Background and Aims Crop yield and nutritional quality are significantly reduced when potassium (K) in soil is deficient. As a beneficial element for plants, silicon (Si) is effective in alleviating the toxic effects of mineral nutrients. However, the roles played by Si in mediating deficiency in essential mineral nutrients in general and K in particular have not been investigated. Methods To evaluate the role of Si in K deficiency-induced inhibition of growth of soybean (Glycine max) seedlings, the effects of K deficiency on shoot and root growth, hydrogen peroxide accumulation, K contents, lipid peroxidation and activities of antioxidant enzymes in the absence and presence of 2 mm sodium silicate (Na2SiO3) were investigated. Key Results Both shoot and root biomass of soybean seedlings were markedly reduced when grown in K-deficient medium (1 mm K) compared with those grown in K-sufficient medium (5 mm). Addition of Na2SiO3 significantly ameliorated the K deficiency-induced reductions in shoot and root growth. Sodium silicate enhanced K concentrations in leaf, stem and root of K-deficient seedlings by 105.4, 83.4 and 58.8 %, respectively. Hydrogen peroxide (H2O2) and malondialdehyde (MDA) contents in soybean seedlings were increased by 25 and 97 %, respectively, when exposed to K-deficient medium. These increases in accumulation of H2O2 and MDA were removed by addition of Na2SiO3. Addition of Na2SiO3 reduced the K deficiency-induced increases in activities of superoxide dismutase, catalase and peroxidase. Conclusions Application of Si to soybean seedlings grown in K-deficient medium markedly enhanced K use efficiency. Therefore, Si not only increases tolerance to nutrient toxicity, but also ameliorates symptoms associated with deficiency in essential nutrients in plants.