How to improve clay soil

Test 1: is it really clay? The ribbon test

Take a small handful of soil from spade depth, remove stones and roots, and add water a few drops at a time. Knead it for a minute or two until it is like putty. Then push it out between your thumb and forefinger into a flat ribbon until it breaks under its own weight. The length of the ribbon tells you the texture:

A clay loam is not a problem soil. It is fertile and holds water well, and it needs only compost and mulch. The advice below matters most for light to heavy clays.

Test 2: does it disperse?

This is the test that decides whether gypsum is worth buying:

1. Let a few crumbs of soil, 3–5 mm across, dry out completely.
2. Drop them gently into a clear glass of rainwater or distilled water. Tap water contains salts that can mask the result. Do not stir.
3. Watch the water around the crumbs over the next two hours, and check again the next day.

Dispersive clay behaves as it does because sodium sits on the clay particles and pushes them apart. When it gets wet, it turns to slurry, seals the surface and dries into a crust. Sodic soils are common across Australia, which is why gypsum has such a reputation here. It is also why it so often disappoints on clays that were never sodic.

How much gypsum

Gypsum (calcium sulfate) supplies calcium that swaps places with the sodium, so the clay particles clump together into crumbs. Water and air can then move between them. Gypsum does not change the soil's pH and is not a fertiliser, although it does supply calcium and sulfur.

• Garden starting rate: about 200 g per m², which is roughly two cupfuls. This is the rate ABC Gardening Australia suggests. Dig it into the top 15–20 cm and re-test after the soil has been wet a few times. If a fresh crumb still clouds the water, repeat.
• For strongly sodic soils: field trials in Australia use 2.5–5 tonnes per hectare, which is 250–500 g per m². Heavier doses are rarely needed in a garden, and re-testing is a better guide than piling more on.
• Top it up with testing, not by habit. Gypsum slowly washes out, so a dispersive soil may need another dose every few years. The glass of rainwater tells you when.

Organic matter: the real fix

Gypsum can stop a sodic clay from dispersing, but only organic matter builds the lasting crumb structure that makes clay easy to work. Soil life turns compost into the natural glues that bind clay into crumbs, and roots and worms open channels through it. On clay that slakes but does not disperse, organic matter is the only thing that helps.

• New bed: spread 75–100 mm of compost or soil conditioner and fork it into the top 20 cm, together with the gypsum if your test called for it. That is 75–100 litres per m². The compost calculator turns it into bags or cubic metres.
• Every year after: 25–50 mm of compost on the surface and 50–75 mm of mulch over it. Worms will take it down for you.
• Grow roots: a winter green-manure crop dug in before it sets seed adds organic matter deep down.

Why not add sand?

It sounds logical: clay is too fine, sand is coarse, so mix them. In practice a few barrows of sand in a clay bed usually makes it worse. The clay fills the spaces between the sand grains and sets like a brick. To actually change the texture you would need to add more sand than there is clay. Add organic matter instead, or build a raised bed over the clay.

When and how to dig clay

• Never when wet. Squeeze a handful. If it smears and holds a shiny shape, it is too wet, and digging will destroy what structure it has. Wait a few dry days.
• Not when bone dry either. It comes up in rock-hard clods. The right moment is when a handful crumbles when you press it.
• Don't walk on beds. Clay compacts under feet. Use paths or boards, and keep beds narrow enough, about 1.2 m, to reach the middle.
• Autumn is a good time to add gypsum and compost in southern Australia and New Zealand. Winter rain then works them in before spring planting.

Working with clay instead of against it

Clay has real advantages. It holds nutrients and water far better than sand, and once it has structure it is some of the most productive soil there is. Where it is heavy and slow to drain, work around it:

• Raise the root zone. A raised bed or a 20–30 cm mound over improved clay gives roots drained soil while they reach down into moisture below. The depth guide covers beds on clay, and the raised bed calculator works out the soil.
• Plant trees high. In heavy clay, a hole dug and filled with rich mix becomes a sump that drowns the roots. Plant on a low mound instead, with only a little compost mixed into the backfill.
• Choose plants that like it. Many Australian natives grow naturally in clay. Avoid high-phosphorus manures and mushroom compost around grevilleas, banksias, hakeas and their relatives, which are sensitive to phosphorus.

Worked example: a new vegetable bed on dispersive clay

1. A 3 m × 2 m bed, 6 m². The crumb test shows a halo within half an hour, so the clay is moderately dispersive.
2. Gypsum at 200 g/m²: 1.2 kg to start, forked into the top 20 cm.
3. Compost at 75 mm: 6 × 0.075 = 0.45 m³, which is 450 L or 15 × 30 L bags, forked in at the same time.
4. Mulch with 50–75 mm of straw. After the first few heavy soakings, test a fresh crumb. If it still clouds, add another 200 g/m².

Questions