Filling a new raised bed: layers, volume and what to buy

A raised bed is only as good as what goes in it. Filling one well takes a bit of planning, but you only do the big fill once. Get it right and the bed will grow good food for years with just a yearly top-up.

Work out how much you need

Measure the inside of the bed and multiply length by width by depth, all in metres. The answer is in cubic metres, which is how landscape yards sell soil.

Worked example: a bed 2.4 m long, 1.2 m wide and 0.4 m deep.

2.4 x 1.2 x 0.4 = 1.15 cubic metres (rounded).

That's a lot more than most people expect. A bulk bag or a trailer load is usually far cheaper than bags at that size. Soil also settles, so it's sensible to order a little extra or plan to top up after the first few months.

If you use the layering method below, the lower layers take up some of that volume, so you'll need less bought mix.

Layering from the bottom up

For shallow beds (up to about 30 cm), you can simply fill with a good mix. For taller beds, filling the whole thing with bought soil is expensive and not needed. Most vegetable roots do their work in the top 30 to 40 cm. Layering fills the space below that with cheaper organic material that slowly breaks down and feeds the bed.

1. Bottom layer: logs and branches (tall beds only)

This borrows from hugelkultur, a method of building beds over buried wood. Lay untreated logs, thick branches and prunings in the base. As they rot they hold moisture like a sponge and feed fungi and soil life.

• Use untreated hardwood or fruit tree prunings if you can.
• Avoid wood from diseased plants.
• Pack the gaps with smaller sticks so there aren't big air pockets.

2. Middle layer: coarse organic matter

Next, add a layer of coarser, half-broken-down material:

• Straw or old pea straw
• Leaves
• Grass clippings (thin layers, mixed in, not a thick mat)
• Half-finished compost
• Aged manure

Water each layer as you go. Dry layers are hard to wet later.

3. Top layer: good growing mix

Finish with at least 25 to 30 cm of a quality garden or vegetable mix. This is where seeds and seedlings grow, so it needs to be good stuff. Mix in some compost if you have it.

Buying bulk soil

Landscape and garden supply yards sell soil by the cubic metre and will often deliver. When you ring around:

• Ask for a vegetable mix or organic garden mix, not plain topsoil or fill.
• Ask whether it meets AS 4419, the Australian Standard for soils for landscaping and garden use. Reputable yards will know what you mean.
• Look at it or ask for a photo. It should be dark, crumbly and smell earthy. A sour or chemical smell, or lots of sticks and plastic, is a bad sign.
• Check the delivery spot. Have it tipped on a tarp, close to the bed, and wheelbarrow it from there.

Wear gloves when handling bulk mix, keep it damp to cut down dust, and wash your hands afterwards.

Top up as it settles

Expect the bed to sink, especially in the first year. The organic layers break down and the soil compacts with watering. A drop of 10 cm or more is normal in a layered bed.

Each autumn and spring, before planting:

1. Add a few centimetres of compost.
2. Top up with more mix if the level has dropped.
3. Mulch over the top.

This keeps the bed full and feeds it at the same time.

What not to fill with

Some materials cause problems that are hard to fix once they're buried.

• Treated timber offcuts. Keep them out of food beds. Use untreated wood only.
• Weedy topsoil. Soil from a weedy patch brings years of weed seeds and runners with it. Couch, onion weed and oxalis are especially hard to get rid of.
• Builders' rubble and fill. No nutrition and poor structure.
• Fresh woodchips in the top layer. Fine buried deep, but in the root zone they can tie up nitrogen.
• Clay dug out of the yard, on its own. It can set like concrete in a bed. Use it only well mixed with plenty of organic matter.
• Plastic, glossy paper or anything you wouldn't want in your veggies.

A note on corrugated iron beds

Metal raised beds are popular and last well. In an Adelaide summer, though, the sides can get very hot in full sun, and the soil near the edges heats and dries out faster. To help:

• Choose lighter colours, which absorb less heat than dark ones.
• Mulch the soil surface well.
• Plant tougher crops near the edges and keep tender roots toward the middle.
• Shade the sunniest side with a low plant or a board if heat is a problem.

Deeper beds hold more water and buffer the heat better than shallow ones.

Planting your new bed

Water the whole bed thoroughly and let it settle for a few days before planting. Easy first crops include lettuce, silverbeet and herbs, and in warmer months, tomatoes, zucchini and basil.

In Patchmates

Add your new bed in My Garden with its size, and set a reminder to top it up each autumn.