Organic materials, more particularly wood and woody parts of plants, are intensely crushed, defibrated, heated and physically broken up in a thermal screw press, an impregnation with previously added auxiliary substances being simultaneously effected. The thus prepared material has a peat-like appearance and provides optimal physical, chemical and biological properties for its use as a peat substitute and substrate component. Inter alia, it is suitable as an additive for the physical upgrading of strongly decomposed peat varieties and for drawing out and better aeration of peat and culture substrates produced on the base of peat. When vaccinated with suitable fungi or microorganism, it is also very advantageously used for the mycorrhization of mycotrophic plants and as an additive for the production of high-grade composts.