Out-of-date finishing plaster to improve clay soil?

[noth...@aolbin.com]

I have a couple of bags of plaster to ditch and I've read differing
views about whether it can be used to improve clay soil so ... do I take
it to the dump or dig it into the soil or the huge piles of grass
cuttings and leaves?

[Chris J Dixon]

Gypsum is recommended as a soil improver where clay is present,
but appears to be sold in garden centres as a proprietary product
costing over £6 for 2.5 kg.

On the other hand, I picked up a 25 kg bag of multi-finish
plaster for under £5, and AFAIK this is essentially the same
material. It worked very well.

Builders plaster is not the same as it used to be many years ago,
when it was all derived from mined gypsum. These days some (a
decreasing amount) of it is the by-product of flue gas
desulphurisation at big coal-fired power stations, where the flue
gases are scrubbed with lime. As the lime is used up, the
efficiency of the scrubbing process drops away, and the spent
lime, now mostly gypsum, is sold off for building use.

It still contains a little residual lime, so whereas in the past,
gypsum could be used around lime-hating ericaceous plants such as
rhododendrons, camellias and heathers to improve and even acidify
the soil (pure gypsum being mildly acidic), it would not be a
good idea these days.

Still OK as a 'clay-breaker' though.

As it happens, I live in an area that features both methods of
production, and have even seen it being mined. However, which one
makes it into the local shops is anybody's guess.

Chris
--
Chris J Dixon Nottingham UK
ch...@cdixon.me.uk @ChrisJDixon1

Plant amazing Acers.

[Martin Brown]

It ought to work to flocculate clay but it might also shift the pH
towards alkaline in the process. Proper horticultural gypsum ought to be
roughly pH neutral but builders plaster tends to be a bit more alkaline.

Try it on a patch of bad clay and see if it does what you want and the
shift i pH is tolerable. You won't be growing azaleas there for quite
while. But anything that likes lime will be perfectly happy.

Clay gets a bad press but apart from being hard work to dig is a
perfectly fertile soil - it just doesn't drain well. I'm on terminal
moraine heavy clay in Yorkshire. There are local ponds where clay for
building bricks was previously extracted including for my house!

--
Regards,
Martin Brown

[Nick Maclaren]

In article <t50mrk$1498$1...@gioia.aioe.org>,

Martin Brown <'''newspam'''@nonad.co.uk> wrote:
>On 04/05/2022 16:25, noth...@aolbin.com wrote:
>> I have a couple of bags of plaster to ditch and I've read differing
>> views about whether it can be used to improve clay soil so ... do I take
>> it to the dump or dig it into the soil or the huge piles of grass
>> cuttings and leaves?
>
>It ought to work to flocculate clay but it might also shift the pH
>towards alkaline in the process. Proper horticultural gypsum ought to be
>roughly pH neutral but builders plaster tends to be a bit more alkaline.

Unless you have some extreme calcifuges, it won't make enough difference
to matter - and, even then, it would matter only if you piled it over
their roots. Scattered over a wider area, it will do no harm (and only
a little good), but it's what I would do.


Regards,
Nick Maclaren.