Gypsum CaSO4.2H2O is recommended as an agent if you don't want to raise the
pH of the soil. With a formula weight 182 it is weight for weight less
effective than hydrated lime. (Formula weight 71). Usually lime is added to
raise the pH as well as cause edge flocculation in the clay.
Gypsum is the main ingredient of plaster board and is the compound which is
formed when water is added to plaster of Paris (CaSO4.0.5H20)
Clay soils are quite well buffered so unless you really want to keep pH down
use lime.
A useful compromise is to use chalk which weathers down, is not strongly
alkaline on application (pH 8 cf 12 for lime) and with a formula weigh of
100 is good value weight for weight.
(As an aside I grew brassiccas successfully in land infested with club root
by planting the seedlings with a ball of powdered chalk mixed with compost
around their roots.)
Regards
David T
"Pam Moore" wrote in message
...
On 29 Jul 2006 20:17:16 GMT, (Nick Maclaren) wrote:


In article ,
"herman" writes:
|
| I've read that Gypsum is the best material for breaking down clay soil.
| But! Where can one buy it? Garden centres try to sell their own
| alternatives and builders supplies have never heard of it.

Well, it is doubtfully effective, but it is the principle component of
building plaster.

Who remembers Acta Bacta? Didn't that contain gypsum?

Pam in Bristol