  We show that the study of the Calcium depletion is a valid an highly
sensitive method for investigating the chemical and physical history of the
very extended ionized nebulae seen around radio galaxies (EELR), massive
ellipticals and `cooling flow' galaxies. By observing the near IR spectrum of
nebular regions characterized by low excitation emission lines (LINER-like), we
can use the intensity of the [CaII]$\lambda\lambda 7291,7324$\AA\ doublet
--relative to other lines, like H$\alpha$-- to infer the amount of Calcium
depletion onto dust grains. The presence of dust in these objects --which does
not necessarily result in a measurable level of extinction-- would favour a
`galactic debris' rather than a `cooling flow' origin for the emitting gas.
Before aplying such test to our data, we study four possible alternative
mechanisms to dust depletion and which could have explained the absence of the
[CaII] lines: a) ionization of Ca$^+$ from its metastable level, b) thermal
ionization of Ca$^+$, c) a high ionization parameter and/or a harder ionizing
contiuum than usually asummed and d) matter bounded models associated to a hard
ionizing continuum. We show that none of these alternative mechanisms explain
the absence of the [CaII] lines, except possibly for the highly ionized EELR
where a high ionization parameter is required combined with a soft power law.
We thus conclude that for the other low excitation emission regions (cooling
flows, liners, low excitation EELR), the abscence of the CaII lines {\it must}
be due to the depletion of Calcium onto dust grains.
