Filling a raised bed is where most of the cost is. Get the mix and the volume right and you spend a fraction of what the garden centre would like you to spend.
| Bed size | 6in deep | 12in deep |
|---|---|---|
| 4 x 4 ft | 8 cu ft | 16 cu ft |
| 4 x 8 ft | 16 cu ft | 32 cu ft (~1 cubic yard) |
| 4 x 12 ft | 24 cu ft | 48 cu ft |
| 1 x 2 m | 0.6 m³ | 1.2 m³ |
Most beginners over-buy. Work out the volume first, then subtract whatever you fill the bottom with.
This is a starting point, not a law. Heavy clay soil wants more compost; sandy soil wants more topsoil and organic matter to hold water.
The cheapest soil is the soil you do not buy. Put the bottom third to work:
Everything you add here reduces the volume you have to buy. It also means the bed settles by 10-15cm over the first season - top it up in spring.
The single most common mistake. Pure compost:
Compost is an ingredient. Treat it as 30% of the mix, not the whole thing.
| Depth | What it suits |
|---|---|
| 15cm (6in) | Salad leaves, herbs, radishes - shallow-rooted only |
| 25-30cm (10-12in) | Most vegetables: onions, beans, brassicas, most roots |
| 40cm (16in) | Carrots, maincrop potatoes, and anything with a long taproot |
You can also build shallower and let roots go into the soil below - but only if that soil is not compacted or waterlogged.
About 60% topsoil, 30% compost and 10% coarse sand or aged manure.
About 32 cubic feet at 12 inches deep - roughly one cubic yard. Filling the bottom third reduces it further.
Yes, over grass or weeds. It suppresses them and breaks down. Avoid glossy printed card.
No. It shrinks by up to a third, dries out fast and can burn seedlings. Use it as 30% of the mix.
If you want the complete small-space growing system: The 5 Foot Farm.
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