  The spectrum of partons in a QCD jet becomes independent of the primary
energy in the low momentum limit. This follows within the perturbative QCD from
the colour coherence in soft gluon branching. Remarkably, the hadrons follow
such behaviour closely, suggesting the parton hadron duality picture to be
appropriate also for the low momentum particles. More generally, this scaling
property holds for particles of low transverse and arbitrary longitudinal
momentum, which explains an old experimental observation (``fan invariance'').
Further tests of the perturbatively based picture for soft particle production
are proposed for three-jet events in e+e- annihilation and di-jet production
events in gamma p, gamma-gamma and p\bar p collisions. They are based upon the
difference in the intensity of the soft radiation from primary q\bar q and gg
antennae.
