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 lOmgC l -z 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
phosphorus. The absorption of micronutrients was
favoured by the lowest doses while doses above
10 mg C l -~ inhibited it, which was probably one of
the causes of the depressed growth observed with the
highest doses.