Aims <p>Soil acidification reduces nutrient availability, increases the activity of harmful metal ions, and inhibits plant nutrient and water absorption. Biochar, a promising soil amendment, has been shown to improve the physical and chemical properties of soil and to alleviate soil acidification. This study aimed to evaluate the effects of tobacco stem-based biochar on improving soil quality, enhancing phosphorus availability, and promoting cucumber growth in acidified soils.</p> Method <p>In this study, Ca and Mg Co-doped biochar was produced by the direct pyrolysis of tobacco stems at a temperature of 800&#xa0;°C. Its effects on phosphorus availability, soil quality, and cucumber growth were assessed under acidified soil conditions.</p> Results <p>Application of tobacco stem-based biochar significantly improved soil pH, electrical conductivity (EC), and acid–base buffer capacity, while reducing total exchangeable acids, exchangeable H⁺, and exchangeable Al³⁺ content in the soil. The in-situ co-doping of Ca and Mg enhanced the slow-release effect when combined with phosphorus fertilizer, leading to improved nutrient retention, increased phosphorus availability, and higher exchangeable calcium and magnesium ion concentrations. In addition, even with a 30% reduction in fertilizer use, cucumber height, stem diameter, and photosynthetic efficiency were significantly enhanced.</p> Conclusions <p>Tobacco stems provide a valuable resource for preparing in situ Ca and Mg Co-doped biochar, which can effectively improve soil acidity and nutrient availability in acidic soils. This study suggests that tobacco-stem-based biochar could be a sustainable and scalable solution to address soil degradation, particularly in regions with high soil acidity, thereby supporting more resilient agricultural systems.</p>