The essential role of humified organic matter in preserving soil health

The soil health notion has been recently expanded to relate soil functions not only to soil fertility for a sustainable primary productivity, but also to the control of water cycling and of the soil carbon storage. Soil humus is fundamental to achieve the objectives of soil health, and it is therefore deemed necessary to enlarge the knowledge of its composition and dynamics, if a modern soil management was to be pursued. This review first describes the contemporary understanding of the supramolecular structure of soil humus and the derived modern method to identify the totality of humic components to an unprecedented extent in soils under different soil management and cropping systems. Then, it accounts on how humus, either native in soil or exogenous from sources rich in organic carbon such as lignite, compost, and lignocellulosic residues, improves the soil physical fertility, limits the risk of erosion, enhances the bioactivity of the rhizosphere microbiome, and directly stimulates plant growth. Moreover, it is highlighted the role played by humus in ecologically sound soil managements, such as in organic and biodynamic agricultural productions, which are progressively growing as alternative to conventional but environmentally unsafe practices.