Soil preparation depends on the soil you have. Sand needs help holding water, clay needs structure and drainage, and a bed for native plants needs less added than most people think. This guide covers how to identify your soil, what each type needs before planting, and the order to do the work in.
Identify your soil first
Australian soils are among the oldest and most weathered on earth, and they vary street by street. Test with a ribbon: moisten a handful taken from 15 cm down and press it out between thumb and finger. If it will not form a ribbon, the soil is sand. A ribbon longer than 5 cm means clay. Anything between is a loam. Our guide to soil types and how to identify them adds a jar test you can run at home.
| Soil | Where it is common | What preparation has to do |
|---|---|---|
| Sand | Perth, the Adelaide coast, much of coastal Queensland and NSW | Add the capacity to hold water and nutrients |
| Clay | Brisbane, western Sydney, Melbourne, inland | Improve structure and drainage |
| Sandy loam | Many established suburbs | Keep the organic matter up |
| Red volcanic soil | Atherton, the Northern Rivers, the Darling Downs | Very little; it is naturally fertile |
The steps, in order
- Clear the weeds and grass. Remove couch, kikuyu and other running grasses completely, roots and all, or smother them under cardboard and mulch for three months or more. Anything left will come up through the new planting.
- Check the drainage. Dig a hole 30 cm deep, fill it with water, let it drain and fill it again. If the second fill is still there after a few hours, plan on raised beds or mounds.
- Loosen the soil. Fork or dig the whole bed to the depth of a spade. Do not work clay while it is wet: that destroys its structure and leaves a compacted layer that takes years to recover.
- Add organic matter. Spread 5 to 10 cm of compost or well rotted manure and work it through the top 20 cm. Organic matter improves both sand and clay: it holds water in the first and opens up the second.
- Check the pH. A test kit costs a few dollars. Most garden plants want 6 to 7. Lime raises a low pH; sulphur lowers a high one, slowly. Our guide on how to lower soil pH covers the acid loving plants that need it most.
- Water, then let it settle for a week or two before planting if you can.
- Mulch after planting, 5 to 7 cm deep, kept clear of stems and trunks.
Clay: gypsum only if it will work
Gypsum is widely sold as a clay breaker, and it works only on some clays. Test before buying it: drop a dry crumb of soil into a glass of clean water and leave it for a few hours. If the water turns cloudy as the crumb breaks down, gypsum will improve the structure. If the crumb sits there and the water stays clear, gypsum will do nothing, and the money is better spent on compost. On heavy clay, build up as well as digging in: a bed raised 20 to 30 cm drains where a flat one does not.
Sand: the water repellence problem
Sandy soils across Western Australia, South Australia and parts of Victoria often become water repellent. Waxy coatings on the grains make water bead and run off. Test by pouring water on a dry patch: if it sits on the surface for more than a few seconds, apply a soil wetting agent and water it in well before planting or mulching. Adding compost, and clay where it is available, reduces the problem over time.
Native plants: leave the phosphorus out
Australian soils are naturally low in phosphorus, and many native plants are adapted to that. Grevilleas, banksias, hakeas and waratahs are damaged or killed by ordinary phosphorus levels. Do not prepare a native bed with mushroom compost, fresh manure or general fertiliser. Use a modest amount of plain compost and a low phosphorus native fertiliser, and keep lawn food well away. Rich soil also produces soft growth on natives, which suffers in heat and wind.
Prepare the whole area, not the hole
A well improved hole in unimproved clay is a bathtub. Water collects in the soft pocket, and the roots circle inside it and do not spread. Improve the entire bed to the same standard, or on heavy ground plant on a raised mound.
Raised beds and pots
Fill raised beds with a blend made for the purpose, or with good garden soil mixed with a third compost. Straight potting mix slumps and dries out in a deep bed, and straight clay sets hard. A bed 30 to 40 cm deep suits most vegetables. In pots, use a premium potting mix, not garden soil. In old inner suburbs, where soil near painted houses and busy roads can carry lead, raised beds of clean soil are the safe way to grow vegetables.
Looking after the soil once it is planted
- Keep it covered with mulch or plants. Bare soil bakes, crusts and washes away.
- Top up compost and mulch each year. The soil life works it in for you.
- Dig as little as possible once the bed is planted. Worms and roots keep the structure open.
- Keep feet and wheels off the beds, especially when wet.
- In the vegetable garden, rotate crops and sow a green manure such as field peas or lupins over bare ground in winter. Peas, beans and wattles add nitrogen.
- Water deeply and less often, so roots go down.
All of this works because of what lives in the soil. Bacteria, fungi and earthworms break down organic matter, release nutrients in forms roots can take up, and bind the soil into crumbs with air between them. Feeding the soil does more than feeding the plant, and our guide to the science behind fertilisers explains how to add to that biology without overriding it.
Timing
Autumn is the best time to prepare and plant across most of Australia, since roots get the cool months to establish before their first summer. In Brisbane and the tropics, preparing before the wet season lets the rain settle the beds. Avoid preparing and planting in midsummer anywhere. Soil preparation is one step of four: how to plant a new garden bed puts it in order with assessing the site, planting and first season care.
Common questions
How deep should I prepare the soil?
To the depth of a spade, about 20 to 30 cm, across the whole bed.
Does gypsum break up clay?
Only some clays. If a dry crumb turns the water cloudy in a glass, gypsum will help. If the water stays clear, use compost.
Should I add fertiliser when preparing soil?
Compost is enough for most beds. Add a slow release fertiliser at planting for vegetables, roses and citrus, and none for native plants beyond a low phosphorus one.
What is the best soil pH for a garden?
Between 6 and 7 for most plants. Azaleas, camellias, gardenias and blueberries want it lower, around 5 to 6.
Ready to plant?
Once the soil is right, the planting is the easy part. Browse our potting mixes, mulches and fertilisers for the compost, gypsum and mulch to get started, and read our guide to planting and care basics before the first plant goes in the ground.
One more thing before the courier turns up: our guide to what to do when your plants arrive covers the first 48 hours, from unboxing to the first watering.
