How to Fill a Raised Bed: The Right Soil Mix for Any Budget
*Quick answer: Fill a raised bed with cardboard on the bottom, optional logs and leaves for deep beds, then a 60% topsoil / 30% compost / 10% perlite mix on top. A standard 4×8 bed at 12 inches deep needs about 32 cubic feet of material. Buying in bulk instead of bags saves $100+.
My first raised bed was a disaster. I grabbed six bags of "garden soil" from Home Depot, dumped them in, and planted tomatoes. Two months later: compacted mud, yellow leaves, and exactly zero tomatoes.
That was four years and twelve raised beds ago. Here's what I've learned since.
The Best Soil Mix for Raised Beds
Forget the fancy "raised bed mixes" in bags. They're overpriced and inconsistent. Here's what I use in every bed:
- 60% topsoil — the heavy stuff. Gives your bed structure so it doesn't dry out in two hours.
- 30% compost — this is where the magic happens. Nutrients, beneficial bacteria, worms love it.
- 10% perlite — those little white rocks. Keeps the soil loose so roots can breathe and water drains properly.
This is a starting point, not gospel. Sandy native soil? Add a bit more compost. Heavy clay topsoil? Bump perlite to 15%. Root crops like carrots and beets? More perlite, less topsoil (55/30/15) — they need looser soil or they'll fork and twist.
Note on containers vs raised beds: if your bed has no bottom and sits directly on soil, roots will grow down into native ground over time. That's a good thing. Pure container mixes (peat-heavy, soilless) are designed for pots — they're too light and dry out too fast in open raised beds.
How Much Soil for a Raised Bed?
Here's the part that trips people up. A 4×8 foot raised bed at 12 inches deep needs 32 cubic feet of soil. That's roughly:
- 32 one-cubic-foot bags ($160–250 at a garden center)
- 1.2 cubic yards in bulk ($35–60 delivered)
Don't want to do the math yourself? I built a soil volume calculator for exactly this — plug in your bed size and it tells you cubic feet, cubic yards, and bags.
How to Fill a Raised Bed, Step by Step
1. Cardboard on the bottom
If your bed sits on grass or weeds, lay flat cardboard on the bottom. Overlapping pieces, no tape or staples needed. The grass dies, the cardboard breaks down in one season, and worms come up through it.
Skip landscape fabric. I used it on my first three beds and regretted all of them. It blocks worms, prevents root growth into the native soil, and creates a soggy layer you can't fix.
2. Fill the bottom 4–6 inches with free stuff
This is the trick that cut my soil costs by a third. For any bed 10 inches or deeper, the bottom layer doesn't need to be premium soil. I use:
- Small logs and branches — break them to fit, pack loosely. Avoid treated lumber, black walnut (toxic to many plants), and anything diseased.
- Dry leaves — I save bags of them every fall for this
- Straw — not hay (hay has seeds that'll sprout everywhere)
One caveat: woody material temporarily ties up nitrogen as it breaks down. That's why your soil mix goes on top, not mixed in with the logs. The compost in the mix provides enough nitrogen for the planting zone. For shallow beds (6–8 inches), skip the bottom layer entirely — there's not enough depth to separate the wood from the roots.
3. Pour in the soil mix
Fill to about an inch below the rim. Don't pack it down — let the water do that.
Pro tip: if you're mixing your own (which I always do), dump the three ingredients in a wheelbarrow and mix with a garden fork. Don't try to mix inside the bed. I've done that. It's terrible. You end up with perlite pockets and compost lumps.
4. Soak it
Water the whole bed deeply. Walk away for 24 hours. Come back — the soil will have settled 2–3 inches. Top off with more mix.
I always overbuild by about 10–15% for exactly this reason. The soil calculator already accounts for this if you add a couple inches to your depth.
5. Mulch the surface
2–3 inches of straw or wood chips on top. This isn't optional. Without mulch, your bed dries out twice as fast in summer and you're watering every single day.
I prefer straw for veggie beds (breaks down in one season, easy to move for planting) and wood chips for perennial beds (lasts longer, looks nicer). Check our mulch calculator to figure out how much you need.
What This Actually Costs
Here's a real cost breakdown for a single 4×8 bed at 12 inches deep.
Prices are typical US garden center and landscape supply ranges. Your area may differ — call local yards for quotes before ordering.
The expensive way (bagged):
Total: ~$224
The smart way (bulk + Hügelkultur):
Total: ~$71
Same bed. Same results. Third of the price. The only catch — you need somewhere to dump a pile of bulk soil and a wheelbarrow to move it.
Mistakes I've Made (So You Don't Have To)
Pure compost beds. My neighbor swore by "100% compost beds." Her plants burned from nitrogen overload in the first month. Compost is amazing at 30%. At 100%, it's too rich and retains too much water.
Cheap topsoil from Craigslist. Got a "great deal" on fill dirt from a construction site. Turned out it was subsoil — basically clay with rocks. Spent a weekend shoveling it all back out. If you buy bulk topsoil, ask where it came from and get a sample first.
Filling to the brim. Looks great for about one watering. Then it settles and spills soil over the sides every time it rains. Leave that inch of space.
Skipping the bottom layer. My first few beds were pure soil all the way down. They worked fine, but I spent twice as much. Once I started with logs and leaves on the bottom, I couldn't go back.
Do Raised Beds Need New Soil Every Year?
No. This is a myth that somehow won't die.
After each season, I do two things:
- Pull out old plant roots and any weeds
- Dump 1–2 inches of compost on top and lightly fork it in
The only time you'd fully replace soil is if something went seriously wrong — contamination, persistent herbicide damage, or heavy pest infestation. In four years across twelve beds, I've never had to do it once.
FAQ
Can I fill a raised bed with only compost?
Don't. Pure compost retains too much moisture, can burn plant roots with excess nitrogen, and compacts into a dense mat by mid-season. Use compost at 30% of the mix — that's the sweet spot.
Should I put rocks or gravel in the bottom of a raised bed?
No. This is old advice that doesn't hold up. Rocks don't improve drainage in a raised bed — they actually create a "perched water table" where water pools above the rock layer. Use logs and leaves instead, or just fill with soil mix.
How deep should a raised bed be?
At minimum 6 inches for lettuce, herbs, and shallow-rooted crops. 10–12 inches for tomatoes, peppers, and root vegetables. Deeper beds also retain moisture better and give roots more room.
Can I use bagged "garden soil" to fill a raised bed?
You can, but it's not ideal on its own. Bagged garden soil is usually heavy and dense. Mix it with compost and perlite (60/30/10) for much better results. And it's expensive — bulk is 3–4x cheaper per cubic foot.
What should I NOT put in the bottom of a raised bed?
Avoid treated lumber, black walnut wood (releases juglins that kills many plants), diseased plant material, meat or dairy scraps (attracts pests), and seed-heavy hay (you'll be weeding for months).
Next Steps
- Use the soil volume calculator to get exact cubic feet, cubic yards, and bag counts for your bed size.
- Use the mulch calculator to figure out how much mulch you need for the top layer.
- Already building the bed frame? Check the lumber calculator for board counts.