Almost every persistent garden problem traces back to soil. Plants that sulk, beds that puddle after rain, sandy ground where water runs straight through, natives that die inexplicably after being fertilised - all of it is soil, and all of it is fixable once you know what you are working with. The good news is that identifying your soil takes about two minutes and no equipment.
The ribbon test
This is the standard field method used across Australia, and it is more reliable than looking at the colour.
- Take a small handful of moist soil from about 150 mm down, not off the surface.
- Work it in your palm until it is uniformly moist and just holds together.
- Press it out between thumb and forefinger into a ribbon.
- Measure how long the ribbon gets before it breaks.
| Ribbon length | Soil type | Feel |
|---|---|---|
| Will not form | Sand | Gritty, falls apart |
| Under 15 mm | Loamy sand | Gritty with slight cohesion |
| 25 mm | Sandy loam | Gritty but holds shape |
| 25-40 mm | Loam | Smooth, slightly spongy |
| 40-50 mm | Clay loam | Smooth and plastic |
| 50-75 mm+ | Clay | Smooth, sticky, shiny when smeared |
The main Australian soil types
Sand
Dominant across the Perth metropolitan area, the Adelaide coastal plain, and much of coastal Queensland and New South Wales. Water and nutrients pass straight through, so plants dry out fast and fertiliser leaches before roots can use it.
What to do: add organic matter constantly, because it breaks down quickly in warm sandy soil. Mulch thickly. Use controlled-release fertilisers rather than soluble ones that wash away. Water more frequently but with less each time.
Clay
Common through Brisbane, western Sydney, Melbourne and much of inland Australia. It holds water and nutrients well, which is a real advantage, but drains slowly, sets hard when dry and cracks in summer.
What to do: apply gypsum to sodic clays to improve structure, dig in compost, and plant on raised mounds so crowns sit above the wet zone. Never work clay when it is wet - you will destroy the structure for years.
Loam
The ideal, holding moisture and nutrients while still draining. Relatively uncommon naturally, and mostly the result of years of adding organic matter. If you have it, maintain it with compost and mulch rather than cultivating heavily.
Red volcanic soils
Found in the Atherton Tableland, the Northern Rivers of New South Wales, parts of the Darling Downs and northern Tasmania. Deep, red, free draining and naturally fertile - some of the best cropping soil in the country, and excellent for gardens.
Two problems specific to Australia
Australian soils are phosphorus-poor, and natives are adapted to that
This is the single most useful thing an Australian gardener can know. Our soils are among the oldest and most weathered on earth, and phosphorus has been leached from them over many millions of years. Native plants evolved to cope, and some - particularly the Proteaceae family, which includes banksias, grevilleas, hakeas and waratahs - are actively damaged by normal phosphorus levels.
Feeding a grevillea with standard garden fertiliser can kill it. Use a low-phosphorus native fertiliser on Proteaceae and on natives generally, and keep lawn fertiliser out of native beds. This kills more Australian native plants in home gardens than any pest.
Water-repellent sands
Widespread in Western Australia and parts of South Australia and Victoria. Waxy organic residues coat the sand grains and water beads on the surface or runs off instead of soaking in, leaving the root zone dry even after heavy rain or long irrigation.
The test is simple: pour water on a dry patch. If it beads and sits rather than soaking in within a few seconds, the soil is hydrophobic. Soil wetting agents fix it, and adding organic matter and clay reduces the problem over time.
Soil pH
Test kits cost very little and take five minutes. Most plants prefer 6.0 to 7.0. Australian gardens vary enormously - coastal sands near limestone can be strongly alkaline, while high-rainfall areas of Queensland and the New South Wales coast are often quite acidic.
- Too acidic (below 5.5) - apply garden lime or dolomite, retest after a few months.
- Too alkaline (above 7.5) - add compost and elemental sulphur, and use chelated iron for plants showing yellow leaves with green veins.
- Yellowing gardenias, citrus and camellias is very often alkaline soil locking up iron rather than a lack of fertiliser.
Improving any soil
- Add organic matter. Compost improves sand and clay alike - it holds water in sand and opens structure in clay. It is the closest thing to a universal fix.
- Mulch to 50-75 mm, kept clear of stems. It moderates soil temperature, holds moisture and feeds the soil as it breaks down.
- Gypsum on sodic clay only. It is not a general clay-breaker; test by dropping a dry crumb into a glass of water and seeing if it clouds. If it does, gypsum will help.
- Stop digging. Repeated cultivation destroys structure and burns through organic matter. Add material on top and let soil life work it in.
- Do not replace soil, improve it. Bringing in a truckload of topsoil over unimproved clay creates a bathtub that holds water and drowns roots.
- Match plants to soil. Cheaper and far more reliable than trying to convert a sandy garden into a clay one.
Which plants suit which soil
| Soil | Plants that thrive |
|---|---|
| Sand | Coastal natives, olives, agaves, banksia, westringia, pandanus, foxtail palm |
| Clay | Lilly pilly, murraya, ornamental pear, callistemon, lomandra, magnolia |
| Wet or poorly drained | Melaleuca, swamp banksia, Carex, Juncus, Lomandra hystrix |
| Dry and poor | Olive, agave, succulents, grevillea, casuarina, westringia |
Frequently asked questions
How do I tell what soil type I have?
Use the ribbon test. Moisten a handful of soil from about 150 millimetres down, press it into a ribbon between thumb and forefinger, and measure how long it gets before breaking. No ribbon means sand; over 50 millimetres means clay.
Why do my native plants die after I fertilise them?
Almost certainly phosphorus. Australian soils are naturally very low in it, and Proteaceae such as grevilleas, banksias and hakeas are damaged by the levels in ordinary garden fertiliser. Use a low-phosphorus native blend and keep lawn fertiliser away from native beds.
What is the best way to improve clay soil?
Compost, gypsum if the clay is sodic, and planting on raised mounds. Do not work clay while it is wet, and do not simply cover it with imported topsoil, which creates a waterlogged layer underneath.
Why does water run off my soil instead of soaking in?
The soil is probably water repellent, which is common in sandy areas of Western Australia and parts of South Australia and Victoria. Waxy coatings on the sand grains stop water penetrating. A soil wetting agent fixes it quickly, and organic matter reduces it over time.
Should I test my soil pH?
Yes, it takes five minutes and explains a surprising number of problems. Most plants prefer 6.0 to 7.0. Yellow leaves with green veins on gardenias, citrus or camellias usually mean alkaline soil locking up iron rather than a fertiliser shortage.
Is it better to improve soil or choose plants that suit it?
Choose plants that suit it, then improve gradually with compost and mulch. Trying to convert sand into clay or vice versa is expensive and rarely lasts, while a well-matched plant thrives with no intervention at all.
Ready to plant?
Whatever soil you have, there are plants that will thrive in it. Browse our Australian natives for low-phosphorus tolerant species, or potting mixes, mulches and fertilisers to improve what you have.
Related reading: our waterwise garden guide and drought tolerant plants.
