On Jun 7, 1:46*pm, Chris J Dixon wrote:
Ragnar wrote:

"Chris J Dixon" wrote in message
.. .
Having just extended a flower bed to incorporate what has been
lawn for 40 years, I am left with some pretty solid clay soil.

Gypsum is recommended as a soil improver, but appears to be sold
in garden centres as a proprietary product costing nearly 5 for
2.5 kg.

http://www.capitalgardens.co.uk/clay...itioner-p-6321.....

On the other hand, I can pick up a 25 kg bag of multi-finish
plaster for under 5, and AFAIK this is essentially the same
material.

Any reasons not to go with plaster?
I don't really know the answer to your question, but I would warn you not to
use gypsum if you want lime-hating plants such as heathers, rhododendrons
etc

Other sources seem to be of the opinion that one of the benefits
of using gypsum is that it is essentially neutral, and will not
alter pH.

Gypsum is calcium sulphate which is more or less neutral but is
calcium rich which will hurt alpines. Nothing seems to hurt
rosydandelions.

Sand is neutral and acid. Use a mix of sand and gypsum with sawdust or
shedded organic stuff. Grow lupins for a while. Any nitrogen loss will
soon come back.

Sand and clay = loam. I think sand and gypsum = marl.