How to Fill a Raised Garden Bed (and Save on Soil)
How deep the soil needs to be for different crops, a simple soil and compost mix, and cheaper ways to fill a tall bed.
Filling a raised bed takes far more soil than most people expect, especially a tall one. The good news is that plant roots don’t need the whole depth to be expensive soil.
How deep does the soil need to be?
An Ask Extension answer gives these minimums:
- At least 8” for leafy greens, beans and cucumbers.
- 12–24” for peppers, tomatoes and squash.
If the bed sits directly on the ground, roots can also grow down into the soil below, so you can loosen that soil with a fork before filling and get away with a shallower bed.
What to fill it with
The same answer recommends a soil and compost mix of about 70% soil to 30% compost, which should be “loose, light and well drained.” In practice:
- Topsoil: buy screened topsoil from a reputable supplier.
- Compost: well-rotted, from a supplier or your own heap.
- Bagged raised-bed or garden soil is convenient for one or two small beds, but costs much more per cubic yard than bulk.
Avoid filling a bed with straight bagged potting mix (it’s expensive and shrinks a lot) or with soil dug from the yard on its own (it compacts and often drains badly). If you’re unsure what your soil needs, a soil test from your local extension office is inexpensive.
How much soil an 8 × 4 ft bed needs
Including 10% extra for settling:
| Bed height | Cubic yards | 1.5 cu ft bags |
|---|---|---|
| 6" | 0.54 | 10 |
| 11" | 1.09 | 20 |
| 12" | 1.2 | 22 |
| 17" | 1.74 | 32 |
| 24" | 2.5 | 45 |
Cheaper ways to fill a tall bed
Logs and branches (hugelkultur). Lay untreated logs and thick branches in the bottom, then smaller sticks, then leaves or grass clippings, then a full depth of soil mix on top. The wood slowly breaks down and holds moisture. Tree services will often give away wood. Allow for the bed to sink as the wood rots, and add soil each year.
Leaves, straw and clippings (lasagna layers). Layers of shredded leaves, straw, untreated grass clippings and kitchen scraps rot down into rich soil over a season or two. Put at least 8–12” of soil mix on top if you want to plant straight away.
Fill the bottom with existing soil. If you’re digging out a path or a patio, use that soil in the bottom half and the good mix on top.
For an 8 × 4 ft bed that’s 24” tall, filling the bottom 12” with wood, leaves or old soil cuts the soil you buy from 2.5 to 1.2 cubic yards, saving about 1.3 cubic yards.
Avoid treated or painted wood, and walnut, which releases a compound that harms many vegetables. Keep anything that hasn’t broken down well below the root zone.
Topping up each year
Raised beds sink as the organic matter breaks down. Each spring, add an inch or two of compost to the top and mix it in lightly.
The Raised Bed, Soil & Mulch Planner works out soil, compost and lumber for any number of beds. For mixes and bags vs bulk, see Raised Bed Soil: How Much and What Mix.