Objective A large number of studies have confirmed that biochar canimprove soil fertility, increase nutrient availability and promote crop growth. Given the low levels of silicon (Si) and phosphorus (P) elements in red soil in southern China, it is imperative to investigate the impacts of biochar addition on chemical speciation of Si and P and plant uptake.

Method Using Si-rich rice straw and husk biocharas test materials, the impacts of adding different doses (0%, 1%, 2%, 4%)of biochar on the transformation and availability of Si and P in red soil, as well as nutrient uptake in soybean plants were studied.

Result Different types and dosages of biochar significantly reduced exchangeable acid content in soil, while increased soil pH and C/N, effective cation exchange capacity and base saturation. The greater the amount of biochar added, the more significant the increase. Straw biochar was more effective than husk biochar. The contents of soil available Si and P in biochar treatments were increased by 4.1%−42.2% and 0.9%−12.6%, respectively compared with control. Biochar application significantly increased dry matter accumulation in soybean plant, improved root morphological characteristics, increased Si concentration of soybean roots and leaves, promoted the uptake and transport of Si and P by roots, and the effect of straw biochar was more significant.

Conclusion The application of rice straw and husk biochar significantly affects the transformation of P and Si in acid soil, increases their availability, absorption and utilization by plants. Straw biochar shows better effect.