Composting and humification

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.