  We extend our previous calibration of the infrared Ca II triplet as
metallicity indicator to the metal-poor regime by including observations of 55
field stars with [Fe/H] down to -4.0 dex. While we previously solved the
saturation at high-metallicity using a combination of a Lorentzian plus a
Gaussian to reproduce the line profiles, in this paper we address the
non-linearity at low-metallicity following the suggestion of Starkenburg et al
2010 of adding two non-linear terms to the relation among the [Fe/H],
luminosity, and strength of the Calcium triplet lines. Our calibration thus
extends from -4.0 to +0.5 in metallicity and is presented using four different
luminosity indicators: V-V_{HB}, M_V, M_I, and M_K. The calibration obtained in
this paper results in a tight correlation between [Fe/H] abundances measured
from high resolution spectra and [Fe/H] values derived from the CaT, over the
whole metallicity range covered.
