Objective To address the problem of low phosphorus (P) fertilizer utilization efficiency in crops caused by low P availability in acidic soils of southern China, this study used a pot experiment to investigate the effects of applying soybean fermentation broth combined with P fertilizer on soil available P.

Method Under the maize pot experiment, the content levels of KH₂PO₄ were set at 0.05, 0.10, and 0.15 g·kg⁻¹ respectively. For each level, four gradients (with dilution ratios of 100, 300, 500, and 700-time, respectively) of soybean fermentation broth were arranged for mixed application with KH₂PO₄.

Result The combined application of soybean fermentation broth and P fertilizer promoted maize growth at the seedling stage, mainly manifested in plant height, stem diameter, net photosynthetic rate, and root growth. Specifically, soybean fermentation broth mixing 0.05 g·kg⁻¹ P significantly promoted the balanced growth of maize plants and roots, with the leaf net photosynthetic rate being significantly increased by 75.03% compared with the P fertilizer alone treatment. In soil chemical properties, soybean fermentation broth mixing 0.05 g·kg⁻¹ P increased soil pH and effectively reduced soil electrical conductivity. At this P level, compared with the P fertilizer alone treatment, 100-time soybean fermentation broth resulted in a substantial increase of 258.86% in available P content. Meanwhile, the contents of Al-P and Fe-P showed the highest increases of 162.48% and 135.91%, respectively.

Conclusion The combined application of soybean fermentation broth and P fertilizer exerts a positive effect on soil P availability, with the 0.05 g·kg⁻¹ P level combined with 100-time soybean fermentation broth exhibiting a more pronounced enhancement effect on P availability.