Chris Hogg wrote:
On Wed, 21 May 2008 12:13:10 +0100, stuart noble
wrote:

Isn't mineral gypsum alkaline as well?

I don't think so, but I may be wrong of course. From my school
chemistry, lime is not quite so strong an alkali as sulphuric acid is
an acid, and the resulting 'salt' (gypsum) ends up slightly acidic
when dissolved in water, even though the stoichiometry is satisfied
(i.e. there is no chemical excess of sulphuric acid). But I've not
actually tested it, for the simple reason I've not come across any,
although I confess I haven't exactly looked hard.

Gypsum used to be recommended for use on clay soils where ericaceous
plants (rhododendrons, azaleas etc.) were grown. These plants cannot
be grown on alkaline soils because some trace elements, notably iron
and manganese, are relatively insoluble at high pH's, and are thus not
available in sufficient quantity for the needs of those plants, and
they become chlorotic with yellowing leaves.

Gypsum would not have been recommended if it were alkaline. But that
recommendation comes from the days when most gypsum was mined or
quarried. Nowadays, the gypsum formed from desulphurisation of flue
gases with lime is widely available, almost a dump material, and is
consequently much cheaper than mined gypsum, and AIUI killed off most
of the mineral gypsum industry some years ago.


Interesting stuff. I seem to remember that clay is slightly alkaline, so
presumably soils that need breaking up aren't usually acidic. Don't
know. I'm not much of a gardener, let alone a chemist :-)