soil analyses included total P, available P, water-soluble P, aluminum-bound P (Al-P), iron-bound P (Fe-P), occluded P (O-P), calcium-bound P (Ca-P), and organic P. Leachate was analyzed for total P, total soluble P, particulate P, soluble inorganic P, and soluble organic P.</p> Results and discussion <p>The 5% application rate of the 3:1:1 CSB + B+HA mixture proved most effective. This treatment significantly reduced the content of readily mobile phosphorus in soil, with water-soluble P, available P, and organic P decreasing by 4.64%, 13.66%, and 30.48%, respectively. Concurrently, it promoted the transformation of soil phosphorus into more stable inorganic fractions (Al-P, Fe-P, O-P, and Ca-P increased by 5.11%, 24.14%, 12.53%, and 7.1%, respectively) and enhanced cabbage P uptake by 18.84%. Consequently, all measured phosphorus forms in leachate were significantly reduced, with total P, total soluble P, particulate P, soluble inorganic P, and soluble organic P decreasing by 16.11%, 18.11%, 15.45%, 18.48%, and 17.16%, respectively, compared to the control (CK).</p> Conclusions <p>CSB + B+HA conditioner at a 5% application rate and 3:1:1 ratio effectively promotes P stabilization in soil, facilitates crop P utilization, and mitigates the risk of P loss to the water environment.</p>