What should I fill a raised bed with?
Something that holds water without turning to concrete. That means a mix, always: a bulky mineral or fibrous base, compost for fertility and water-holding, and something coarse so it can't compact. Every recipe you'll read is a different answer to how much of each, and they all grow vegetables. What separates them is cost — but not in the way the recipe arguments imply. Bought in bags, all four common mixes land within a third of each other. What changes the number by multiples is whether you can buy the ingredients loose, and that's decided by which recipe you picked.
A standard 8 × 4 ft bed filled 12 in deep holds 32 cubic feet, about 1.2 cubic yards. Here is what filling it costs four different ways.
The four mixes, priced for one bed
| Mix | What goes in (per bed) | Bags | Bagged cost | $ / cu ft |
|---|---|---|---|---|
| 60 / 30 / 10 — topsoil, compost, aeration | 21.1 cu ft topsoil + 10.6 cu ft compost + 3.5 cu ft perlite | 42 | $189 | $5.37 |
| Mel’s mix — ⅓ compost, ⅓ peat/coir, ⅓ vermiculite | 11.7 cu ft compost + 11.7 cu ft peat + 11.7 cu ft coarse | 17 | $207 | $5.88 |
| 50 / 50 — topsoil and compost | 17.6 cu ft topsoil + 17.6 cu ft compost | 42 | $180 | $5.11 |
| Bagged raised-bed mix | 35.2 cu ft bagged | 24 | $240 | $6.82 |
One 8 × 4 × 12 in bed, 35.2 cu ft ordered (geometry plus 10% settling), computed by the raised bed calculator. Illustrative retail prices — topsoil $3 per 0.75 cu ft bag, compost $6, perlite $18 per 2 cu ft, peat $15 per 3 cu ft bale, vermiculite $35 per 4 cu ft, bagged mix $10 per 1.5 cu ft. Put your own prices in the calculator.
In bags, there's remarkably little in it: the cheapest of the four (50 / 50, $180) and the dearest (Bagged raised-bed mix, $240) are only 1.3× apart — bagged topsoil is expensive too, which is the part the "Mel's mix is pricey" arguments miss. The real gap opens when you can buy loose: the 50 / 50 delivered by the cubic yard is about $59 of material for this bed, $121 less than the same recipe in bags and 3.5× cheaper than Mel's mix. And Mel's mix can't follow it there: no landscape yard sells vermiculite by the yard. That is the decision — not the ratios.
Two ingredients lie about how much is in the bag
Peat moss and coco coir are sold compressed: a 3 cu ft bale fluffs up to about 6 cu ft once you break it apart and wet it. Buy by the label and you end up with twice the peat you needed — or, working the other way, people who allow for it and then don't break the bale up properly come up short. Mel's mix for this bed needs 11.7 cu ft of loose peat, which is 2 bales, not 4. The calculators here do that conversion; most don't.
The second thing that moves the number is settling. Fresh mix drops around 10% after the first few waterings as the air gaps close, so order that much over the geometric volume or accept a bed that looks half-empty by June. It keeps settling more slowly after that as the organic fraction breaks down, which is what the annual top-up below is for.
Mel's mix: what it is, and whether it's worth it
From Mel Bartholomew's Square Foot Gardening: equal parts by volume of compost, peat moss (or coir) and coarse vermiculite. It is genuinely excellent to work with — light, weed-free because there's no soil in it, never compacts, and holds water while draining freely. It is also, at $207 for this bed, the expensive option, and the vermiculite is most of that.
There's a real argument against it that's worth knowing rather than parroting. Mel's mix is roughly two-thirds organic matter by volume; ordinary productive soil is around 5–10%. Critics — Robert Pavlis of Garden Myths most prominently — argue that much organic matter is unnecessary, releases nutrients faster than plants can use them, and guarantees the bed sinks every year as it decomposes. Against that, side-by-side trials (Jill McSheehy's three-mix comparison is the best-documented) have found Mel's mix out-yielding a soil-based blend on tomatoes while costing far more.
The honest summary: it works, it's pleasant, and it is not several times better than a soil-based mix — it's a bit better, and it locks you into the bag aisle, which is where the cost is. A sensible compromise many people land on is Mel's mix for one or two small beds where the weed-free lightness matters most (seed-starting beds, salad beds), and a soil-based blend for the rest.
The soil-based blends: 60/30/10 and 50/50
60% topsoil, 30% compost, 10% aeration (perlite, pumice or bark fines) is the extension-and-garden-center default, and for most vegetable beds it's the right answer: real soil holds nutrients and water in a way soilless mixes don't, drains fine once it's above grade, and costs $189 bagged for this bed — $18 less than Mel's.
A plain 50/50 topsoil and compost is simpler and cheaper still ($180 bagged, about $59 bulk), and it's what most people should build a vegetable garden out of. If your topsoil is clay-heavy, add the 10% of something coarse and you're back at 60/30/10. Either way buy the topsoil and compost loose once you're past a single bed: bagged topsoil works out around $108 a cubic yard against $40 from a landscape yard.
Why not just bagged "garden soil" or straight topsoil?
Bagged raised-bed mix is a legitimate product — pre-blended, no mixing, and at $240 and 24 bags for one bed it's defensible for a single small bed on a balcony. Bagged garden soil and topsoil are different things and are not intended to be used alone: they're formulated to be dug into existing ground. Fill a bed with them and the fine particles settle into a dense, airless slab within a season — the classic "my raised bed turned into a brick" outcome.
Straight compost is the opposite failure: too rich for many crops, and it slumps to half its volume in a year. It's a third of a mix, not the mix. And free fill from a construction site is a gamble on what's in it — for food beds, know where the soil came from.
What to put in the bottom of a deep bed
Anything over about 18 inches doesn't need buying soil all the way down: most vegetable roots live in the top foot, and the rest is expensive ballast. Fill the bottom with logs, branches, twigs, leaves, straw and cardboard — the hügelkultur approach — and top with mix.
The saving is the whole point: a 8 × 4 ft bed at 24 in wants 70 cu ft of mix (about $375 bagged). Fill the bottom half with wood and it needs 35 cu ft ($189) — $186 saved on one bed. The woody layer holds water like a sponge and rots down into something very like compost over three to five years. Two caveats: don't do it in a shallow bed, where roots need the depth, and expect the bed to drop noticeably as the wood breaks down.
Don't put gravel in the bottom "for drainage"
It is the most repeated piece of raised-bed advice and it does the opposite of what it promises. Water moving down through soil will not cross into a coarse layer until the soil above is saturated — the physics is capillary tension, and the result is called a perched water table. A gravel layer therefore holds a soggy zone at the bottom of the soil, and by raising that saturated band closer to the roots it makes drainage worse, not better. The same effect is why a pot with gravel in the base drains worse than one without.
This is not the same as the log-and-branch fill above, which works for a different reason: wood is absorbent and rots into the profile rather than sitting under it as a permanent barrier. If your bed genuinely drains badly, the fixes are loosening the ground underneath before you fill, more coarse material through the mix, or more height — never a layer of anything.
Topping up, year after year
Whatever you fill with, the organic fraction is being eaten by soil life and the level falls an inch or two a year — faster for organic-heavy mixes like Mel's, slower for soil-based ones. That is normal and it is not the soil "wearing out". Top up each spring with an inch or two of compost, which replaces the volume and the fertility at once; there's no need to empty and refill a bed, ever, unless something contaminated it.
One inch of compost over this bed is 2.7 cu ft — a couple of bags, or a wheelbarrow from a bulk pile. That annual habit matters more to how the bed performs in year three than which recipe you chose in year one.
So which should you use?
- One small bed, no truck, want it easy: bagged raised-bed mix. You're paying $6.82 a cubic foot for convenience, and for one bed that's a fair trade.
- A vegetable garden of two or more beds: 50/50 or 60/30/10, bought loose by the cubic yard. Cheapest per bed by a wide margin and the best long-term structure.
- A precise salad or seed bed where weed-free and light matter: Mel's mix, and accept the price.
- A deep bed: any of the above over a foot of wood and leaves.
Then put your own bed dimensions and local prices into the raised bed soil calculator — it does the recipe split, the compressed-bale conversion and the bags-versus-bulk comparison for your numbers rather than these.
Sources: Mel Bartholomew, All New Square Foot Gardening (Mel's mix); University of Minnesota Extension and Oregon State Extension, Raised bed gardens (soil-based blends, depths); Robert Pavlis, Garden Myths — Soil for Raised Beds (the organic-matter critique); Jill McSheehy, 3 Raised Bed Soil Mixes Compared (side-by-side trial). Quantities, bag counts and costs computed by this site's engine from the bed dimensions and prices stated above.