What a good raised bed mix does
Soil in a raised bed drains faster and warms sooner than ground soil, but it can also dry out and compact. A good mix balances three jobs:
- Body and minerals from screened topsoil or loam, which holds nutrients and gives roots something firm to anchor in.
- Organic matter from compost, which feeds soil life, holds moisture and improves structure.
- Air and drainage from coarse material such as perlite, pumice, fine bark or coarse sand, which keeps the mix from packing down.
Common mixes compared
| Mix | Recipe by volume | Good for | Watch out for |
|---|---|---|---|
| 60/30/10 | 60% topsoil, 30% compost, 10% aeration material | Most vegetable beds; balanced and fairly economical | Topsoil quality varies; ask for screened loam |
| 50/50 | Half topsoil, half compost | Simple, rich mix for heavy feeders | Can settle and compact more |
| Equal thirds | Compost, coir or peat, vermiculite | Shallow beds, small beds, no topsoil available | Lightweight, dries out faster, costs more per cubic foot |
| Bagged raised-bed mix | As packed | One or two small beds, no mixing | Check the bag volume; costly for large beds |
These are widely used starting points rather than exact science. Local compost and topsoil vary a lot, so look at what you are buying: it should be crumbly and free of large stones, roots and trash.
Many gardeners avoid filling a raised bed only with native clay soil, which can compact and drain poorly in a bed, or only with compost, which can be too rich and shrink a lot as it breaks down. Potting mix designed for containers works but is expensive to use in volume.
How deep should the soil be?
| Crops | Typical soil depth |
|---|---|
| Lettuce, salad greens, herbs, radishes | 6 to 8 in |
| Most vegetables: beans, peppers, brassicas, onions | 10 to 12 in |
| Tomatoes, carrots, parsnips, potatoes, squash | 12 to 18 in |
If the bed sits on loosened ground, roots can grow down into the soil below, so a shallower bed works. On a patio or hard surface, give the plants the full depth in the bed.
How much of each ingredient?
Work out the bed volume first, add about 10 percent for settling, then split it by percentage.
ingredient volume = bed volume × (1 + settling %) × ingredient %
Worked example
A tall 3 × 6 ft bed, filled 18 inches deep so it is easier on the back, holds 27 cu ft. With 10 percent extra for settling that is 29.7 cu ft, or about 1.1 cubic yards. A 60/30/10 mix needs:
| Ingredient | Share | Cubic feet | Liters |
|---|---|---|---|
| Topsoil | 60% | 17.8 | 505 |
| Compost | 30% | 8.9 | 252 |
| Aeration material | 10% | 3.0 | 84 |
| Total | 100% | 29.7 | 841 |
In a bed this deep, some gardeners fill the bottom few inches with coarser, cheaper material such as extra topsoil, and keep the richer mix in the top 12 inches where most roots grow.
The raised bed soil calculator does this for several beds at once and gives bag counts for 1, 1.5 and 2 cu ft bags.
Bags or bulk for your mix?
For one small bed, bags are simple. For several beds, bulk usually works out cheaper. Four 4 × 8 ft beds filled 12 inches deep with 10 percent for settling need 140.8 cu ft, which is 5.21 cubic yards, or 71 bags of 2 cu ft. Many landscape suppliers sell premixed garden blends by the cubic yard; ask what is in them, since the proportions vary.
Mixing tips
- Mix on a tarp or in the bed in layers, turning each layer in with a fork. Pockets of pure compost or pure sand cause uneven drainage.
- Moisten dry peat or coir before mixing; dry material repels water at first.
- Fill in fall if you can. The mix settles over winter and you can top it up before spring planting.
- Each year, add a couple of inches of compost to replace what plants and soil life use.
- Cover bare soil with mulch to hold moisture. The mulch calculator works out how much.
Safety notes
Buy topsoil and compost from suppliers who can tell you where it came from, especially for food crops. If you build beds from reclaimed wood, avoid old railroad ties and wood of unknown treatment. Wear gloves and a dust mask when handling dry peat, perlite, vermiculite and bagged mixes, and wash hands after gardening.
Frequently asked questions
What is the 60/30/10 raised bed mix?
It is 60 percent topsoil, 30 percent compost and 10 percent coarse aeration material such as perlite, pumice or fine bark, measured by volume.
Can I fill a raised bed with just compost?
It is usually better not to. Pure compost is very rich, can hold too much water and shrinks noticeably as it breaks down. Blend it with topsoil.
How much soil mix do I need for a 4 × 8 bed?
At 12 inches deep, 32 cu ft, or about 35.2 cu ft with 10 percent extra for settling: roughly 21.1 cu ft of topsoil, 10.6 of compost and 3.5 of aeration material in a 60/30/10 mix.
Do the math for your own project.
Open the raised bed soil calculator