Natural Gypsum from Queensland Organics is certified organic by Biofarmers and contains 16% Calcium and 20% Sulfur which adds vital minerals and helps to breakdown clay. It also aids water retention and at pH 7.5 is neutral and so does not change the existing pH of the soil. If you have compacted soil is will help to open it up and although it may require annual application for around 3 years it will help to provide good growing conditions for gardeners with heavy clay soils and/or subsoils. If you are unsure simply dig a hole around 400mm deep and if you hit clay then use the gypsum. This type of gypsum has no cadmium, the heavy metal found in many gypsums.
It comes in 25kg plastic bags (shake clean to render the bag recyclable at your supermarket) which is sufficient for around 100m2 (at the manufacturer’s recommended rate of 200g or 1 cupful per m2). A higher rate for heavy soils may be increased up to 500g per m2.