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% native clay can bake into a brick. You want something in between.
A practical default mix
For a general vegetable bed in much of the US, this is a sane starting point by volume:
- About 50% screened topsoil or high-quality garden soil (mineral bulk, not bagged “fill dirt” full of rocks)
- About 30–40% finished compost from more than one feedstock if you can (yard waste plus manure compost is better than one tired source)
- About 10–20% aeration material if the blend is heavy: coarse horticultural perlite, vermiculite, or pine bark fines — not playground sand dumped into clay
Match the mix to the situation
| Situation | Lean toward | Watch out for |
|---|---|---|
| New bed on a patio (closed bottom) | Lighter mix with more compost and bark fines so it drains | Straight native soil in a sealed box stays waterlogged |
| Open-bottom bed on decent ground | More mineral soil; roots can go down | Still add compost so the top 12 inches are fertile |
| Hot, dry climate | Slightly more compost and mulch on top to hold water | Very sandy mixes dry out by afternoon |
| Wet, clay native soil below | Open bottom plus a mix that drains; do not add a rock layer | A water-tight liner turns the bed into a tub |
What to avoid
- 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)
- Assuming “organic” on the bag describes nutrient content. It does not replace compost quality.
After the first season
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?
A bed of 100% compost can be too rich, too fluffy, and prone to shrinking. Blend it with mineral soil for structure.
Can I use native soil from my yard?
Yes, if you screen it and mix in compost. Heavy clay benefits from compost and a drainage component. Do not use soil from an area treated with persistent herbicides.