"It's regarded as rather old-fashioned," explained Chris, the nice lady at Rootwise who answered the phone. "And people don't understand what it's for."
My arrival at Rootwise had taken time, because tracking down gypsum is not easy. According to this thread on Allotment Gardening, because people don't understand gypsum and it's so bulky and hard to shift, garden centres are now reluctant to stock it. Rootwise supply it in 25kg bags to gardeners by mail-order (and to agriculture in massive tonnage), but are finding it extremely difficult to persuade garden centres to take it.
This is a shame, because when I first used gypsum it seemed almost sinfully easy. All you do is sprinkle it around; you don't even have to dig it in. I first tried it about ten years ago. I gave one of the beds a topping that looked as if I'd iced it with sugar, thought, "What on earth is that going to do?" and forgot about it. When I came back to the soil the following spring, it didn't seem as heavy as I'd expected, and then I remembered the gypsum.
How gypsum works
It's this sort of effect that can get you hooked on soil science. The calcium contained in gypsum reacts with the chemically active clay particles and encourages flocculation - the binding of the particles into slightly larger aggregates. This means the spaces between soil particles are larger, so the soil looser and, immediately, drainage is improved. The effect doesn't last forever, which is why gypsum needs reapplying every so often.
Rootwise recommend an application in spring and autumn for really heavy clay, dropping down to one application every other year once it's in good condition.
Nothing, of course, substitutes for adding bulky organic matter, or compost (equally old-fashioned but, arguably, better understood) but anything that aids the break up of the sort of massive, solid clods that I chip my way through in the garden has to be embraced. As long as you can find it.
If you have clay soil, can I ask that you beetle down to your local nursery or garden centre and ask for gypsum? Only customer demand will make it more widely available.
Incidentally, occasionally you find references to using it as a general soil improver. Read the ever-helpful Linda Chalker-Scott to see why it's pointless to use gypsum on any soil but clay.
NB And Michelle from Vegplotting has kindly pointed out that this post (completely coincidentally, I have to admit) coincides with World Soil Day (always December 5th). Find out more at the Global Soil Partnership.