How to fill a raised garden bed cheaply

The rule: pay for the top 25 cm

Most vegetables do their feeding in the top 20–30 cm of soil. The soil below that holds water, anchors deep roots and supports the layer above. It needs to be organic and free-draining, but it does not need to be the $150-a-cubic-metre vegetable mix. That leaves a lot of room for cheaper material:

The raised bed soil calculator lets you enter each layer's depth and prices them separately, so you can see how much of the bill is the premium mix.

1. Hugelkultur: logs and branches in the bottom

Hugelkultur ("hill culture") buries wood under the soil. As the wood rots over several years it feeds the soil life, and once it has started to break down it holds water like a sponge. For a raised bed, it is simply a way of filling the bottom third with something free.

1. Lay the biggest logs first, then branches and prunings to fill the gaps. Aim for 15–30 cm of wood in a bed 45 cm or deeper.
2. Fill the air gaps with soil, leaves or compost and water it in. Big voids collapse and the bed sinks unevenly.
3. Add a nitrogen layer of manure or fresh grass clippings over the wood. Rotting wood borrows nitrogen from the soil, and this pays it back.
4. Keep the wood at least 25 cm below the surface, with the vegetable mix on top.

Which wood: untreated hardwood, fruit-tree and garden prunings. Half-rotten wood is ideal. Avoid treated, painted or glued timber, anything that can re-sprout from a cutting (willow, poplar) and weeds carrying seed. In termite-prone areas, build wood-filled beds well away from the house and other timber structures.

2. No-dig layers: the Esther Deans method

No-dig gardening has deep roots in Australia. Sydney gardener Esther Deans popularised it in the 1970s with Esther Deans' Gardening Book: Growing Without Digging, and it has been a staple of Australian vegetable gardening ever since. It builds a bed out of layers instead of soil:

1. Overlapping newspaper, about 5 mm thick (or plain cardboard), laid straight over lawn or tired ground to smother it.
2. Pads of lucerne hay, then a sprinkle of organic fertiliser such as aged chicken manure.
3. About 20 cm of loose straw, and another sprinkle of fertiliser.
4. Circles of good compost, 8–10 cm deep and about 45 cm across, wherever you want to sow or plant.

Inside a raised bed, the same layers make a cheap bottom half under 20–25 cm of vegetable mix. On their own they collapse fast, a 30 cm stack of fluffy straw ending up a fraction of that within a season. That suits gardeners who add a fresh layer every season, and it makes a good bed for leafy crops in the first year. Deep-rooted crops do better once the layers have rotted into soil.

3. Use the soil and turf you already have

• Dig the paths. Lowering the paths between beds by 5–10 cm produces a surprising amount of topsoil. Two 3 m paths 60 cm wide, dug 8 cm deep, give about 0.3 m³.
• Turn the turf upside down. Lawn cut out where the bed goes can go grass-side down in the bottom of the bed, where it rots into good loam. The exception is couch and kikuyu: their runners grow up through almost anything, so remove them completely.
• Improve rather than replace. If your garden soil is a decent loam, mix it about two parts soil to one part compost and it will match many bagged "garden soils". It is poor value only if it is heavy clay or pure sand. For those, see the guides to clay and sandy soil.

4. Buy the bulk smartly

• Buy bulk once you pass about a dozen bags. Bagged vegetable mix commonly works out at $300–$450 per cubic metre; bulk is a third of that. See bagged vs bulk soil for the break-even.
• Two products, one truck. Many yards will deliver a cheaper compost or soil conditioner for the middle layer and a premium mix for the top in one load, separated by a board. You pay one delivery fee.
• Share a delivery. Split a cubic metre with a neighbour or your community garden and halve the delivery fee on each share.
• Free organic matter. Autumn leaves, grass clippings and coffee grounds from cafés are all good middle-layer material. Some councils give away mulch or compost made from green waste, and arborists often need somewhere to drop wood chip. Use chip on paths or as mulch, not mixed into the root zone.

5. Don't build deeper than you need

The cheapest soil is the soil you never buy. On open ground a 30 cm bed grows almost every vegetable, because roots continue into the loosened ground below. A 45 cm bed on the same spot needs 50 % more fill. Build deep where there is a reason to: concrete underneath, contaminated ground, root crops, or less bending. The depth guide goes through each case.

What not to put in a raised bed

• Treated, painted or glued timber offcuts, and particleboard. They are not the same as untreated branches.
• Glossy or plastic-coated cardboard, and packing tape. Plain brown cardboard and newspaper are fine.
• Weedy soil and couch or kikuyu runners. You will be pulling them out of your carrots for years.
• Manure, hay or straw of unknown origin. Some farm herbicides (clopyralid, aminopyralid, picloram) survive in hay and straw, and even pass through animals into manure. Traces distort tomatoes, potatoes, beans and peas. Ask where it came from. If in doubt, sow a few bean seeds in a pot of it first: cupped, twisted new leaves mean trouble. Lucerne and pea straw are less risky, because those herbicides would kill the crop they come from.
• Builder's fill and rubble. They are fine under a path, useless under roots.
• Fresh wood chip mixed into the top layer. Buried wood deep down is fine, but chip mixed through the root zone ties up nitrogen right where seedlings need it.

Worked example: what it saves

A 2.4 m × 1.2 m bed (inside), 45 cm deep, filled with vegetable mix at $10 per 30 L bag or $120/m³ bulk plus $80 delivery. These are illustrative prices; put your own into the calculator.

The saving is biggest for people who buy bags, because every bag saved is $10. For bulk buyers the fixed delivery fee narrows the gap, but you still save more than half a cubic metre of your best soil. On a larger project with four beds, that is over 2 m³.

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