This is part 4 of The Garden Guide — the studio's field manual for building a native garden from bare ground in Canberra and the Southern Tablelands. Start at the overview if you've just arrived.
Picture clay soil as layers and layers of dinner plates, all packed tight and overlapping every which way at random. There's no gap anywhere for water or air to get through — every route is blocked by another plate lying across it.
Gypsum stacks the plates. Tidy piles, with clear gaps between the stacks — and water and oxygen move freely down the gaps. That's what it does: it binds the individual clay particles together into crumbs, and the space between the crumbs becomes the plumbing. But it only works on some soils, and on the rest it's money poured on the ground — hence the test.
The test
- Roll a 1cm ball of soil taken from 10cm down. Add a little distilled water if it won't bind.
- Fill a clean jar with distilled, bottled or rainwater — never tap water.
- Lower the ball in gently. Don't shake it. Read it at 2 hours, then again at 24 hours.
| Result | Means |
|---|---|
| Water goes cloudy or milky around the ball | Dispersive clay. It will respond to gypsum — the cloudier, the stronger the response. |
| Water stays clear, ball holds together | Stable soil. Unlikely to respond — don't buy gypsum. |
| Ball crumbles into a cone, water stays clear | That's slaking, not dispersion. Gypsum won't fix it. It needs organics — lots. |
The water tells you, not the ball.
Is this test actually reliable?
Yes — more than it looks. It's the Emerson Aggregate Test, a standard soil science method used by agronomists, and a version of it is standardised as ASTM D6572. What it detects is dispersion caused by sodium on the clay, which is precisely the thing gypsum fixes: the calcium in gypsum swaps out the sodium and lets the particles clump.
A cloudy result is trustworthy. It's a reliable positive — if it goes milky, gypsum will help.
A clear result is less certain. High organic matter, high iron or acidic soil can all stop a genuinely sodic soil from dispersing in the jar, so you can get a false "no". If everything else says your clay is tight and airless but the jar stays clear, it's worth a lab test before ruling gypsum out.
The tap water trap: tap water already contains calcium — the same ion gypsum supplies. Run the test in tap water and you may well suppress the dispersion you were trying to detect, and get a clear jar from a soil that needed gypsum all along. Distilled, bottled or rainwater only.
If the jar went cloudy, gypsum goes on at about two cups per square metre as part of the soil renovation.
This series comes from the Garden Guide I give my design clients. I write one letter a week on spatial design, craft and technology — and subscribers read the full guides first.
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