Clay-improver bags are mostly gypsum, and the label promises to break up heavy ground. The careful advice (RHS) is: lime for acid clay, gypsum elsewhere, and test a small area first, because only some clays respond. In Scotland, where much clay is acid, it's usually lime and organic matter that do the work.
Quick answer
On acid clay, which a lot of Scottish ground is, use lime rather than gypsum. Gypsum is for clay that isn't acid, and even then only some clays respond, so try a small patch first. Organic matter and drainage are the usual first steps for clay.
What gypsum does
Gypsum is calcium sulphate, and it's the active ingredient of many commercial clay improvers. The idea is flocculation: calcium makes clay particles clump together into larger crumbs, which opens the soil so air and water can move through it. Some, but not all, clay soils respond to extra calcium, so test on a small area in the first instance to make sure it works on your type of clay. A clay whose particles are already bound by calcium and magnesium has little for extra calcium to do, which is one reason it often disappoints.
Unlike lime, gypsum doesn't raise pH. So where the soil is acid, lime can be applied, and where it isn't, gypsum is the one to add.
Where gypsum works best: sodic soil
Gypsum's best-documented job is fixing sodic soil. Soils high in exchangeable sodium relative to calcium and magnesium are called sodic, and their pH usually exceeds 8.5. They're often marked by a dark or white crust on the surface and slow water penetration. Sodium can destroy the aggregate structure of fine- and medium-textured soils, which cuts porosity so the ground can't hold enough air and water, and it can damage roots by direct toxicity.
To reclaim sodic soil, you apply gypsum first so calcium displaces the sodium, and then leach it out with water. A lab test of the sodium against calcium and magnesium is the way to find out whether soil is sodic, and the same advice is to test before applying gypsum to see whether it will help and how much to use.
What this means in Scotland
Sodic soil forms where evaporation beats rainfall, or where mineral-rich irrigation water builds up sodium. Scotland's heavy rain does the opposite: it washes soluble salts through the soil, which is why much of Scotland's soil is naturally acidic. That's reasoning from how sodic soil forms, not a finding from Scottish trials, but it points the same way: acid clay wants lime, not gypsum.
A coastal garden is a different case. Sea spray can scorch leaves, with damage on the windward side of plants, but that's sodium on the foliage, not sodic clay, and gypsum won't fix it.
If you want to try it
Test a small patch first, using the rate printed on the bag, since no home-garden dose is published, and see whether the clay really improves before you treat a whole bed. The right amount depends on a soil test. If a lab test does show sodium dominating the soil, gypsum is the right treatment.
The appeal of gypsum is that granules are cheap and need no digging. But organic matter and drainage are the usual first steps on clay, so put the money and effort there rather than gambling on a test patch. A bag is not a wasted trip if a soil test shows sodium or a calcium shortage is the problem. A bed beside a road or the shore may pick up sodium from winter road grit or sea salt, and a clay already dominated by calcium and magnesium gives gypsum nothing to displace. Gypsum does not shift pH, so it is safer than lime near rhododendrons or blueberries, though there is little reason to use it there.
What to do instead
Raised beds or mounding improve drainage, and repeated additions of organic matter improve structure, whether or not the clay is sodic. Do not count on sand: diluting clay with sand or grit takes very large volumes, so a few bags will not fix it. The clay soil guide covers both, and when and how much lime covers acid ground.
Common questions
Is gypsum the same as garden lime?
No. Both supply calcium, but lime raises soil pH and gypsum doesn't, so lime is for acid soil and gypsum for soil that isn't acid.
Will gypsum harm my soil if it does nothing?
It doesn't raise pH, so applying it where it isn't needed is mostly a waste of money, not a risk to the bed.
How do I know if my soil is sodic?
A lab test of sodium against calcium and magnesium is the reliable way. A dark or white crust and slow water penetration are warning signs, and the pH is usually above 8.5. A crusted or waterlogged clay can look similar without being sodic.
Is gypsum safe near rhododendrons or blueberries?
Gypsum does not raise soil pH, so it is safer there than lime, but there is little reason to use it unless a soil test shows a sodium problem.
If gypsum will not fix my clay, what will?
Raised beds or mounding for drainage, and repeated additions of organic matter for structure. Lime helps acid clay.
Sources
Show the 2 sources
- Clay soils: Calcium makes clay particles flocculate; some but not all clay soils respond; lime where soil is acid, gypsum elsewhere; gypsum is the active ingredient of many clay improvers; test on a small area first
- Nutrient and mineral excesses, salinity, and salt toxicity (UC IPM): Sodic soil: pH over 8.5, crust, slow water penetration, damage to aggregate structure; gypsum then leaching; lab testing; sea spray on foliage