Biochar, made by pyrolysis of various organic materials such as plants, can amend soil physicochemical properties and improve the efficiency of pre-planted incorporated (PPI) herbicides. The excessive consumption of PPI herbicides results in environmental predicaments that improving PPI herbicides' efficacy by changing soil biological properties might solve this problem. Trifluralin, a PPI, is recommended against annual broadleaf weeds such as velvetleaf in soybean fields. In the present study, treatments included normal soil (NS) (sand 30% + silt 30% + clay 35%) and manipulated soil (MS) (sand 27% + silt 32% + clay 32% + biochar 9%). Two blocks of NS received recommended dose (RD) (1.7 lit/ha) (NS+RD) and a reduced dose (1.2 lit/ha) (NS+ReD) of trifluralin. Meanwhile, the block of MS was exposed to the reduced dose (MS+ReD) of trifluralin. Two days after herbicide treatments, seeds of Abutilon theophrasti were sowed. Then, seven days after sowing, the growth of weeds was monitored and weed control percentage was calculated using the arcsine model based on observed data. Results showed that the NS+RD was the most effective treatment in velvet control (100%), followed by MS+ReD (93.5%) with no significant difference. The NS+ReD treatment resulted in 81% weed control showing significant differences with the NS+RD and MS+ReD. Thus, it seems that biochar acts as a neutral buffer and decreases the necessity of PPI herbicides application in soybean fields. The biochar application can potentially reduce soil contamination, weed resistance, environmental pollution, and the adverse effects of PPI herbicides on the soil microbial population.