Raised bed soil mix, Mel's Mix vs the alternatives
Filling a new raised bed is the single most expensive thing you'll do the first year of a garden. The soil costs more than the lumber, more than the seeds, and if you pick wrong, you'll spend the next three seasons fighting drainage or nutrient deficiencies.
I've filled six 4x8 beds and two 2x8 beds over the last six years, and I've made pretty much every mistake available. Here's what I know now.
The original recipe: Mel's Mix
Mel Bartholomew's formula from All New Square Foot Gardening is exactly this:
- 1/3 compost (by volume), ideally blended from five different sources
- 1/3 peat moss OR coconut coir
- 1/3 coarse vermiculite (NOT perlite, NOT sand)
A 4x8x12 bed holds about 32 cubic feet. So you need roughly 11 cubic feet of each component. At 2026 prices in Indiana, that's about:
- Compost: $60-80 (5 bags of Black Kow at Menards plus a yard from the local composter)
- Peat moss: $35 (3-cubic-foot compressed bale, Premier Horticulture)
- Vermiculite: $90 (two 4-cubic-foot bags of horticultural grade)
Total: about $200 for one 4x8 bed.
That's a lot. It's also the best-draining, best-aerating, most root-friendly mix I've ever worked with. The vermiculite is the expensive part, and it's also the component you can't substitute cleanly.
Why vermiculite and not perlite
Perlite is cheaper. It also floats to the surface when you water, and within two seasons your bed looks like a popcorn factory threw up on it.
Vermiculite holds water and nutrients in its flaky structure, releasing them as roots pull. Perlite doesn't; it's purely for drainage. In a raised bed, you want both drainage AND moisture buffering, and vermiculite does both. Sand is too dense and compacts over time.
If you're going to skimp somewhere in the recipe, don't skimp on vermiculite. Skimp on the "five different compost sources" part.
Alternatives that actually work
Fox Farm Happy Frog raised bed mix
This is what I used for my 2022 expansion beds. 1.5 cubic feet per bag, about $18 at most garden centers. A 4x8x12 bed needs about 22 bags = $400. More expensive than Mel's Mix, but zero mixing, zero measuring, and the bag is nicely balanced.
Downside: out of the bag, it's heavy on peat and light on compost by SFG standards. I amend with a 2-inch layer of my own compost on top of each bed in year two.
Coast of Maine Bar Harbor Blend
What I've filled my last two beds with. 1 cubic foot per bag, about $12. Similar formulation, a little lighter than Happy Frog. Made with lobster shell compost (yes, really), which adds calcium and trace minerals.
Downside: harder to find outside the Northeast and Midwest. Online shipping is expensive.
DIY budget mix: topsoil + compost + peat
If you're filling a big bed and can't stomach the Mel's Mix price, here's a budget recipe:
- 50% topsoil (the cheap bulk kind from a local soil yard, $20-40 per cubic yard)
- 30% compost (screened, aged, local)
- 20% peat moss or coir
It's not as airy or well-draining as Mel's Mix, but it's maybe $50 for a 4x8 bed instead of $200. Plants will grow in it. You'll fight drainage and compaction a bit more, and you'll need to top-dress with compost every year.
I used this for my back-yard overflow bed in 2021 and it's produced respectable harvests every year since. Not as productive as my Mel's Mix beds, maybe 70% of the output, but at 25% of the cost.
What NOT to fill a bed with
- Bagged "garden soil" from a big box store. Usually lightly amended topsoil at 3x the price of bulk. Waste of money.
- Fresh manure of any kind. Burns plants, introduces weed seeds, carries pathogens. Let manure compost for at least 6 months before using on an edible bed.
- Pure compost. Compost alone is too rich. Plants grow leggy and lodge (fall over). Use compost as part of a mix, not the whole mix.
- Native soil from your yard. Defeats the point of a raised bed. If your native soil was great, you wouldn't be building a raised bed.
- Sand. Drains too fast, no nutrient retention, and it compacts into cement over time.
The compost question
Mel says five different compost sources blended together. The logic is that every compost pile has its own balance of microbes, nutrients, and organic matter sources. Blending smooths out the variation.
In practice, two good sources are fine. I use my own compost (kitchen scraps + yard waste, composted 18 months) plus Black Kow composted manure from a bag. That covers both the microbial diversity and the nitrogen load.
If you only have one source, use Black Kow or a similar aged manure compost. Don't use mushroom compost (too alkaline for most vegetables) or pure leaf mold (not enough nitrogen).
Top-dressing in year two and beyond
Here's the good news: the vermiculite and peat in Mel's Mix are essentially permanent. You paid for them once. In year two, you just add a 2-inch layer of compost on top of each bed in early spring and mix it in with a garden fork.
That's it. No more $200 refills. Maintenance cost per bed per year: about $15 in compost.
I'm in year six on my original 2019 bed. Soil is still beautiful. Slightly lower and more uniform than year one (the peat compresses over time), but still the best garden soil I've ever worked with.
pH and soil testing
Most vegetables want pH 6.0-7.0. Peat moss is acidic (pH 3.5-4.5), so a fresh Mel's Mix bed usually comes in around pH 5.5-6.0, slightly low. Add a cup of garden lime per 8 square feet to bring it up to 6.5.
I test every bed every spring with a Bluelab pH meter ($120, lasts forever). Soil that's 5.5 in spring will be 6.2 after a lime amendment, which is where tomatoes and most Brassicas thrive.
Test kits from your local extension office are more accurate than a pH pen. Cost about $15 per sample and give you NPK plus micronutrients. Worth doing every 2-3 years.
The short version
First bed, money's no object: Mel's Mix from the original recipe. You'll have the best soil in the neighborhood.
First bed, budget constrained: Fox Farm Happy Frog or Coast of Maine, bagged from a garden center. 80% of the performance, 70% of the price, zero measuring.
Filling a big bed cheap: DIY topsoil + compost + peat at 50-30-20. Not as good, but plants will grow.
Year two and after: top-dress with compost every spring, never refill.
Ready to plan what goes in your new bed? Calculator's here.