  Infrared JKs-band photometry of the Galactic globular clusters M15 and, for
the first time, M56 is presented. We estimated the reddening
(E(B-V)=0.18+/-0.08 mag) and distance modulus ((m-M)_V=15.43+/-0.30 mag)
towards the poorly studied globular cluster M56. We combined our data with
observations of other clusters from the literature (12 in total) to extend the
[Fe/H] vs. Red Giant Branch (RGB) slope relation towards metal-poor clusters.
Our best fit yields to [Fe/H]=-3.40(+/-0.22)-27.74(+/-2.35)x(RGB Slope), with
an r.m.s.=0.20. The broader metallicity baseline greatly reduced the
uncertainties compared to other existing calibrations. We confirmed a
previously obtained calibration of the relation between the RGB color
(J-Ks)_0(RGB) at M_Ks=-5.5 vs. [Fe/H]:
[Fe/H]=-6.90(+/-0.99)+6.63(+/-1.05)x(J-Ks)_0(RGB) with an r.m.s.=0.33. Finally,
using the new RGB slope calibration we estimated the abundance of the super
metal-rich cluster Liller 1 [Fe/H]=+0.34+/-0.22.
