Raised beds fail more often from the wrong mix than from the wrong lumber. Mixes that are 100% potting soil can be too light and expensive at bed scale. Mixes that are 100% compost can be too rich, too fluffy, and prone to shrinking as organic matter decomposes. You want mineral bulk plus compost. That is different from potting mix, which should stay soilless.
Balanced raised-bed mix
For a general vegetable bed, this is one practical point inside the University of Minnesota Extension range, and the default starting recipe in the raised bed soil calculator, by volume:
- 60% screened topsoil (bulk delivery is often practical for large volumes; bagged garden soil varies and is not identical to native yard soil)
- 40% finished plant-based compost
More-topsoil mix
Another practical point in the same Extension range when you lean harder on bulk topsoil:
- About two-thirds screened topsoil
- About one-third finished plant-based compost
Neither preset is a mandatory universal formula. Local topsoil texture and drainage differ. Oregon State University Extension also discusses other mineral blends (for example equal thirds topsoil, sand, and compost, or equal quarters with vermiculite or lava rock). Those examples are editorial context here — not extra calculator presets in this tool.
Match the mix to the situation
| Situation | Lean toward | Watch out for |
|---|---|---|
| New bed on a patio (closed bottom) | Balanced raised-bed mix and a mix that still drains | Straight native soil in a sealed box stays waterlogged |
| Open-bottom bed on decent ground | Balanced or More-topsoil; roots can go down into loosened native soil | Still keep compost in the top 12 inches |
| Hot, dry climate | Mulch on top to hold water; watch sandy topsoil dry out | Assuming more compost is always better |
| Wet, clay native soil below | Open bottom plus a mineral-heavy fill that drains | A water-tight liner turns the bed into a tub |
What to avoid
- Filling the entire bed with compost alone
- Uncomposted manure that can burn roots and carry weed seeds
- Bagged products that are mostly shredded wood dyed brown
- Mixing in fresh wood chips throughout the root zone (they can tie up nitrogen as they decompose)
- Filling the whole frame with bagged potting mix (costly and often too light for tall crops)
- Assuming “organic” on the bag describes nutrient content. It does not replace compost quality.
After the first season
Newly filled beds may sink as organic material decomposes — top off after watering. You rarely need to dump the bed and start over. Top-dress ½–1 inch of compost each year, and use the compost calculator for the volume. If plants stall, a soil test is more useful than adding random fertilizer. See how to fertilize a raised bed garden.
Frequently asked questions
Can I use only compost?
No. Oregon State University Extension advises against filling raised beds with compost alone. Blend finished plant-based compost with topsoil or another mineral component — as in the Balanced or More-topsoil recipes.
Can I use native soil from my yard?
Yes in open-bottom beds if you screen it and mix in finished plant-based compost. Heavy clay needs patience and good compost quality. Do not use soil from an area treated with persistent herbicides. Native soil belongs in beds, not in ordinary pots.
How do I turn bed volume into bag counts?
Enter the bed size in the [raised bed soil calculator](/calculators/raised-bed-soil), pick Balanced raised-bed mix or More-topsoil mix, and read the ingredient table. It splits cubic feet (and estimated 2 cu ft bags) for each part of the recipe.
Sources
- Raised bed gardens — University of Minnesota Extension
- How to use compost in gardens and landscapes — Oregon State University Extension