Gravel, V., Dorais, M. and Ménard, C. 2013. Organic potted plants amended with biochar: its effect on growth andPythiumcolonization. Can. J. Plant Sci. 93: 1217-1227. Even though the benefits from the use of charcoal in agriculture have been known for a long time, little biochar is utilized in agriculture. Therefore, we hypothesized that biochar amendment to an organic potting soil improves plant growth without promoting plant root pathogens such as Pythium ultimum. Growth of sweet pepper, lettuce, basil, geranium and coriander grown in an organic potting soil containing a commercially available biochar (1:1 vol:vol) was compared with an unamended potting soil. Macronutrients (NPK) were supplied through application of an organic liquid fertilizer three times a week via injection irrigation. The effect of biochar amendment on P. ultimum colonization and infection was also evaluated in a sub-sample. No effect of the biochar amendment on growth was observed for sweet pepper and geranium. On a dry weight basis, coriander shoot growth was 45% greater in the biochar-amended potting soil, while a decrease of 44% in shoot biomass was observed for lettuce. The negative growth impact of biochar was not related to a phytotoxicity effect as water extract from biochar did not affect seed germination. For Pythium-inoculated plants, root colonization by the pathogen was higher for all crops in potting soil amended with biochar, except for coriander. However, despite the fact that biochar offered a good environment for P. ultimum development as shown by a higher root colonization rate, no visible signs of damage to the root system or to plant development were observed. Soil respiration was lower when biochar was present in the growing medium, which could be related to a lower root biomass and the biochar-specific properties on greenhouse gases rather than to a reduction in the potting media biological activity. In conclusion, replacement of an important proportion of organic growing media with biochar may be beneficial in terms of plant growth and CO₂ emission, but may also offer a good environment for Pythium ultimum development.