What to fill raised garden beds with
By Muzamil Ali · Last updated
What to fill raised garden beds with — roughly 60% topsoil, 30% compost and 10% coarse material, why pure compost fails, and how to work out the volume.
Fill raised garden beds with a mixture, not one material: roughly 60% topsoil, 30% compost and 10% coarse material such as bark, perlite or pumice. Pure compost is the common mistake — it holds water beautifully in year one, then collapses as the organic matter finishes breaking down. No single material does all three jobs a bed needs.
What a bed actually has to do
Three things, and they pull against each other.
It has to hold water, so a plant is not stressed between rains. It has to drain, so roots are not sitting in water and suffocating. And it has to hold its structure, staying open enough for air and roots to move through it for years rather than compacting into a brick.
| Material on its own | Holds water | Drains | Keeps structure |
|---|---|---|---|
| Compost | Very well | Adequately at first | Poorly — it disappears |
| Topsoil | Adequately | Poorly inside a frame | Well |
| Coarse material | Poorly | Very well | Very well |
| The mixture | Well | Well | Well |
That table is the whole argument for a mix. The raised bed soil calculator splits whatever total volume your bed needs into those three parts, and the percentages are editable if your situation calls for different ones.
Can you fill a raised bed with just compost?
You can, and it will disappoint you by the second season.
Compost is 30 to 50% organic matter that is still decomposing. As that finishes, the volume goes with it — a bed filled with pure compost commonly drops a third in the first year. The structure goes at the same time: what began as a fluffy, water-holding mix slumps into something dense and airless.
There is a second problem that gets less attention. Compost that dries out completely can turn hydrophobic, shedding water instead of absorbing it. A bed can then be soggy on top and bone dry two inches down.
Compost is the best thing you can add to a bed and a poor thing to fill it entirely with. Use it as the 30%, and add more every spring.
A starting point, and how to adjust it
For a general vegetable bed:
- 60% topsoil or a bagged garden soil. The bulk and the mineral structure. It holds nutrients in a way pure organic matter does not, and it does not disappear.
- 30% compost. The biology and the water-holding. Municipal compost, bagged compost, well-rotted manure, leaf mould, or your own.
- 10% something coarse. Perlite, coarse bark, pumice or sharp sand. The part people skip, and then wonder why the bed is heavy. It keeps air pockets open as the organic matter settles.
Adjust from there:
| Your situation | Change |
|---|---|
| Heavy clay native soil | Coarse fraction to 15–20%; be generous with compost |
| Sandy native soil | Use more of it; lean on compost for water holding |
| Bed on a patio, deck or liner | Treat as a large container — more coarse material, real drainage holes |
| Bed deeper than 18 in | Bottom third can be coarse woody material, upturned turf or native soil |
University of Maryland Extension publishes a mix along similar lines, and it is worth reading alongside this one — the proportions published by different extension services vary, because the soils they are written for vary.
What to put in the bottom of a raised bed
Over grass: cardboard, which smothers the turf and rots away within a season. Over soil: nothing. In a deep bed: coarse woody material for the bottom third, if you want to save money.
Three things not to put there:
A gravel layer "for drainage". This does not work, in a bed or a pot. Water will not cross from fine material into coarse until the fine material above is saturated, so a gravel layer raises the waterlogged zone rather than lowering it.
Woven landscape fabric. It stops roots reaching the ground below and stops worms coming up. Both are things you want happening.
Fresh wood chips mixed into the soil. On the surface as mulch they are fine. Dug in, they tie up nitrogen while they break down — wood chips run around 400:1 carbon to nitrogen against the roughly 30:1 a pile needs, which is why the microbes decomposing them take nitrogen from everything nearby. Greens and browns in compost explains the mechanism.
Also avoid fresh manure, which burns roots and causes forked growth in root crops, and cheap topsoil of unknown origin, which is sometimes subsoil, sometimes full of perennial weed root, and occasionally contaminated. Ask what it is and buy a small amount first if unsure.
Work out the volume before you shop
This is where most of the frustration comes from.
| Bed size | 6 in deep | 12 in deep | 18 in deep |
|---|---|---|---|
| 4 × 4 ft | 8 cu ft | 16 cu ft | 24 cu ft |
| 4 × 8 ft | 16 cu ft | 32 cu ft | 48 cu ft |
| 3 × 6 ft | 9 cu ft | 18 cu ft | 27 cu ft |
A 4 × 8 ft bed at 12 inches deep needs 32 cubic feet — about 22 bags at 1.5 cubic feet, or 1.2 cubic yards loose. People routinely buy four bags and are surprised.
Two practical points. Measure the inside of the frame: two-inch lumber makes a bed four inches narrower inside than it looks. And buy about 10% more than the calculation says, because a fresh mix settles as its organic matter continues to break down.
How deep to build in the first place is a separate question with real cost consequences — how deep should a raised bed be for vegetables works through it crop by crop.
Topping up, year after year
A raised bed is not filled once. Expect the level to drop an inch or two a year as the organic matter is consumed, and top it up each spring with compost rather than a full mix.
That annual compost is what keeps the bed alive: it feeds the soil biology, replaces what the crops took, and maintains structure. Mulching the surface through the season slows the loss and does the same job more gradually.
A bed filled this way and topped up will grow better vegetables in year five than in year one — including hungry crops like tomatoes, which are the usual reason people build a bed in the first place.