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Short answer: Plants use the top 12 to 18 inches of soil. In a bed 24 inches or deeper, fill the bottom third to half with logs, branches, leaves and straw instead of purchased soil. This cuts the soil bill by 30 to 50 percent, and the material breaks down into good soil over the next few seasons. Two things to get right: never use gravel, and expect significant settling in year one.
The soil is often more expensive than the bed. A 36-inch bed needs three times the soil of a 12-inch one, which is the reason most people talk themselves out of the height they actually wanted.
You don't have to buy three feet of soil.
Why this works
Root depth requirements are finite. Lettuce and herbs work the top 6 to 12 inches. Tomatoes, peppers and carrots are comfortable in 18 to 24. Almost nothing a home gardener grows needs 36 inches of premium soil mix.
The bottom foot of a tall bed isn't doing horticultural work. It's doing structural work — holding the soil above it at a height that's comfortable for you. Anything that occupies volume, drains, and eventually turns into soil can do that job.
Better than "anything", actually. Buried wood absorbs water like a sponge during rain and irrigation, then releases it slowly as the bed dries. A bed built this way needs watering less often, not more. As the wood decomposes it also feeds soil biology and adds organic matter from below.
This approach has a name — hugelkultur, from the German for mound culture — and it long predates raised beds. The traditional version is a free-standing mound of buried wood. Inside a raised bed it's the same principle with a frame around it.
What it actually saves
Using an 8 × 4 ft bed as the example:
| Bed | Total volume | Soil to buy, filled normally | With bottom third layered | Bags saved |
|---|---|---|---|---|
| 8 × 4 rectangular, 36" | 96 cu ft | 64 bags | 43 bags | 21 |
| 8 × 4 oval, 36" | 85.7 cu ft | 58 bags | 39 bags | 19 |
| 8 × 4 rectangular, 24" | 64 cu ft | 43 bags | 32 bags | 11 |
| 8 × 4 oval, 24" | 57.1 cu ft | 39 bags | 29 bags | 10 |
Bags are 1.5 cubic feet. Fill the bottom half rather than the bottom third and a 36-inch bed drops to about 29 bags — roughly half.
The saving compounds across a multi-bed garden. Four tall beds is 80 bags you don't buy, and it's the difference between a 36-inch garden being affordable or not.
What to use, ranked
Best: logs and thick branches
Hardwood is ideal — oak, maple, fruit wood, ash. It holds water well and breaks down slowly, giving years of benefit. Softwoods like pine and fir work too and decompose faster.
Partly rotted wood is better than fresh. It's already colonised by fungi and holds water immediately.
Good: smaller branches, twigs, prunings
These fill the gaps between logs and decompose faster, which gets the bed producing soil sooner.
Good: fallen leaves
Free, abundant in autumn, and excellent filler. Pack them in — they compress a lot.
Good: straw, hay, grass clippings
Straw is better than hay, which carries seed. Grass clippings should go in thin layers rather than a thick mat, which turns anaerobic and slimy.
Fine: untreated wood chips, cardboard, kitchen scraps
Plain brown cardboard with tape and labels removed. Kitchen scraps are fine buried deep, though avoid meat and dairy unless you want to meet your local wildlife.
What not to use
Gravel or rocks. This is the most persistent myth in raised bed gardening, and it's worth being blunt: a layer of coarse material under finer soil does not improve drainage. It makes it worse.
Water doesn't move readily from a fine-pored material into a coarse-pored one — it has to saturate the fine layer first before it will cross the boundary. The result is a perched water table, a permanently soggy zone sitting right above the gravel, exactly where you didn't want one. You've also permanently lost that volume, since rock never becomes soil.
Pressure-treated, painted or stained wood. No reclaimed lumber of unknown history.
Black walnut. It contains juglone, which is toxic to tomatoes, peppers, eggplant and potatoes. Avoid it entirely.
Cedar and redwood. Not harmful, just useless here. They resist decomposition — that's the point of them — so they'll still be logs in a decade instead of becoming soil.
Diseased plant material. Don't bury blight or wilt problems in a bed you plan to grow in.
Plastic of any kind. Bags, sheeting, pots. It won't break down and it blocks water movement.
How to build it
36-inch bed
- 0–12 in: logs. Largest pieces on the bottom, packed as tightly as you can. Don't worry about gaps yet.
- 12–18 in: branches, twigs, leaves, straw. Work these down into the gaps between the logs. Water each layer as you go — dry material takes a long time to wet through once buried.
- Add a nitrogen source. A few inches of compost, aged manure or grass clippings scattered through the woody layers. This matters — see below.
- 18–36 in: soil mix. Roughly 60% topsoil, 30% compost, 10% aeration material. This is your actual root zone, so don't economise here.
24-inch bed
- 0–6 in: branches and leaves. Skip large logs — you don't have the depth.
- 6–24 in: soil mix. Eighteen inches of real soil covers every common crop.
12-inch bed
Don't layer. Twelve inches is already the minimum root zone for most crops. Filling any of it with wood means you're growing in six inches of soil.
The nitrogen problem, honestly
This is the part most articles leave out.
Decomposing woody material consumes nitrogen. The microbes breaking down carbon-rich wood pull nitrogen from their surroundings to do it, and while that's happening, the nitrogen isn't available to plants. It's called nitrogen immobilisation, and it shows up as yellowing lower leaves and stunted growth.
Three things keep it from being an issue:
- Depth. The wood is a foot or more below the root zone, so most roots never contact the active decomposition front. This is the main reason hugelkultur works in a bed but fresh wood chips mixed into soil cause obvious problems.
- Nitrogen added at build time. Compost, aged manure or grass clippings layered in with the wood gives the microbes their own supply.
- A heavier feeding schedule in year one. Especially for leafy crops, which are the most nitrogen-hungry.
If you want to be cautious, plant year one with beans and peas — legumes fix their own nitrogen — and save the heavy feeders for year two.
Settling: plan for it
A layered bed settles far more than a soil-filled one. Six inches or more in the first year is normal, and it continues at a slower rate for several years as the wood breaks down.
Three consequences:
- Overfill slightly at build time. Mound the soil an inch or two above the rim.
- Budget for annual topping up. An inch or two of compost each spring, which the bed wants anyway.
- Don't panic in month three. Settling is the system working, not failing.
What happens over time
| Year | What's going on |
|---|---|
| 1 | Most settling happens. Wood starts breaking down. Nitrogen may need supplementing, especially for leafy crops. |
| 2–3 | Settling slows. Wood is well colonised by fungi and holds water noticeably — you water less often than a conventional bed. |
| 4–6 | Woody material is largely soil. Organic matter content is high, structure is excellent. This is when a hugelkultur bed outperforms the bed you'd have built with purchased soil. |
| 7+ | Fully converted. Top up with compost as you would any bed. |
The trade is real and worth stating plainly: you save money now and take on a slightly harder first season, in exchange for a better bed from year three onward.
When not to do this
- Beds 12 inches or shallower. No spare depth.
- Growing long root crops immediately. Parsnips, daikon and full-length carrots want uninterrupted depth. Give the bed a couple of years first.
- You want maximum yield in year one. A bed filled entirely with good soil performs better in season one. It just costs more and is worse by year four.
- You have no access to organic material. Buying logs and straw defeats the purpose. This works best when the filler is free — your own prunings, autumn leaves, a neighbour's storm damage, a tree service that will drop chips for nothing.
Frequently asked questions
What should I put in the bottom of a raised garden bed?
In beds 24 inches or deeper, logs, branches, leaves, straw and untreated wood chips. These fill volume you don't need for roots, hold water, and break down into soil. In beds 12 inches or shallower, use soil throughout.
Can I put gravel or rocks in the bottom of a raised bed for drainage?
No. A coarse layer beneath finer soil creates a perched water table — a saturated zone above the gravel — which makes drainage worse rather than better. It also permanently occupies volume that will never become soil.
How much soil can I save by layering?
Filling the bottom third saves roughly a third of the soil cost; filling the bottom half saves about half. For an 8 × 4 ft bed at 36 inches, that's 19 to 29 fewer bags of purchased soil.
What is hugelkultur?
A method of building growing beds over buried wood. The wood holds water and releases it slowly, and decomposes into rich organic matter over several years. In a raised bed it means filling the lower portion with logs and branches before adding soil.
Will buried wood steal nitrogen from my plants?
It can, but the effect is limited when the wood sits well below the root zone. Adding compost or aged manure with the woody layers, and feeding a little more heavily in year one, handles it. Planting legumes the first season is a simple alternative.
How much will a layered bed settle?
Six inches or more in the first year, then more slowly for several years. Mound the soil slightly above the rim when you build, and top up with compost each spring.
What wood should I avoid in a raised bed?
Pressure-treated, painted or stained wood. Black walnut, which contains juglone and is toxic to tomatoes, peppers, eggplant and potatoes. Cedar and redwood aren't harmful but resist decomposition, so they never become soil. Also avoid diseased plant material.
Working out quantities? Our soil volume chart covers every bed size, and the height guide explains when a 36-inch bed is worth it.