The short answer
Most garden crops need at least 10 inches of soil, according to the University of Georgia Extension. How much more depends on what is under the bed.
Over poor or compacted ground, the University of Maryland Extension says to spread at least 8 inches of mix on top, and Colorado State University Extension calls 8 to 12 inches minimal where the soil below is not suitable for crops. Where the whole root zone sits in the fill, Washington State University Extension says to build beds at least 15 to 18 inches deep. On a patio or other hard surface, Maryland gives at least 8 inches for leafy greens, beans and cucumbers, and 12 to 24 inches for peppers, tomatoes and squash.
How deep the roots go
The UC Master Gardener program groups vegetables by where most of their roots grow in good soil. In a bed on open ground, roots can carry on into the soil below the bed; on paving they cannot.
| Group | Main roots in the top | Examples |
|---|---|---|
| Shallow-rooted | 6 to 12 in | Cabbage, cauliflower, lettuce, celery, sweet corn, onion, white potato, radish |
| Moderately deep-rooted | 1 to 2 ft | Snap bean, carrot, cucumber, eggplant, peas, pepper, summer squash |
| Deep-rooted | 2 to 4 ft | Asparagus, globe artichoke, cantaloupe, pumpkin, tomato, watermelon |
On open ground, loosen what is underneath
If a bed is shallower than 10 inches, UGA says to till the existing soil below it so roots can go on down. Colorado State adds a warning about layers: a sharp change from one soil to another slows water and air and can stop roots, so mix the first few inches of fill into the ground below.
What each depth costs to fill
For a 4 × 8 ft bed measured inside the walls, every inch of depth adds 2.67 cubic feet of soil. Bags are 1.5 cubic feet, rounded up.
| Soil depth | Cubic feet | Cubic yards | Bags of 1.5 ft³ |
|---|---|---|---|
| 8 in | 21.33 | 0.79 | 15 |
| 10 in | 26.67 | 0.99 | 18 |
| 12 in | 32 | 1.19 | 22 |
| 18 in | 48 | 1.78 | 32 |
| 24 in | 64 | 2.37 | 43 |
The 4 × 8 ft page and the other bed sizes give the same table for your size, and the deep-bed guide covers ways to fill a tall bed with less bought soil.
Wall height is not soil depth
Fill a little short of the top. Iowa State University Extension suggests leaving 1 to 2 inches for mulch, and notes that new soil often settles, leaving a lip. Washington State University Extension adds that bed soil keeps subsiding as it settles and breaks down, so plan to top it up every year or two.
References
- Raised Garden Bed Dimensions (Circular 1027-4), University of Georgia Cooperative Extension
- Growing Vegetables in Raised Beds, University of Maryland Extension
- Block Style Layout in Raised Bed Vegetable Gardens (GardenNotes #713), Colorado State University Extension
- Raised Beds: Will They Benefit Your Vegetable Garden? (FS075E), Washington State University Extension
- Irrigation, UC ANR Master Gardener Program
- Creating Raised Bed Planters, Iowa State University Extension and Outreach