Re: CULT: Gypsum (was Old sawdust)

From: "John Bruce" <jbruce@infinet.com>


>All the advice which I have been given is that gypsum will
>NOT acidify soil. I can't pretend to be a chemist but gypsum
>is hydrated calcium sulphate and it seems to me that it will
>produce one negative and one positive ion. Thus it shouldn't
>do much to change the pH, or am I overlooking something.


I have never had a problem with gypsum acidifying my soil.
But then I add it in sparing amounts. It is strongly recommended
to not add gypsum if your soil pH is less than 5.8 . Extension agents
in our area recommend gypsum for mildly acidifying soil and agricultural 
sulfur for a stronger acidifying effect. Thes are much quicker acting than
peat moss and other organic additives.

Gypsum is one of the ingredients of Portland cement. It acts 
to greatly extend the hardening time of the silicates while they are 
in solution. Presumably this is the action it performs in soil.

Ionic charges are not the primary driver of pH. A good example is 
hydrochloric acid. It is a strong acid made up of a single positive and
a single negative ion. It is still ionized enough to conduct electrical 
current when dissolved in water.  pH is more strongly related to 
concentration, equilibrium and relative strength of systems in solution.
I think the sulfate ion in the gypsum is far more strongly acidic than 
the calcium ion is a basic one, and that is where the acidifying action 
of the gypsum would lie.

OK....now everyone can wake up....chemistry class is over <G>

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
John Bruce
jbruce@infinet.com
President, Miami Valley Iris Society
AIS Region 6 / SW OH Zone 5 / Sunset Zone 35
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~





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