A terrestrial biotic ligand model (t-BLM) was developed to predict nickel toxicity to wheat (Triticum aestivum) root elongation in hydroponic solutions.The competitive effects of five major cations (Ca2+,Mg2+,Na+,K+ and H+) on Ni toxicity were investigated and Mg2+was found to be a strong competitor,while H+ showed less competing effect.Besides free Ni2+,the toxicity induced by the species NiHCO3+ was non-neglect able at pH ＞ 7 because NiHCO3+ occupied a significant fraction of total Ni under such condition.Thus,a t-BLM including Ni2+,NiHCO3+,Mg2+,and H+ could successfully predict the nickel toxicity to wheat root elongation and it performed better prediction than the conventional free ion activity model.In addition,the model was examined with two sets of independent experiments,which contained multiple cations and low-molecular-weight organic acids to mimic the rhizosphere condition.The developed t-BLM well predicted nickel toxicity in both experiments since it can account in both complexation and competition effects,suggesting its potential to be used in a complicated matrix like soil solution.This study provides direct evidence that the t-BLM is a reliable method for the risk assessment of nickel in terrestrial system.