We compared the effect of humic substances extracted from municipal wastes (sewage sludge and a compost) and those extracted from more humified materials (leonardite, peat and a commercial humic acid) on plant growth and nutrient absorption during hydroponic cultivation. The results showed that doses representing less than 10 mg C l(-1) favoured plant growth, while higher doses sometimes inhibited it. Humic substances favoured the development of the aerial part rather than the root The effects of both groups of humic substances were similar both as regards plant growth and nutrient absorption. The absorption of macronutrients (nitrogen, phosphorus and potassium) was significantly affected by the addition of humic substances, but differed for each nutrient. Nitrogen absorption, for example, was stimulated by the lowest doses, such stimulation decreased as the dose increased, while the opposite was true for The absorption of micronutrients was the lowest doses while doses above 10 mg C l(-1) inhibited it, which was probably one of the causes of the depressed growth observed with the highest doses. Copyright (C) 1996 Elsevier Science Ltd.