Method of preparing wood fibers for a cultivation substrate

A method of preparing wood fibers for use in the composition of a cultivation substrate, wherein wood chips are fed into a defibration sheath containing two parallel screws driven in rotation in such a manner as to mesh with each other via their respective threads, said threads having, in succession, in the direction going from upstream to downstream, at least an upstream series of segments and a downstream series of segments, each of which comprises a drive upstream zone in which the chips are driven downstream and a braking downstream zone through which the chips are forced under the drive effect procured by the upstream zone, the chips thus being driven through the sheath while being braked in the braking zones and while being transformed into fibers that are recovered at the outlet of the defibration sheath, the transit time for which the chips remain in the sheath lying in the range 15 s to 80 s, and provision is made for the pressure to be at least substantially equal to 90 bars upstream from the braking downstream zone of the upstream series, so that a temperature lying in the range 120° to 150° C. is reached inside the sheath without supplying external heat.