The development of humic substances in the course of four composting proces
ses was monitored quantitatively, recording both relative and absolute cont
ents. Relative data showed contrasting results if the humic substances (HS)
were related to the dry matter (d.m.) or to volatile solids (VS). Humic su
bstances were apparently formed because of a concentration effect due to or
ganic matter degradation. If absolute contents were considered, a decrease
in the humic substances was observed, above all in the early stages of the
process, due, probably, to degradation of the organic material, such as pro
teins, carbohydrates and lipids, coextracted with the humic substances. Pro
cessing of the data in respect of humic substance content over 13 compostin
g processes and one study on the degradation of plant residues in soils, co
nfirmed that no net humic substances are formed during composting and that
the humification should be interpreted merely as degradation of the organic
matter associated with the humic substances, after uncovering what is know
n as the core of the humus. The concept of humification during the composti
ng process therefore needs to be reviewed, bearing in mind that neither the
method commonly used for humic substance extraction nor the relative resul
ts obtained enable it to be interpreted satisfactorily.