Soil amendment with biochar has been proposed as effective in improving agricultural land fertility and carbon sequestration, although the characterisation and certification of biochar quality are still crucial for widespread acceptance for agronomic purposes. We describe here the effects of four biochars (conifer and poplar wood, grape marc, wheat straw) at increasing application rates (0.5, 1, 2, 5, 10, 20, 50 % w/w) on both germination and root elongation of Cucumis sativus L., Lepidium sativum L. and Sorghum saccharatum Moench. The tested biochars varied in chemical properties, depending on the type and quality of the initial feedstock batch, polycyclic aromatic hydrocarbons (PAHs) being high in conifer and wheat straw, Cd in poplar and Cu in grape marc. We demonstrate that electrical conductivity and Cu negatively affected both germination and root elongation at ≥5 % rate biochar, together with Zn at ≥10 % and elevated pH at ≥20 %. In all species, germination was less sensitive than root elongation, strongly decreasing at very high rates of chars from grape marc (>10 %) and wheat straw (>50 %), whereas root length was already affected at 0.5 % of conifer and poplar in cucumber and sorghum, with marked impairment in all chars at >5 %. As a general interpretation, we propose here logarithmic model for robust root phytotoxicity in sorghum, based on biochar Zn content, which explains 66 % of variability over the whole dosage range tested. We conclude that metal contamination is a crucial quality parameter for biochar safety, and that root elongation represents a stable test for assessing phytotoxicity at recommended in-field amendment rates (<1–2 %).