  We report on isofield magnetic relaxation data on a single crystal of
$Ba_{1-x}K_xFe_2As_2$ with superconducting transition temperature $T_c$= 32.7 K
which exhibit the so called fish-tail effect. A surface map of the
superconducting transition temperature shows that the superconducting
properties are close to homogeneous across the sample. Magnetic relaxation
data, M(t), was used to obtain the activation energy U(M) in order to study
different vortex dynamics regimes. Results of this analysis along with time
dependent measurements as a function of field and temperature extended to the
reversible region of some M(H) curves demonstrate that the irreversibility as
well the second magnetization peak position, $H_p(T)$, are time dependent and
controlled by plastic motion of the vortex state. In the region delimited by a
characteristic field Hon (well below $H_p$), and $H_p$, the vortex dynamics is
controlled by collective pinning. For fields below Hon the activation energy,
$U_0$, increases with field as expected for collective pinning, but the pinning
mechanism is likely to be in the single vortex limit.
