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

Authors: GHOSH, RAKESH - Auburn University; PUROHIT, NISITH - Auburn University; Prior, Stephen; MAITY, ANIRUDDHA - Auburn University

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/9/2026
Publication Date: N/A
Citation: N/A

Interpretive Summary: The application of biochar has emerged as a potential soil amendment owing to its impact on soil improvement, restoration, soil carbon build up, and cropping system resilient strategies. However, poor weed control of some pre-emergence herbicides in biochar-amended soils have gained serious attention and could become a challenging issue for growers applying biochar. Present work indicated that biochar application at 2 t ha-1 could reduce the weed control efficiencies of pre-emergence herbicides, but the impact varied depending upon the type of biochar and herbicide used. Commercial pinewood biochar (2 t ha-1) could offer promising results to growers with 100% efficiency to control A. palmeri and D. aegyptium using label-recommended doses of metribuzin and S-metolachlor. Whereas, metribuzin at label-recommended dose should be selected instead of S-metolachlor to achieve 100% control of A. palmeri and D. aegyptium if sugarcane biochar (2 t ha-1) is selected as a soil amendment.

Technical Abstract: A dose-response trial was conducted in two greenhouse experimental runs in 2025 (Plant Science Research Center, Auburn University, Alabama) to determine weed control efficiencies of two pre-emergence herbicides (S-metolachlor and metribuzin) 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). In sugarcane biochar-amended soils, complete control of both weeds was achieved with metribuzin at the label-recommended dose. Whereas the label-recommended dose of S-metolachlor attained respective weed control efficiencies (WCE) of 45.8 and 74.8% for A. palmeri and D. aegyptium in sugarcane biochar-amended soils. Germination (%) of A. palmeri was strongly and negatively correlated with total (r = –0.96, p = 0.035) and adsorbed residues (r = –0.93, p = 0.068) of S-metolachlor, while desorbed residues showed no effect (r = 0.18, p = 0.82). Likewise, D. aegyptium germination showed strong negative correlations with total (r = –0.97, p = 0.032) and adsorbed residues (r = –0.98, p = 0.024), but no influence of desorbed residues (r = 0.51, p = 0.48). This study also demonstrated a differential sensitivity of the two weed species to S-metolachlor residues. At a total residue level of 23.89 µg 10 g soil'¹ on day 7 in sugarcane biochar-amended soil, D. aegyptium showed only 2.42% germination, whereas A. palmeri remained far less affected with 50.57% germination. Complete control of both weeds was observed with the label-recommended doses of S-metolachlor and metribuzin in pinewood biochar-amended soils. Results from this study indicated that the differential distribution of biochar particles in biochar-amended soils, coupled with inherent herbicide chemistry, could influence the weed control efficiencies of studied herbicides even at a low biochar dose (2 t ha-1).