Quick answer
Fill the bottom third to half of a deep raised bed with logs, branches, leaves, grass clippings and cardboard, then top it with at least 6 to 8 inches of a planting mix of roughly 50 to 60 percent topsoil and 40 to 50 percent compost. Buy that mix in bulk if you need more than about a cubic yard. A 4 x 8 ft bed that is 18 inches deep holds 48 cubic feet; filling half with free material means buying only about 24 cubic feet of soil.
How much soil a raised bed holds
Volume in cubic feet = length x width x depth, all in feet. Divide by 27 for cubic yards.
| Bed size | Depth | Cubic feet | Cubic yards | 1 cu ft bags |
|---|---|---|---|---|
| 4 x 4 ft | 12 in | 16 | 0.59 | 16 |
| 4 x 8 ft | 12 in | 32 | 1.19 | 32 |
| 4 x 8 ft | 18 in | 48 | 1.78 | 48 |
| 4 x 8 ft | 24 in | 64 | 2.37 | 64 |
| 3 x 6 ft | 12 in | 18 | 0.67 | 18 |
At about $8 to $12 for a 1 cubic foot bag of raised bed mix, a single 4 x 8 x 18 inch bed would cost roughly $380 to $580 in bags. That is why layering pays off. The raised bed soil calculator gives volume and bag counts for several beds at once.
How much real soil do plants need?
Most vegetables root mainly in the top 6 to 12 inches. Lettuce, herbs and other shallow crops do fine in 6 to 8 inches of good mix. Tomatoes, squash and root crops such as carrots and parsnips benefit from 12 inches or more. Below that, the bed just needs material that holds moisture and slowly breaks down.
The layering method
This combines ideas from hugelkultur (burying wood) and lasagna or sheet composting (stacking brown and green layers). From the bottom up:
- Cardboard (optional). If the bed sits on lawn, lay plain brown cardboard with tape and labels removed. It smothers grass and breaks down within a season. Skip it if you want roots to reach native soil right away, or if you are using hardware cloth for gophers.
- Logs and thick branches. Rotting hardwood works best. Pack them tight and fill gaps with smaller sticks. In an 18 to 24 inch bed this layer can be 6 to 10 inches thick.
- Twigs, wood chips and leaves. Fill the gaps between logs so there are no big air pockets that will collapse.
- Green material. Grass clippings (from untreated lawns), garden trimmings, aged manure or kitchen scraps add nitrogen that helps break down the wood.
- Water each layer as you build it. Dry material wicks moisture away from roots later.
- Planting mix on top, at least 6 to 8 inches deep and 12 inches for deep-rooted crops.
What to avoid in the bottom layers
- Pressure-treated, painted or stained wood.
- Black walnut, which contains juglone that can stunt many vegetables.
- Diseased plant material or weeds that have gone to seed.
- Grass clippings from lawns treated with herbicides, which can harm vegetables.
- Rocks or gravel as "drainage." A gravel layer actually holds water in the soil above it rather than draining it.
Soil mix ratios
| Mix | Recipe | Notes |
|---|---|---|
| Simple bulk mix | 50 to 60% screened topsoil, 40 to 50% compost | Cheapest; many suppliers sell it pre-blended |
| Lighter mix | 60% topsoil, 30% compost, 10% perlite or coarse bark | Better drainage in heavy topsoil |
| Equal-parts mix | 1/3 compost, 1/3 peat moss or coconut coir, 1/3 vermiculite | Light and fluffy but costly; best for small beds |
Avoid filling a raised bed with bagged potting mix alone. It is made for containers, costs more, and often shrinks as the organic matter breaks down.
Worked example
A 4 x 8 ft bed, 18 inches deep, holds 48 cubic feet. Fill the bottom 8 inches with logs, branches and leaves (about 21 cubic feet). The top 10 inches takes 4 x 8 x 10 / 12 = about 27 cubic feet, or 1 cubic yard, of soil mix. A yard of bulk topsoil and compost blend often costs about $40 to $70 plus delivery, compared with roughly $220 to $320 for 27 bags.
Building three beds? Order the mix for all of them in one bulk delivery. Use the compost calculator if you are blending your own.
Expect settling
Wood and leaves shrink as they decay, so the bed surface can drop several inches in the first year. Overfill by 2 to 3 inches and top up with compost each spring. Fresh wood also ties up nitrogen as it breaks down; keeping the planting layer thick and adding compost or a balanced organic fertilizer prevents pale, slow growth.
Other ways to save
- Ask local tree services for free wood chips and logs.
- Collect bagged leaves from neighbors in fall.
- Check whether your city sells or gives away compost.
- Build shallower beds directly on loosened native soil, so roots can grow downward without filling a deep box.