Installation for preparing wood fibers for a cultivation substrate

An installation for preparing wood fibers for use in the composition of a cultivation substrate, which installation comprises a defibration sheath having an inlet for wood chips and an outlet for the fibers obtained from said chips, the sheath containing two parallel screws suitable for being 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 having forward threads, and a braking downstream zone having reverse threads, the threads in the braking zones having notches, the notches in the threads of the braking downstream zone of the downstream series being smaller than the notches in the threads of the braking downstream zone of the upstream series, the number of notches per thread in each braking zone lying in the range 2 to 6, and the section-to-output ratio of the sum of the sections of the notches of a thread of the braking downstream zone of the downstream series to the fiber output rate lying in the range 60 mm2/m3h−1 to 80 mm2/m3h−1.