nasties in gypsum??

I read something about nasties in gypsum, but it was a while ago and I can't remember the details. Does anyone know anything?
I would like to use gypsum as clay breaker. Our soil is alkaline.

Gypsum is calcium sulfate.  I see no problem.

Can you give more details?

Naw, not at all. I use it all the time. Nothing but positive resultants.
Check this segment: http://youtu.be/f86K58P7xS8?t=3m15s
Quite informative...

Maybe it was gypsum from wallboard (gib), which people are starting to use from the waste stream. It can have various additives in it that you might not want in your land.

I don't really remember, it was something on most gypsums commercially available the stuff they sell at Bunnings.

I've read plenty about gypsum really only being of much use in sodic soils, which are pretty rare.
When I think of 'claybreaker', I think of loads of organic matter rather than calcium sulfate, but plenty of people swear by it.

According to "Hands On Agronomy" (book about Albrecht-style soil science), gypsum is to be used with caution because of its sulfur content.  Apparently it is very important to use it in some specific cases, but in general I believe he recommends using a different form of calcium (calcitic limestone):

"Gypsum is a source of calcium, but it should only be used when there is a need for calcium and the calcium saturation is above 60% in that soil."

According to the Albrecht system, Ca and Mg together should total up to about 70-80% of the total cation exchange capacity for the soil, so if you're already at 60% calcium it's pretty uncommon to need more of it I think.

I think the issue is that adding sulfur to a soil that doesn't need it binds up other minerals, but I'm not 100% sure I understood it right.  Anyway, it looks like it's not a case of gypsum being "bad", just being called for in specific circumstances.  I don't really have first hand experience with it though, so I'm just parroting what I read from a book here...

The sulfur will help to acidify the soil, which should be OK given the original poster has an alkaline soil. In its sulfate form (as found in gypsum) it may compete for uptake with other anions e.g. nitrates.

Some info here on sulfur: http://www.spectrumanalytic.com/support/library/ff/S_Basics.htm

As permacaper says, gypsum is mainly useful with sodic soils.  These soils have a high amount of sodium compared to calcium.  By adding calcium a better balance is established, which improves the soil texture, helping to break up clay.  But calcium and sodium are salts, so adding too much can cause problems with plant growth.

http://www.soilduck.com/2010/09/soil-myths-3-clay-breaker-is-not-always.html

Follow the steps in the link above to do some simple tests to see if your soil is sodic.  You already know it is alkaline so that is step 1 taken care of.

apparently there are heavy metals in gypsum too. This might be stated on the packet (only if there are none obviously)

http://cenvironment.blogspot.com/2009/12/flue-gas-desulfurization-gypsum-used-on.html

Don't buy FGD gypsum which is a byproduct of burning coal.  It can contain heavy metals.

Pure gypsum is CaSO4, i.e. all calcium and sulfate (sulfur+oxygen).