Ezequiel
I think your grandmother was basically correct.  Many soils are deficient
in lime (calcium), in other words they are too acidic.  I cannot remember
what the plant uses the lime for or whether it is needed to allow other
nutrients to be utilized.  I would think that it would be better to crush
the shells rather than leave them whole, but perhaps whole shells leach
their calcium quickly enough in soil.
Where I live, the opposite is true, we have too much lime in the soil and
this prevents iron from being available to the plants, so serious gardeners
here look for ways to make the soil more acidic. So adding shells here
would not help the plants.

Nora
Calgary, Alberta
CANADA

"Ezequiel M. Pinto-Guillaume" wrote:

> Hi everyone,
>
> When I was a child I remember seeing my grandmother placing shells in
> planting pots in order to give the plants calcium (This was in Italy in
> the sixties). Almost every single planting pot in the garden had its
> share of shells. A month ago, I saw at a friend's house (Uppsala,
> Sweden) also a potted plant into which half-a-dozen shells had been
> placed. When I asked about it, I was told that it was good for the
> plants.
>
> Has anyone heard, seen or know more about this curious custom?
>
> Best regards,
>
> Ezequiel
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