A biotic ligand model (BLM) was developed to predict nickel toxicity, affecting root growth of barley (Hordeum vulgare), in nutrient solutions. The extent to which Ca2+, Mg2+, Na+, K+ ions and pH each influenced nickel toxicity was determined. Higher activities of Mg2+ linearly increased the 4 d EC50Nl2+, while Ca2+, Na+, K+ and H+ activities did not significantly influence Ni2+ toxicity. Stability constants for the binding of Ni2+ and Mg2+ to the biotic ligand were obtained: log KNiBL = 5.27 and log KMgBL = 3.47. Further, it was calculated that on average 57% of the biotic ligand sites needed to be occupied by nickel to induce 50% root growth inhibition. Auto-validation of the BLM indicated that predicted EC50s differed from the observed EC50s by a factor of less than 2, indicating that the BLM concept may also be used to predict metal toxicity to terrestrial plants.