Raised Bed Soil Mix: A Simple Recipe That Works
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.
How much soil you actually need
| 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.
The basic mix
- 60% topsoil - the bulk; provides structure and holds moisture
- 30% compost - nutrients and biology
- 10% coarse sand or aged manure - drainage, and a slow feed
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.
Fill the bottom first
The cheapest soil is the soil you do not buy. Put the bottom third to work:
- Cardboard over grass or weeds - suppresses them for a season and breaks down. Avoid glossy printed card.
- Logs, branches, prunings - they rot slowly and hold water. Expect settling, so plant shallow-rooted crops the first year.
- Turf sods, upside down - free organic matter
- Kitchen compost - only if it is already broken down; raw scraps attract rodents
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.
Do not use pure compost
The single most common mistake. Pure compost:
- Shrinks dramatically - a bed filled with compost alone can drop by a third
- Dries out fast and becomes hydrophobic - water runs off instead of soaking in
- Can burn young roots if it is not fully matured
Compost is an ingredient. Treat it as 30% of the mix, not the whole thing.
Depth: how deep does it need to be?
| 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.
Topping up and feeding
- Add 5cm of compost each spring rather than replacing the whole bed
- Do not dig it in - lay it on the surface and let worms work it down
- Feed through the season if you are growing heavy feeders (courgettes, tomatoes, brassicas)
Common questions
What is the best soil mix for a raised bed?
About 60% topsoil, 30% compost and 10% coarse sand or aged manure.
How much soil do I need for a 4x8 raised bed?
About 32 cubic feet at 12 inches deep - roughly one cubic yard. Filling the bottom third reduces it further.
Should I put cardboard in the bottom of a raised bed?
Yes, over grass or weeds. It suppresses them and breaks down. Avoid glossy printed card.
Can I fill a raised bed with just compost?
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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