Background and aims Biochar has demonstrably im-proved crop yields in weathered and acidic soils, but studies in calcareous soils are particularly lacking, so biochar effects on plant growth was investigated under these conditions. Methods Six biochars were obtained from different feedstocks and production technologies. Chemical char-acterization of fresh biochars included total and extract-able nutrients, labile carbon, and Fourier transform in-frared spectroscopy. Extractable nutrients were also evaluated in biochar-soil mixtures with a basic (pH >8.2) test soil. Bioassays with lettuce and ryegrass were carried out to relate biochar chemical properties to ef-fects on plant biomass. Results A sewage sludge slow pyrolysis char was stim-ulatory to plant growth, as was a slow pyrolysis pine wood char at an intermediate concentration, while gasification and fast-pyrolysis pine and poplar wood chars were strongly inhibitory, with reduc-tions in biomass at realistic application rates of 5– 19 t ha −1 . Conclusions Statistical comparison of plant responses with biochar composition led to the assessment that plant responses were most correlated with volatile mat-ter content and total P content, whose availability was likely regulated by pH and Ca content. Potential effects of phytotoxins were considered, but these were seen to be much less probable than effects due to nutrient availability.