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Raised Bed Soil Mix: How to Fill a Raised Bed Cheaply (Ratios and Volumes)
Ratios, bag counts per bed size, and the log trick that halves the cost of a deep bed.
Updated 2026-09-11
The mix
60% topsoil or bagged garden soil, 30% compost (mix two sources), 10% aeration (perlite, coarse bark fines or coir). Pure compost or pure bagged potting mix both fail: one slumps and stays wet, the other dries out and blows away.
What the bags mean: "topsoil" is screened mineral soil, cheap and heavy, the body of the mix. "Garden soil" is topsoil with some compost in it and replaces topsoil one for one. "Potting mix" is peat or coir with perlite and no soil; it belongs in containers. Bagged "raised bed mix" is usually a fair 60/30/10 already, just expensive by the yard.
Compost: mix two kinds, say municipal leaf compost and bagged composted manure, so the bed does not depend on one pile. Mushroom compost is fine up to a quarter of the compost share; more and its salts set greens back.
How much you need
Length × width × depth in feet gives cubic feet; divide by 27 for cubic yards. A 4×8 bed 17 in deep is 4 × 8 × 1.42 = 45 cu ft, or 1.7 cu yd.
| Bed | 12 in deep | 17 in deep |
|---|---|---|
| 4×4 ft | 0.6 cu yd (16 bags of 1 cu ft) | 0.85 cu yd (23 bags) |
| 4×8 ft | 1.2 cu yd (32 bags) | 1.7 cu yd (45 bags) |
| 2×8 ft | 0.6 cu yd (16 bags) | 0.85 cu yd (23 bags) |
Bulk delivery of a screened "garden blend" by the yard costs half of bagged soil once you need more than about 1 cu yd.
Bagged vs bulk, with numbers
Bagged garden soil runs $5–8 per cubic foot, so the 45 bags for a 4×8×17 bed cost $225–360 and two trips in a hatchback. A bulk "garden mix" from a landscape yard costs $40–70 a yard plus $50–100 delivery: about $150 for the same bed, dropped on a tarp. Ask what is in it; a good blend is screened topsoil and compost that smells like a forest floor, not sour. If it arrives as pure black compost, add topsoil yourself.
The raised bed kit comparison lists soil volumes for each kit size, so you can price the soil before you buy the bed.
The deep-bed trick
In a 17 in bed, fill the bottom 6–8 in with logs, branches, leaves and grass clippings, water it, then add the mix on top. It saves a third of the soil cost, holds moisture and settles a couple of inches over the first year: top up with compost. Use hardwood that is already a bit rotted; avoid walnut, cedar and treated lumber. Keep at least 10 in of real mix on top for root crops.
Filling and settling
Fill to the rim, water thoroughly and let it sit a day; it drops an inch or two. Top up and plant. Every fresh mix settles 10–20% in its first year, so a bed 3 in low next spring is normal.
What not to put in a raised bed
- Fill dirt or subsoil from a construction site: no life, no structure, often clay.
- Fresh manure: burns roots and can carry pathogens. Composted for six months is fine.
- Fresh wood chips or sawdust mixed in (fine as a mulch on top or in the bottom layer).
- Soil from a bed that had blight or wilt problems.
Each spring
Add 1–2 in of compost on top. Do not dig it in. Test pH every couple of years (6.0–7.0 is right for almost everything). Feed heavy feeders (tomatoes, squash, brassicas) with an organic slow-release fertilizer mid-season. A $15 test kit also tells you whether nitrogen is low, which in a raised bed it usually is by the third year; a balanced organic granular feed at the bag rate each spring fixes it.
Plan the top-up as part of the fall cleanup in the fall vegetable garden guide: compost spread in October is worked in by worms by April.
Mistakes to skip
- Filling a bed with 100% compost because "plants love compost". It stays soggy, slumps by half and starves roots of air.
- Using the deep-bed trick in a 12 in bed. There is not enough soil left on top.
- Buying "topsoil" in bags and stopping there. It is mineral soil with almost no organic matter; add the compost.
- Skipping the aeration share on heavy clay topsoil.
- Adding lime or sulfur without a test.
FAQ
What is the best soil mix for a raised bed?
60% topsoil or garden soil, 30% compost from two sources, 10% perlite, bark fines or coir. This holds water without waterlogging, feeds plants for the season and costs about half of bagged raised-bed mix.
How many bags of soil for a 4×8 raised bed?
32 bags of 1 cu ft for a 12 in deep bed, 45 bags for 17 in. With 1.5 cu ft bags, 22 and 30 bags. Above about 1 cubic yard, bulk delivery is cheaper.
Can I use garden soil from my yard in a raised bed?
Yes, as the topsoil share, if it is screened of roots and rocks and it drains. Heavy clay yard soil needs the full 10% aeration and a bit more compost. Do not use it on its own.
Do I need to replace raised bed soil every year?
No. Add 1–2 in of compost each spring, feed heavy feeders and the same soil improves for decades. Replace soil only after a serious soil-borne disease.
Can I use Mel's mix (square foot gardening mix) instead?
Yes, and it is excellent, but a third each of peat or coir, vermiculite and compost costs two to three times as much per yard. It makes sense for a 6 in square-foot bed, not for filling a 17 in bed.
What I'd buy first
Luster Leaf Rapitest Soil Test Kit
Cheap pH/NPK check before you buy amendments.
Check price via Amazon Associates (US)Espoma Organic Garden-tone Fertilizer 4 lb
Slow-release organic feed for heavy feeders mid-season.
Check price via Amazon Associates (US)
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