Raised Bed Soil Mix
Raised beds require a manufactured soil mix — in-ground garden soil compacts too much for good root development when confined. This guide covers the proven base formula, soil type variations, and pH management.
Mel's Mix — the standard raised-bed formula
Developed by Mel Bartholomew in All New Square Foot Gardening (3rd ed., Cool Springs Press). Designed specifically for raised beds: the three components provide drainage, moisture retention, and nutrients without compaction.
At least 5 different types if possible (mushroom, cattle, worm casting, leaf mold, etc.). Compost diversity improves microbial activity.
Provides moisture retention and lightness. Coco coir is a sustainable peat substitute with comparable water-holding capacity.
Improves drainage and aeration. Use coarse-grade (#2 or #3) — fine-grade vermiculite compacts over time.
Source: Bartholomew, Mel. All New Square Foot Gardening, 3rd ed. Cool Springs Press.
Soil type profiles
For specialty crops or existing amended beds, these profiles describe the best mix type to use.
A loamy mix (60% screened topsoil + 30% compost + 10% perlite) — the all-purpose raised-bed standard.
A sandy / well-draining mix (50% sandy loam + 30% compost + 20% coarse sand or perlite). Great for root crops and drought-lovers.
A heavier loam (skip extra sand; rely on compost + topsoil). Most raised beds avoid pure clay — amend generously.
A rich silty loam (topsoil + leaf mold + compost). Holds moisture well; add a bit of perlite for drainage.
An alkaline-leaning mix (loam + a little garden lime if needed). Rare for raised beds — most stores sell a neutral mix.
A peaty / acidic mix (loam + peat moss or pine bark fines + compost). Good for blueberries and other acid-lovers.
Soil pH guide
Most raised-bed mixes sold in the US land at pH 6.5–7.0 (neutral) out of the bag. Test annually with a soil meter or home test kit.
Slightly acidic (pH ~5.5–6.5). Add elemental sulfur or peat moss if your mix tests too high.
Neutral (pH ~6.5–7.0) — the sweet spot for most vegetables and herbs.
Slightly alkaline (pH ~7.0–7.5). Add garden lime if your mix tests too low.
Source: UMN Extension — “Soil pH for the Home Garden.”
Common amendments
| Amendment | Effect on pH | Use case |
|---|---|---|
| Garden lime (calcitic) | ↑ Raises pH | Correct overly acidic soil; common in Pacific Northwest |
| Elemental sulfur | ↓ Lowers pH | Prepare beds for blueberries or other acid-loving crops |
| Peat moss | ↓ Slightly lowers | Moisture retention + mild acidification |
| Aged manure | Neutral to ↑ slightly | Nutrient boost (nitrogen, phosphorus, potassium) |
| Worm castings | Neutral | High biological activity; gentle slow-release nutrition |
| Perlite | Neutral | Drainage improvement in heavy or compact mixes |
Source: OSU Extension — Raised Bed Gardening. Oregon State University.
Sources
- Bartholomew, Mel. All New Square Foot Gardening, 3rd ed. Cool Springs Press.
- University of Minnesota Extension — “Soil pH for the Home Garden.” extension.umn.edu
- Oregon State University Extension — “Raised Bed Gardening.” extension.oregonstate.edu