Perth soils: identifying Bassendean, Spearwood and Karrakatta on your block

Ask a Perth landscaper what the soil is like and you get a one-word answer: sand. That is true, and it is not useful. There are several quite different sands under this city, they behave differently, and the one under your block sets a large part of your budget before you have drawn anything.

This lesson is about finding out which one you have. It takes one hole, twenty minutes, and no equipment you do not already own.

Why a professional checks this first

Soil type drives four expensive decisions:

1. How much money goes into the ground before a single plant does. Amending deep pale sand to a useful standard costs real money per square metre. On shallow sand over limestone you may be paying to take material out instead.
2. What you can plant. Alkaline sand over limestone and slightly acidic grey sand suit noticeably different palettes, and getting this wrong shows up about eighteen months later as yellowing leaves and slow decline.
3. How the irrigation must be designed. Water moves through these sands fast. Application rate, run time and whether you cycle-and-soak all depend on what is underneath.
4. Whether you can dig at all. Hitting cemented limestone at 400 mm changes your excavation method, your machinery hire and your drainage design.

None of that is visible from the surface. All of it is visible in a hole.

The Swan Coastal Plain in one paragraph

Perth sits on a coastal plain made of ancient dune systems laid down in sequence, oldest inland and youngest at the coast. Moving from the hills toward the sea you cross the Bassendean system, then the Spearwood system, then the narrow coastal Quindalup dunes. The Karrakatta soil association is the familiar shallow-brown-sand-over-limestone found across much of the Spearwood system — which is why you will hear “Karrakatta sand” used in Perth as a soil name rather than a suburb name.

The consequence that matters: the further inland you are, the longer that sand has been sitting in the rain. Bassendean sand has been leached for a very long time. It has no limestone left in it, negligible clay, and holds almost nothing.

Digging the test hole

Pick somewhere representative — not the strip beside the driveway where fill was dumped during construction, and not the one bed someone has been composting for a decade. If your block has obviously different areas, dig a hole in each.

Go down at least 600 mm if you can. Lay the material out on a tarp in order as you go, so you can read the profile as a sequence rather than a pile.

Then record four things.

1. Colour, and where it changes

This is the single most diagnostic observation you will make.

What you see	Likely system	What it means for you
Pale grey or near-white sand, often paler with depth, no limestone	Bassendean	Slightly acidic. Very low nutrient retention. Prone to severe summer water repellency. Budget for clay and organic matter, and expect to keep adding it.
Yellow or orange-brown sand, deep, no rock	Spearwood, deep phase	Better than Bassendean but still free-draining and low in nutrient. Usually neutral to mildly alkaline.
Shallow brown or grey-brown sand over cream rock at 300–800 mm	Karrakatta / Spearwood over limestone	Alkaline. Restricted rooting depth. Excavation gets hard and possibly mechanical. Plant selection genuinely matters.
Cream, shelly, obviously calcareous sand near the coast	Quindalup	Strongly alkaline. Salt and wind exposure as well. The most constrained palette of the four.
Dark, cemented, coffee-coloured hard layer at depth	Coffee rock, common under Bassendean	An impeding layer. Water perches on it. Matters enormously for drainage and for anything deep-rooted.

2. Depth to anything hard

Limestone, coffee rock, clay or compacted fill. Measure it and write it down. This number comes back in your drainage plan, your tree selection and your excavation costs.

3. Whether it repels water

Drip water onto a dry sample from near the surface and time how long the droplet sits there before soaking in. If it beads and sits for more than a few seconds, you have a water-repellent soil — which in a Perth summer means irrigation can run off or channel straight past the root zone while the plant dies in what looks like wet ground. The soil testing lesson covers this properly.

4. How fast it drains

Fill the hole with water, let it drain completely, fill it again, and time the second fill. Sand usually drains very fast. If yours does not, you have found something worth knowing about.

Check the watertable while you are out there

The Perth Groundwater Map gives you the estimated depth to the watertable at your address, free, in about five minutes. On parts of the coastal plain it is shallow enough to affect deep-rooted trees, soakwell performance in winter, and whether a garden bore is worth considering at all.

Do it now, while you are recording soil. It belongs with the rest of your site analysis.

What this changes downstream

Write your findings into your workbook, because these four lines get referenced again and again:

• Soil type and profile → the plant palette and the soil budget
• Depth to hard layer → excavation method, drainage, tree choice
• Water repellency → wetting agent programme and irrigation design
• Depth to watertable → drainage, deep planting, bore viability

The next lesson tests this soil properly — pH, repellency and drainage rate — and turns it into the soil report you will cost from in Phase 7.