Three soils of mediterranean countries were supplied with the humic extracts of two different organic soils. The HA extracts, characterized by Py-GC, resulted to be similar from a qualitative point of view but showed quantitative differences concerning N-compounds and phenolic and aliphatic molecules. An increase of carbon dioxide emission in the soils with added HA extracts, an inhibition of nitrification and a stimulating effect on denitrification were verified. During thirty days of incubation biomass C decreased shaprly in control samples but increased in the samples supplied with humic extracts. © 1987.