Soil mix ratio for raised beds (starting points)
Practical starting ratios for raised beds and what to adjust for drainage and fertility.
Common starting mixes
| Mix (by volume) | Good for | Note |
|---|---|---|
| 50% compost + 50% topsoil | General beds | Simple starting point |
| 40% topsoil + 40% compost + 20% aeration | Better drainage | Adjust for climate |
How to adjust the mix (simple rules)
- If the bed stays wet or drains poorly, increase aeration/structure (not just compost).
- If the mix dries too fast, increase water-holding materials and plan mulching/topdressing.
- If compost is very fresh or very fine, reduce the percentage and add structure so the bed does not slump.
Avoid over-amending
- Too much compost can reduce structure in some soils.
- If drainage is poor, address structure (aeration) not just fertility.
Practical planning tip
Treat soil as a “base mix” and plan to topdress later. Beds settle over time, and adding a thin layer of compost or topsoil is often easier than trying to build the perfect mix on day one.
What “aeration” usually means (options and tradeoffs)
Aeration is the part of the mix that creates pore space so roots can breathe and water can drain. The best choice depends on what is available locally and what you’re planting.
| Material (common) | Why people use it | Watch-outs |
|---|---|---|
| Coarse composted bark | Structure + drainage | Can float or break down over time |
| Pumice / perlite | Lightweight pore space | Can be dusty; may cost more |
| Coarse sand | Adds weight and drainage | Too fine can make mixes dense |
| Rice hulls / biochar | Structure and water handling | Quality varies; start small |
How to calculate your ingredient volumes
Step 1: calculate bed volume (length × width × depth). Step 2: convert to cubic yards or cubic feet. Step 3: multiply by each percentage in your mix.
Example: a 4 ft × 8 ft bed filled to 12 inches is 32 cu ft (about 1.19 cu yd). If you use a 40/40/20 mix, that’s ~12.8 cu ft topsoil, ~12.8 cu ft compost, and ~6.4 cu ft aeration material.
- If you’re using bagged products, convert everything to cubic feet first and round up to whole bags.
- Plan for settling: many beds lose volume after watering and the first few weeks (project dependent).
Crop and climate adjustments (starting guidance)
- Vegetables and annuals typically like a fertile mix but still need structure so roots don’t sit wet.
- Perennials and shrubs often prefer a more mineral mix (more topsoil, less compost) for long-term stability.
- Hot/dry climates benefit from mulching and water-holding strategy; wet climates benefit from more structure and drainage focus.
Common mistakes that cause bed problems
- Using only compost: it can shrink, get hydrophobic when dry, or get soggy depending on compost quality.
- Using very fine materials that compact easily, reducing drainage and oxygen.
- Skipping mulch: the top of a raised bed can dry fast and form crust without protection.