How to fill a raised bed cheaply (without ruining it)

First, know how much you are filling

Raised bed volume is length × width × depth. With length and width in feet and depth in inches, divide by 12 for cubic feet. A 4 × 8 ft bed is 32 cu ft at 12 in and 48 cu ft at 18 in. Add 10–20% for settling if you are filling with fresh, fluffy material, and measure the depth you will really fill: a "12 inch" wooden bed built from two 2×6 boards is 11 in tall, so it holds about 10 in of soil once you leave room below the rim. The common sizes are worked out with bag counts: How much soil for a 4×8 raised bed, the deeper 4×8 raised bed 18 inches deep, and the metric 1.2 × 2.4 m raised bed.

Only the top part of the bed needs to be good growing mix. Most vegetable roots do the bulk of their work in the top 8–12 in, especially when the ground under the bed has been loosened. Every inch of cheaper material below that is an inch of soil you don’t have to buy.

Option 1: buy bulk instead of bags

The simplest saving. Landscape yards sell "garden blend," "raised bed mix" or topsoil–compost blends by the cubic yard. One yard is 27 cu ft — nearly enough for a 4 × 8 bed at 12 in, before settling. Bulk usually costs a fraction of the same volume in bags, even after delivery, once you are filling two or more beds.

• Ask what is in the blend and whether it is screened. You want a crumbly mix with plenty of organic matter, not heavy fill dirt.
• Ask the supplier how the blend drains; some are heavy on fine compost that stays wet.
• Split a load with a neighbor if you only need a yard or two and the minimum is higher.

Option 2: loosen the ground and use a shallower bed

If the bed sits on native soil, you can dig or fork the ground beneath it before filling. That lets roots go down into the ground and means an 8–10 in bed performs like a deeper one. It is free, and it is often the biggest saving of all. Remove grass first, or smother it with cardboard (below).

Option 3: logs and branches at the bottom (hugelkultur-style)

Hugelkultur is the practice of burying wood under soil. In a raised bed, that usually means a layer of logs, thick branches and brush in the bottom third, then leaves or grass clippings to fill gaps, then good soil on top. Fans report that the rotting wood holds moisture and slowly feeds the soil. The evidence for dramatic benefits is mostly anecdotal, but as a way of using up material you already have instead of buying soil, it is widely used.

• Use wood that is already starting to rot if you can; it settles less over time than fresh wood and holds water sooner.
• Avoid black walnut, treated lumber and painted wood. Cedar and other rot-resistant woods break down very slowly.
• Keep at least 8–12 in of real soil on top. Seedlings planted directly into woody layers struggle.
• Expect settling. As the wood breaks down over years, the bed will drop; top up with compost each season.
• Fresh woody material can tie up nitrogen where it contacts soil as it decomposes. Keeping the wood deep, under a good soil layer, limits the effect in the root zone, but watch for pale, slow growth in the first season.

Option 4: lasagna layering

Lasagna (sheet) gardening builds soil by stacking alternating layers of "browns" (dry leaves, straw, shredded paper, cardboard) and "greens" (grass clippings, vegetable scraps, manure, green plant material), then letting it break down. In a raised bed, you can use this for the bottom half and put finished soil or compost on top so you can plant immediately.

The catch is shrinkage. Uncomposted layers can lose a large share of their volume over a season, so beds built this way need topping up. Many gardeners build them in fall so the layers settle over winter before spring planting.

Option 5: cardboard over grass

Instead of digging out sod under a new bed, lay plain cardboard over mown grass, overlap the edges well, soak it, and build the bed on top. The grass dies in the dark and the cardboard breaks down within a season or so. Remove plastic tape and glossy labels first. This saves the work of stripping turf and adds a little organic matter, though it doesn’t reduce how much soil you need.

Option 6: mix your own

Buying components separately can beat a pre-bagged "raised bed mix," especially if you can get bulk topsoil and compost. Common starting points:

• Topsoil and compost, about two-thirds to one-half topsoil: the University of Minnesota Extension’s recommendation for raised beds; Penn State Extension suggests about 70% soil to 30% compost.
• 60/30/10: topsoil, compost, and an aerating material such as fine bark, perlite or pumice.
• 50/50: topsoil and compost. Simple and widely used.
• Equal thirds (Mel’s Mix): compost, peat or coir, and vermiculite. Light and loose, but the bagged peat/coir and vermiculite make it one of the more expensive options for big beds.

Avoid filling a bed entirely with bagged "potting mix" or pure compost. Potting mixes are designed for containers and can be costly at this scale; pure compost can settle a lot and may be very rich in nutrients and salts. A mineral component (topsoil) gives the bed structure.

What not to use

• Plastic, rocks or gravel at the bottom "for drainage." A layer of coarse material under fine soil actually holds water in the soil above it (a perched water table): as Washington State University’s Linda Chalker-Scott explains, water won’t move from a fine layer into a coarse one until the fine layer is saturated. Open-bottom beds on loosened soil drain better.
• Treated wood scraps, painted wood, or anything from a demolition site.
• Hay (full of seeds), diseased plant material, or pet waste.
• Fresh wood chips mixed into the growing layer. Keep wood below or on the surface as mulch.

A worked example

Two 4 × 8 ft beds, 18 in deep, would take 96 cu ft of soil — about 3.6 cubic yards (one bed is worked out on the 4×8 at 18 inches page). If you fork the ground underneath, fill the bottom 6 in with logs, branches and leaves you already have, and buy only the top 12 in, you need 64 cu ft (about 2.4 yards) of purchased mix instead. With 10% for settling that is about 70 cu ft (2.6 yards), so order 3 yards from a yard that sells in half-yard steps. That is a third less soil — and far less than buying 96 cu ft in bags.

Keep the bed full

Whatever you fill with, the bed will settle, and organic matter breaks down each year. Rutgers Cooperative Extension notes that mineral soils tend to hold their volume after the first settling, while beds heavy in organic material keep shrinking and need regular additions. Plan on an inch or two of compost every season. It keeps the soil level up and replaces the nutrients the last crop used.