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Raised Garden Bed Soil: How Much Soil and Mulch to Fill It

The formula

Length × width × height in feet = cubic feet, ÷ 27 for yards. An 8×4×1 ft bed = 32 cu ft = 1.2 cu yd. Fill 60% topsoil + 30% compost + 10% aeration (perlite or rice hulls), then top with 2 inch mulch. Size it with our Mulch Calculator.

Measure the fill height, not the frame height. A bed built from two stacked 6-inch boards holds about 10 inches of soil, not 12, and leaving the last couple of inches empty is normal — it keeps mulch off the rim and stops soil washing over the edge in a storm.

Volume by bed size

Split each volume in the 60/30/10 ratio before you shop. For a single 8×4×1 ft bed that is roughly 19 cu ft topsoil, 10 cu ft compost, and 3 cu ft perlite — three separate purchases, and the topsoil is by far the heaviest and the one worth buying in bulk.

Worked example: two 8×4×1 ft beds

Each bed 32 cu ft: 19 topsoil + 10 compost + 3 perlite. Both beds ≈ 2.4 cu yd total — order 3 yards bulk or 48 two-cu-ft bags. Water deeply after filling; the mix settles 10–15% in two weeks, then top up before planting.

Fill in stages and water between them. Dry-mixed soil looks full until the first real soaking collapses it, and topping up a settled bed after planting is far more awkward than topping it up before the seedlings go in.

Best mix, and what to avoid

• Do not use pure garden soil — it compacts into brick
• Bagged raised-bed mix works but costs 3× bulk ingredients
• Never use pressure-treated railroad ties; modern ACQ lumber or cedar instead
• Line weedy ground with cardboard, not plastic — worms need passage
• Keep the compost fully finished; raw manure burns young roots
• Skip the sand-and-topsoil “recipe” unless your native soil is already sandy

If you are unsure about your own soil, the squeeze test settles it in seconds: damp soil that forms a loose ball and crumbles is loam and can go straight into the mix; soil that ribbons out like putty is clay and belongs nowhere near a raised bed.

Depth by crop

Salad greens and herbs thrive in 6–8 inch. Tomatoes, peppers, and carrots want the full 12 inch — short beds stunt taproots and halve yields. On concrete or contaminated soil, add a weed-block bottom and treat the bed as a giant container with extra drainage holes.

Root crops are the honest test of bed depth. A raised bed at 8 inches will grow a respectable lettuce and a sad, forked carrot; if carrots and parsnips are the plan, build to 12 inches from the start rather than rebuilding in a season.

Watering raised beds

Raised soil drains fast and dries a day earlier than ground soil. Drip lines on a timer beat hand watering: 30 minutes every other morning in summer, adjusted for rain. Mulch the surface 2 inch with straw to cut watering by a third.

Cost reality

Two cedar 8×4 beds with bulk fill run $250–$400 DIY versus $800+ installed. Lumber is half the cost — price cedar, ACQ pine, and galvanized steel kits before committing. Soil from a landscape yard at $35–$60 per yard beats 48 retail bags every time.

FAQ

• Bottom needed? Only on concrete or contaminated ground; open bottoms drain better.
• How long does fill last? Top up 2 inch compost yearly; full replacement rarely needed.
• Snails and slugs? Copper tape on bed walls plus evening patrols beats pellets near food.
• Can I fill with native soil? Only if it drains well and is not clay — otherwise the bed becomes a bathtub in a wet spring.
• Do I need landscape fabric? No, and under fill it is worse than nothing: it blocks worms and traps water above it.

Limits

Beds over 4 ft wide waste the middle — you cannot reach it without stepping in. Wood under constant moisture lasts 5–8 years; plan rebuilds into the budget. More in our garden guides.

One last rule: label each bed with the fill date and mix ratio on a weatherproof tag. Three seasons later that tag tells you exactly what to top up instead of guessing — and guessing is how beds slowly turn back into plain dirt.

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