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[EarthNSky <erthnsky AT bellsouth.net>]

Regarding gypsum use on clay, this is my experience. After adding gypsum to
a Beaumont Clay soil, starting pH of 6.5, the pH increased to around 8.0
within a month.  The clay is highly weathered, semi-tropical, compact and is
submerged several times a year (~ 60 inches rain/yr)

Attached are a couple of interesting documents.  Please refer to the calcium
speciation diagram.

The exact reason for the increase in pH is difficult to determine. The
proposed scenario is as such:
Gypsum is dissolved by acidic soil water containing carbonate species.
Carbonate species are supplied by active soil bacteria oxidizing organic
matter, which is ongoing in warm semi-tropical soils.  Calcium speciation
shifts to calcium carbonate (see speciation diagram).

Anaerobic sulfur reducing bacteria in the clay interstices utilized the
available sulfur as a terminal electron acceptor. The reduced sulfide either
combines with iron, is volatilized or contributes to soil toxicity. The
sulfate is removed from the soil and does not contribute to acidity. The
calcium increases alkalinity in the soil.

The result was vegetables would not
grow at all in this site for a year after adding gypsum. Only heavy mulching
with compost improved the soil's growing capacity years later.

I should have specified my soil type and rainfall pattern,
since these conditions vary around the globe and are critical to soil
chemistry . I apologize for any misleading statements
I may have given.


--
Bevanron of EarthNSky Farm   Sleep well tonight, for somewhere in the 
ocean's depths, MY son is protecting YOUR freedom.