Physicochemical properties of biochar influence weed control efficiency of pre-emergence herbicide

A trial involving two types of biochar was conducted in two greenhouse experimental runs in 2025 to determine weed control efficiencies of two pre-emergence herbicides (metribuzin and S -metolachlor) at respective label-recommended rates to control Palmer amaranth ( Amaranthus palmeri S. Watson) and crowfootgrass [ Dactyloctenium aegyptium (L.) Willd]. Plants were grown in a loamy soil amended with commercially available sugarcane and pinewood biochars (2 t ha -1 ). Complete control of both weeds was observed with the label-recommended doses of metribuzin and S -metolachlor in pinewood biochar-amended soils. Whereas in sugarcane ( Saccharum officinale L.) biochar-amended soils, complete control of both weeds was achieved only with metribuzin at the label-recommended dose. The label-recommended dose of S -metolachlor provided nearly 47 and 74% control of A. palmeri and D. aegyptium , respectively, in sugarcane biochar-amended soils. Application of sugarcane biochar decreased the total residue of S -metolachlor as compared to soil with no or pinewood biochar and showed poor weed control efficiency (WCE). On day 7, nearly 3 and 24% germination for D. aegyptium and A. palmeri , respectively, were observed with S -metolachlor in sugarcane biochar-amended soils, whereas no germination was recorded in other treatments, which indicated biochar-herbicide-species specificity. Pearson correlation analysis showed negative associations between weed germination and soil herbicide residues across sampling times, though these relationships were not statistically significant (p > 0.05). Results from this greenhouse study indicated that commercial biochars from different raw materials could influence the WCE of pre-emergence herbicides even at a low biochar dose (2 t ha -1 ). Field evaluations are necessary to understand the weed control dynamics.