What Goes In A Raised Bed
A productive raised bed is rarely a single material — it's a blend. The recipe this calculator suggests is 60% topsoil, 30% compost, and 10% amendments, and each part does a distinct job. Topsoil supplies the bulk and the mineral content that gives roots something to anchor in and draw from. Compost adds organic matter, a slow release of nutrients, and the microbial life that turns inert dirt into living soil. Amendments — vermiculite, perlite, peat, or coco coir — are the smallest share but they tune how the bed handles water, loosening dense mixes for drainage or helping sandy ones hold moisture.
Treat the proportions as a baseline, not a law. If your native topsoil is heavy clay, lean toward more compost and a drainage amendment; if it's sand, more compost and a water-holding amendment. The goal is a crumbly, dark mix that drains freely yet stays evenly moist between waterings.
Buying Soil: Bags vs Bulk
How you buy depends on volume. Under roughly 30 cubic feet, bagged soil is convenient and easy to handle — you can carry it through a gate, store the surplus, and avoid a delivery fee. The trade-off is cost per cubic foot, which is always higher in bags.
Over 30 cubic feet, bulk delivery by the cubic yard is usually cheaper per unit and far less plastic to dispose of. As a rough capacity guide, a pickup truck bed holds about 1.5 to 2 cubic yards of soil, and a small landscape trailer holds 3 to 5 cubic yards. One cubic yard equals 27 cubic feet, so a single yard fills a 4×8 bed nearly a foot deep — bulk adds up fast.
Saving On Soil For Deep Beds
Deep beds — anything past about 18 inches — don't need premium growing mix all the way to the bottom. You can fill the lower third with a base layer of logs, branches, leaves, and cardboard, then cap it with your real soil blend. This is the hugelkultur principle: the woody base decomposes over several seasons, slowly feeding the bed and improving drainage while it cuts the volume of purchased soil. Expect the bed to settle as that material breaks down, and plan to top it up in year two.
Frequently Asked Questions
How deep should a raised bed be?
Six inches is the practical minimum for most vegetables, but 8 to 12 inches is better — it gives roots room and holds moisture more evenly. Root crops like carrots, parsnips, and potatoes appreciate 18 inches or more. Deeper beds also dry out and overheat more slowly, which matters in hot climates. If your bed sits on solid ground rather than a hard surface, roots can extend into the soil below, so the bed depth is a floor, not a ceiling.
What's a good soil mix for raised beds?
A reliable starting recipe is 60% topsoil, 30% compost, and 10% amendments. Topsoil provides bulk and mineral content; compost adds organic matter, nutrients, and the microbial life that makes a bed productive; amendments like vermiculite, perlite, peat, or coco coir tune drainage and water retention. This calculator splits your total volume along those proportions so you know roughly how much of each to buy. Adjust to taste — sandy native soil wants more compost and water-holding amendments, heavy clay wants more drainage.
Should I fill a deep bed with full soil, or use a base layer?
For beds deeper than about 18 inches, you usually don't need premium soil all the way down. Many gardeners fill the bottom third with a base layer of logs, branches, leaves, and cardboard — the hugelkultur principle — then top it with the real growing mix. The base layer decomposes slowly, adding nutrients and improving drainage, and it cuts the volume of purchased soil substantially. Just expect the bed to settle over the first year or two as that material breaks down, and plan to top it up.