Purpose <p>Agricultural P loss is a major contributor to surface water eutrophication. However, the effects of repeated applications of biochar and compost on potential risk of P loss in salt-affected acidic soils remain unclear.</p> Methods <p>A field experiment was set up in two salt-affected acidic paddy fields with a different level of soil available P in Vietnamese Mekong Delta. Each site had four treatments: i) three rice crops per year (RRR), ii) two rice crops rotated with fallow (FRR), iii) FRR plus compost (FRR + Comp), iv) FRR plus biochar (FRR + BC). Soil samples were collected after two years of rice cultivation in RRR (crop sixth) and P adsorption–desorption parameters and Olsen P were determined for each soil sample. The Langmuir maximum adsorption capacity (Qmax) and degree of P saturation (DPS) were calculated. The risk of P loss was estimated using the equation: P concentration in leachate (mg L⁻¹) = 0.0323 × ln(DPS) + 0.128.</p> Results <p>Biochar and compost amendments did not alter Qmax at either site.&#xa0;By contrast, the amount of P desorption and the DPS were significantly higher in FRR + BC than in FRR, indicating a greater potential risk of P loss. Nevertheless, estimated P concentrations in leachates from biochar-amended soil were up to 0.03&#xa0;mg L⁻¹ at both sites, well below the Vietnamese surface water quality threshold of 0.1&#xa0;mg L⁻¹.</p> Conclusion <p>These findings suggest that biochar and compost can be applied to salt-affected acidic paddy soils without causing significant environmental P losses, provided that DPS is monitored and managed.</p>