Graber, E.R., Institute of Soil, Water and Environmental Sciences, The Volcani Center, Agricultural Research Organization, POB 6, Bet Dagan 50250, Israel
Tsechansky, L., Institute of Soil, Water and Environmental Sciences, The Volcani Center, Agricultural Research Organization, POB 6, Bet Dagan 50250, Israel
Gerstl, Z., Institute of Soil, Water and Environmental Sciences, The Volcani Center, Agricultural Research Organization, POB 6, Bet Dagan 50250, Israel
Lew, B., Institute of Agricultural Engineering, The Volcani Center, Agricultural Research Organization, POB 6, Bet Dagan 50250, Israel

Background and Aims: Amendment of soil by biochar may reduce efficacy of soil-applied herbicides due to sorption. Methods: Bioassays with Green Foxtail (Setaria viridis) tested the influence of two biochars on phytoavailability of S-metolachlor and sulfentrazone under biochar amendment of 0, 13, 26 and 52 Mg ha -1. Results: Adsorption of both herbicides was an order of magnitude greater on a high specific surface area (SSA) biochar (EUC-800; SSA 242 m 2 g -1) than on a low SSA biochar (BC-1; SSA 3.6 m 2 g -1). Herbicide doses near the lowest recommended label rates controlled the weed at 13 and 26 Mg ha -1 of BC-1; sulfentrazone was also effective at 52 Mg BC-1 ha -1. These same herbicide doses controlled weed germination and development only at 13 Mg ha -1 of EUC-800; at herbicide doses near the highest label rates, weed control was also achieved at 26 Mg EUC-800 ha -1, but not at 52 Mg EUC-800 ha -1. Conclusions: Increased doses of soil-applied herbicides cannot necessarily offset decreases in herbicide phytoavailability in biochar-amended soils, particularly if the biochar has a high SSA. Considering the long half-life of biochar in soil, pest control needs will be best served by low SSA biochars. © 2011 Springer Science+Business Media B.V.