"WiGard" wrote in message
news

This will alter the pH.

Gypsum does not alter the pH of soil.


http://www.awgypsum.com/why_use_gypsum.htm
It does effect pH.


The effect quoted here is called buffering, which is the tendency to modify
excesses of high or low pH towards a middle value. The value depends on the
type of buffer, for gypsum I think it is around neutral. To have a
significant effect will probably take a great deal of gypsum and the closer
the pH gets to neutral the less change adding more gypsum will produce. I
would expect as buffering substances go gypsum would be quite a poor one.

So while you are technically correct that gypsum _can_ alter soil pH but
practically speaking if you wanted to modify soil pH you wouldn't choose
gypsum to do it as there are much more effective ways. If you wanted to
modify pH *away* from the buffer level, from neutral to either acid or
alkali, you are completely wasting your time with gypsum.

I would like to see this vendor produce figures showing how much gypsum
would be required to raise the pH of a variety of common soils from say 5 to
6, which would be a typical sort of thing you would want to do.

This is a complex issue. From the point of view of the practical gardener,
if I had to choose between two simplified views of the world (1) "gypsum is
useful to modify soil pH" and (2) "gypsum has no significant use in
modifying soil pH" I would take the second every time.

You should keep in mind that these people are trying to sell gypsum!

David