The method for preserving the initial fermentable potential of an organic substrate over time, said potential being represented by the methanogenic potential of the biomass, or BMP, of the organic substrate, comprises at least the following steps: a) determining the moisture content (HM) of the organic substrate (2) collected (1), b) comparing (8) the determined moisture content (HM) to a predefined moisture level (HD) of between 0% and 40%, c) storing, in conditions limiting any activity of the microorganisms present in the substrate, and limiting any variation in the moisture level (HM), only the organic substrate (2) of which the moisture content (HM) determined in step a) corresponds to the predefined moisture content (HD), d) if necessary, during steps a) and c), taking measurements of the methanogenic potential of the biomass, e) comparing, if necessary, the measurements of the methanogenic potential of the biomass obtained during step c) to that of step a) and f) adapting the storage conditions to keep the moisture content (HM) of the organic substrate (2) collected (1) at a value that is equal, within 5%, to that of the predefined moisture content (HD). The invention also concerns a facility for implementing the method.