The effects of soluble silicon (Si) on the toxicity of aluminium (Al) to soybean roots were investigated in solution culture. A weak nutrient solution which included a low concentration of phosphorus (P) was shown to be necessary to allow the full expression of Al toxicity. As solution pH decreased below 6, reductions in root growth in the presence of Al became increasingly severe. Silicon alleviated the symptoms of Al toxicity, but the effective rate was dependent on pH. Greater concentrations of Si were required at lower pH where Al was more toxic. These results support the hypothesis that the pH‐dependent affinity of Si for Al in dilute solutions, and the consequent formation of sub‐colloidal inert hydroxyaluminosilicate species, is the basis for the alleviation of Al toxicity by Si.