Perth soils

Almost every job in the Perth metropolitan area sits on one of three wind-blown dune systems, laid down youngest at the coast and oldest inland — or, on the eastern edge, on the river flats and scarp footslopes behind them. Work out which one you are on before you price anything, because it decides whether you hit limestone at 800 mm, whether the bed will drown in August, and whether the plant list on the drawing will survive its second summer.

The short version

Roughly west to east, with DPIRD’s soil-landscape system codes:

System	Where it sits	What you dig into	Reaction	Water behaviour
Quindalup (211Qu)	The active and recently stabilised coastal dunes — foredunes, blowouts and the first stretch behind the beach	White calcareous sand of quartz and shell debris; calcrete at 60–90 cm in some phases	Alkaline — fizzes hard with acid	Drains instantly; salt-laden wind; effectively no nutrient holding
Spearwood (211Sp)	The limestone ridge immediately inland of the coastal dunes, and it runs further inland than people expect (Cottesloe and Scarborough are both on it)	Yellow to yellow-brown sand over Tamala limestone, commonly at 1–2 m, with outcrop and pinnacles on the ridges	Neutral to alkaline where limestone is shallow	Free draining, deep watertable, but the rock stops your excavator
Bassendean (212Bs)	The central sandplain and low dunefields inland of the limestone ridge (Bassendean itself; DPIRD also names a Jandakot phase)	Pale grey to bleached sand, often over an iron-organic hardpan — the trade calls it coffee rock	Acid to neutral	Repellent when dry; swales waterlog in winter; shallow watertable in places
Pinjarra (213Pj)	Alluvial flats along the Swan, Canning and Helena (Guildford sits on it)	Variable alluvial and aeolian soils; sand over mottled gravelly clay	Variable	Poorly drained plain; clay perches water
Forrestfield (213Fo)	Undulating footslopes of the Darling Scarp	Duplex sandy gravels, pale deep sands and grey sandy duplexes over clay	Acid	Water sits on the clay; gravel eats blades

Boundaries are gradational and they interleave — Quindalup sand blows over Spearwood limestone in places, and DPIRD maps a unit for exactly that. Treat the table as a first guess. Do not trust a suburb list, including this one; look the address up.

Which one are you on?

Look it up. Open NRInfo, zoom to the lot, and read the soil-landscape map unit description. DPIRD publishes these at scales from 1:20,000 to 1:3,000,000, so a boundary can be tens of metres out — and DPIRD says plainly that this is a soil-landscape map, not a soil map, and that you should not make significant decisions on it alone. Note that across much of urban Perth the map unit is carried from the geological mapping, so you will get a description like “SAND — very light grey at surface, yellow at depth” rather than a named soil phase.

Then dig. A 1.2 m hand auger or a post-hole shovel in three spots tells you more than any map:

• Colour. White and shelly is Quindalup. Yellow or yellow-brown is Spearwood. Bleached grey over paler grey is Bassendean.
• The acid test. This is the state’s own field method: drip weak (1 M) hydrochloric acid on a sample. Visible or audible fizzing as the carbonate breaks down means calcareous: Quindalup, or Spearwood where limestone is near the surface. Nothing happens on Bassendean sand.
• What stops the auger. Limestone gives a hard, grating refusal at 0.6–2 m on Spearwood. Coffee rock — DPIRD maps it as an iron-organic hardpan and calls the soils iron-humus podzols — gives a dark brown to near-black cemented layer that a sharp auger meets with a smooth rotational feel. On Bassendean it sits deeper than 2 m in the dune phases (B1), 1–2 m on the sandplain (B2), and less than a metre in swales and depressions (B3a).
• Smell and staining. Peaty black sand and a sulfur smell means an old wetland margin. Stop and read the acid sulfate soil note below.

Depth to watertable

Use the Perth Groundwater Map. It estimates depth to the watertable under a specific property, depth to the base of the superficial aquifer, and groundwater salinity at bore sites.

Read it with two corrections. First, the contours were drawn to end-of-summer levels and DWER states it cannot guarantee accuracy because groundwater level varies seasonally by up to 3 metres. Second, the level is at its highest at the end of winter and falls through summer. A pool excavation quoted off a February hole is not the same job in August. On the low-lying Bassendean flats — the phases DPIRD describes as having a shallow winter watertable — that swing is the difference between a dry dig and a dewatering line item.

Where you are excavating below the watertable near a former wetland, check acid sulfate soil risk. DWER’s classes are defined by depth: high risk means the material may occur within 3 m of the surface, moderate means below that. Disturbing those sediments and letting air reach them oxidises the pyrite to sulfuric acid, which DWER says then dissolves aluminium, iron and heavy metals such as arsenic into the groundwater. Ring DWER before you dig, not after, and ask your council whether it wants an assessment with the application.

pH, and reading a soil test

WA labs report pH in 0.01 M calcium chloride at 1:5. That reads about 0.7 units lower than a hardware-store water test — pH(CaCl2) ≈ pH(water) − 0.7 — so a “6.5” from a kit is roughly 5.8 to a lab. The state ratings are: strongly acid below pH(CaCl2) 4.5, acid 4.5 to 5.5, neutral 5.5 to 7.0, alkaline above 7.0. Most plants do best between 5.5 and 7.5.

In practice: Bassendean sand runs acid to neutral, which suits banksias, hakeas and most of the Swan Coastal Plain natives. Shallow Spearwood over limestone and all of Quindalup run alkaline, which locks up iron, manganese, zinc and copper — that is why gardenias, camellias and blueberries yellow off on the limestone and behave themselves on the acid lateritic soils of the hills. You cannot economically acidify a limestone site. Choose plants that want it, or build raised beds.

Water repellency

Perth sand goes non-wetting because waxes from decomposing leaf litter and root exudates coat the grains. Test it rather than assuming: scrape the top 5 mm off dry topsoil, place water droplets, and time them. Under 10 seconds is non-repellent, 10 to 60 seconds is low, over 60 seconds is moderate, and anything that beads for minutes is severe.

Wetting agents mask it for a season. Clay fixes it. The research rate is 100 to 450 tonnes per hectare to lift topsoil clay to 3 to 6 per cent — for a garden bed, Perth soil yards translate that to roughly 500 g to 1 kg per square metre of bentonite or 5 to 15 kg per square metre of kaolinite, worked to about 300 mm. Water Corporation’s baseline is organic matter through the top 30 cm plus 5 to 10 cm of mulch, which it credits with up to 20 per cent water saving.

Nutrients

All three systems leach. Fertilise Wise, run by SERCUL with local government backing across the Swan and Canning catchments, recommends a 10:1:6 N:P:K ratio, a maximum of 25 g per square metre on couch and 12 g per square metre on kikuyu and buffalo, and applications confined to September to November and March to April. Its reasoning is blunt: nutrients added to our sandy soils are easily leached, so anything applied outside those windows ends up in the groundwater and the drains rather than in the plant. Proteaceae — banksia, hakea, grevillea, dryandra — need low-phosphorus product regardless.

Where this varies

The soil is state-mapped; what your council does about it is not. Deep soil area, tree canopy, verge planting palettes, soakwell sizing for sand versus clay, and whether an acid sulfate soil assessment is required with a development application are all set locally, and they differ. Find your own answer: search your local government’s site for its local planning policy on landscaping, then ring the planning counter and ask what soil and drainage information they want lodged. Do that before you draw, not after.