Humic substances extracted from vine-cane mature compost with a solution of 0.1 M Na4P2O7 plus 0.1 M NaOH and separated into humates (SH), humic (HA) and fulvic (FA) acid according to their solubility in acid and alkaline solutions, were purified and tested for their effects on growth of tomato seedlings. The humic substances were beneficial to shoot- and root-growth at intermediate concentrations (100-300 ppm), but inhibitory at high concentrations 1000-2000 ppm). The beneficial effects of humic substances were highest for shoot development; the highest optimum range of concentration was with FA and the lowest with HA.