How to turn a soil recipe into a shopping list
Every mix recipe — "60/30/10," "one part compost to one part peat," "⅓ each" — is by volume, not weight, because the ingredients weigh wildly different amounts: a cubic foot of sand is 100 lb, a cubic foot of perlite 8. So the math is: total volume × each percentage = cubic feet of that ingredient; then divide by the bag size, remembering that compressed bales (peat, coir) roughly double when you open them. The calculator does the scaling for you even when your percentages don't add up, and knows which ingredients are worth buying by the yard.
27 cu ft × 60% = 16.2 cu ft of topsoil (22 forty-pound bags, or ⅗ of a bulk yard); 30% = 8.1 cu ft of compost (9 bags); 10% = 2.7 cu ft of perlite or bark fines (2 bags). It weighs about 1,682 lb mixed — a trailer, not a hatchback.
Common blends, per cubic yard
| Blend | Per cubic yard | Bags | Weight |
|---|---|---|---|
| Raised bed 60 / 30 / 10 | 16.2 cu ft topsoil + 8.1 cu ft compost + 2.7 cu ft perlite | 33 | ≈ 1,682 lb |
| Mel’s mix ⅓ / ⅓ / ⅓ | 9.0 cu ft compost + 9.0 cu ft peat + 9.0 cu ft coarse | 15 | ≈ 558 lb |
| Potting mix ⅓ / ⅓ / ⅓ | 9.0 cu ft peat + 9.0 cu ft compost + 9.0 cu ft perlite | 16 | ≈ 567 lb |
| Lawn leveling 70 / 30 | 18.9 cu ft coarse + 8.1 cu ft compost | 47 | ≈ 2,255 lb |
| Bed amendment 50 / 50 | 13.5 cu ft topsoil + 13.5 cu ft compost | 32 | ≈ 1,688 lb |
Engine figures at the component table's bag sizes (topsoil 0.75 cu ft, compost 1, perlite 2, peat 3 compressed → 6 loose, vermiculite 4, sand 0.5). Presets are starting points, not gospel.
Measuring "parts" without a calculator
On the day, the unit is whatever container you have: a 5-gallon bucket is about ⅔ of a cubic foot, a wheelbarrow 3–4. Count buckets in the ratio — six of topsoil, three of compost, one of perlite — onto a tarp, and mix by lifting the tarp corners. Wet the peat first (dry peat sheds water for days), and mix in batches small enough to actually blend; a pile of ingredients layered on top of each other is not a blend, it's a stratigraphy.
The instinct is to "lighten" clay soil with sand. In the amounts a homeowner adds — 10 or 20 percent — sand plus clay makes something closer to concrete than to loam; it takes well over 50% sand by volume to change the structure, which means replacing the soil, not amending it. What loosens clay is organic matter — compost, year after year. The 70/30 sand-compost in the presets is for topdressing a lawn on top of the soil, which is a different job.
Which ingredient does what
| Ingredient | Bag | lb / cu ft | Role |
|---|---|---|---|
| Topsoil | 0.75 cu ft | 80 | Screened; bulk by the yard is far cheaper. |
| Compost | 1 cu ft | 45 | Any finished compost; mix sources if you can. |
| Peat moss or coco coir | 3 cu ft → 6 loose | 10 | Sold compressed — a 3 cu ft bale makes ~6 cu ft. |
| Coarse vermiculite | 4 cu ft | 7 | The expensive part of Mel’s mix; perlite substitutes. |
| Perlite / pumice / bark fines | 2 cu ft | 8 | The "soilless mix" or aeration fraction. |
| Coarse sand | 0.5 cu ft | 100 | Builder’s / coarse sand, never play sand. Heavy. |
| Pine bark fines | 2 cu ft | 15 | The aeration fraction in container and azalea mixes. |
| Composted manure | 1 cu ft | 45 | Aged; a partial compost substitute, richer in N. |
Topsoil is the body; compost is the fertility and water-holding; the aeration fraction (perlite, bark, pumice, vermiculite) keeps it from compacting in a box. Peat or coir is the water-holding in a soilless mix — coir is the renewable one and is sold in the same compressed bricks. Sand is for drainage in a container mix or for leveling a lawn; in beds it mostly adds weight.
Sources: University of Minnesota Extension, Raised bed gardens; Cornell Cooperative Extension, Soil Mixes for Containers; Mel Bartholomew, All New Square Foot Gardening; USDA NRCS on sand amendment of clay soils. Bag sizes and densities from product specifications.