The addition of activated charcoal to a nutrient solution for the hydroponic culture of cucumber resulted in significant increases in the dry weight of the plant and fruit yield. Hydrophobic root exudates were collected at different growth stages with Amberlite XAD-4 resin and bioassayed with lettuce seedlings. The exudates at the reproductive stage were more phytotoxic than those at the vegetative stage. The exudates contained organic acids such as benzoic,p-hydroxybenzoic, 2,5-dihydroxybenzoic, 3-phenylpropionic, cinnamic,p-hydroxycinnamic, myristic, palmitic, and stearic acids, as well asp-thiocyanatophenol and 2-hydroxybenzothiazole, all of which, except 2-hydroxybenzothiazole, were toxic to the growth of lettuce.