Six DIY Soil Tests to Run Before You Plant a Garden

This is part 3 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.

Most of the work in your garden is soil work. Build the soil and the soil will look after the plants.

Roughly fifty-nine percent of the species on Earth live in soil. That makes it the most biodiverse habitat there is — more than the oceans, more than the forest above it. Effectively every terrestrial organism depends on it: the whole food chain starts there, and we wouldn't have evolved without it. Since colonisation, Australian soils have been degraded year on year by European farming practice and development.

(The 59% figure comes from Anthony, Bender & van der Heijden, Enumerating soil biodiversity, PNAS 2023. Their estimate is 59 ± 15%, so treat it as an order of magnitude rather than a precise count. The direction is not in doubt: soil is where most life is.)

There is no such thing as bad soil. People say "I've got awful clay soil" as though it were a life sentence. It isn't — and the irony is that clay holds the highest nutrient content of any soil type. The nutrition is already there. It's just locked away where roots can't reach it, and the soil work is what makes it available.

Soil has a physical composition (clay, sand, silt), a chemical one (pH), and a microbial community. We're not trying to turn your clay into something else. We're opening it up so the plants can get at what it's already holding. Everything below costs less than one lab test.

1. pH

Plants need nutrients, and those nutrients become inaccessible outside their ideal pH range. You're aiming for 6 to 6.5, and 7 at the most alkaline.

The test is read by colour. Get a friend to help — it needs more colour accuracy than you'd think, and it's easy to talk yourself into the reading you were hoping for. Match it against the chart in daylight, never under a kitchen light and never under fluorescent.

To raise pH, use garden lime. To lower it, sulphur soil acidifier. Both in small increments.

2. Hydrophobicity

Bone dry soil supports neither plants nor microbes. Some soil actively repels water — clay particles get a waxy film on them, shed from eucalypts overhead or produced by a fungus in the soil, and water simply will not enter.

The test: scrape the top centimetre off and pour water on. Watch what the water does. In healthy soil it disappears — you see a dark wet patch spread out. In hydrophobic soil it sits on top and just stays there. A splash beads up into little round drops, like water on a waxed car. The dust underneath stays dust.

The other tell is simpler still: dry soil directly after rain. If you dig a day after a downpour and it's dry an inch down, you have a problem.

3. Soil life

Dig, and look. What insects are there? Are there worms — many, or none? What does it feel like in the hand?

Take some photos of the soil for your own reference. Write down what you see. The point is the comparison: what you find now against what you find two years after the garden goes in. The change is usually dramatic, and you'll only really believe it if you have the picture to look back at.

4. Compaction

Anywhere that's been driven on, walked over, or had something heavy sitting on it. Compacted soil is hard for roots, water, oxygen and microbes to move through. Note those areas — they need loosening.

The simplest test you have: if the soil is especially hard to dig, it will be equally hard for plants. Your spade is the instrument. Where you have to lean on it, roots have to fight too.

5. Drainage

Dig a hole 30cm deep by 30cm across. Fill it with water and let it drain completely. Fill it again, and time the second drain — the first fill only wets the soil, the second is the reading.

While you're there, use your nose. Anaerobic soil — soil with no oxygen in it — smells sour and bad, like a wet compost bin. Anywhere that smells like that needs structure introduced before anything will grow in it.

6. Structure — the jar test

Dig up soil, put it in a jar, fill with water, shake it hard, and let it settle. The particles fall out by weight — heaviest first — so the soil sorts itself into bands up the glass.

You'll need a permanent marker. Each time a layer settles, draw a line on the outside of the jar at the top of it. Set a timer, because the intervals matter:

Three lines, three layers. Measure the depth of each band up the side and you have your proportions — and with them, your soil type.

Read the top of the jar too. Organic matter floats — it stays up in the water and on the surface. A good amount of floating material means real organic content in your soil. Not much floating means not much organic matter, and that tells you where to start.

There's one more test worth its own post: the gypsum jar test, which tells you whether gypsum will actually fix your clay — or whether you'd be pouring money on the ground.