What to Put in a Raised Bed: Soil Mix Guide

Quick answer: The most reliable raised bed soil mix is one-third compost, one-third peat moss or coco coir, and one-third coarse vermiculite. If that’s over budget, a simpler mix of 60% topsoil, 30% compost, and 10% perlite or coarse sand works well for most home gardens. Use our raised bed soil calculator to find out exactly how much of each ingredient you need.

The soil you put in a raised bed matters more than nearly any other decision you’ll make. Raised beds drain differently than in-ground gardens, dry out faster, and don’t benefit from natural soil amendments that work their way up from below. Getting the mix right from the start sets up every season that follows.

The Classic Mix: Mel’s Mix

Mel’s Mix comes from Mel Bartholomew’s Square Foot Gardening method and has become the gold standard for raised bed soil. It’s built around three equal parts:

1/3 compost — blended from multiple sources if possible (mushroom compost, leaf compost, garden compost). This is the fertility engine of the mix.
1/3 peat moss or coco coir — lightweight, improves moisture retention, and keeps the mix from compacting over time. Coco coir is the more sustainable option; peat moss is more widely available.
1/3 coarse vermiculite — improves drainage and aeration, prevents compaction, and holds some moisture without waterlogging roots.

Why it works

Mel’s Mix is light, loose, and drains well while still holding enough moisture for plant roots to access between waterings. It won’t compact the way garden soil or topsoil does, which means roots can spread easily and you don’t need to till between seasons. The high compost content means the mix is nutrient-rich from the start, and regular compost additions keep it that way year after year.

The tradeoff

Vermiculite is the most expensive ingredient — and for larger beds it adds up fast. A 4×8 bed filled to 12 inches deep needs roughly 8 cubic feet of each component (about 0.9 cubic yards total). If your project is large, the budget-friendly mix below is worth considering.

The Budget-Friendly Mix

For homeowners filling a larger bed or multiple beds at once, a simpler mix using materials you can buy in bulk delivers good results at a lower cost:

60% topsoil — use screened topsoil or a topsoil/compost blend, not unscreened fill dirt
30% compost — bagged garden compost, aged manure, or mushroom compost all work
10% perlite or coarse sand — perlite is lighter and better for drainage; avoid fine sand (it makes the mix drain poorly rather than better)

This mix is heavier than Mel’s Mix and will compact slightly more over time, but it performs well for vegetables, herbs, and flowers. It’s also easier to source — all three ingredients are available at most garden centers and landscape supply yards.

Layering a Deep Raised Bed

If your raised bed is deeper than 12 inches, you don’t need to fill the entire depth with your premium mix. A layered approach saves money and puts your best materials where the roots are.

Bottom layer: rough organic material

Fill the bottom 6–12 inches with coarser organic material: wood chips, straw, fallen leaves, cardboard, or rough compost. This layer improves drainage, breaks down slowly over time to feed the layers above it, and keeps costs down on a deep bed. This approach is sometimes called “hugelkultur” when larger wood pieces are used, but even a simple layer of leaves and cardboard works.

Avoid using anything that might be contaminated — no pressure-treated wood scraps, no diseased plant material, no sod from herbicide-treated lawns.

Middle layer: garden mix

The middle 4–6 inches can be a good-quality topsoil blend or garden mix — something between the rough bottom layer and the premium top layer. A bagged garden mix or a topsoil/compost blend from a landscape yard works well here.

Top layer: fine, compost-rich mix

The top 6–8 inches is where seeds germinate, roots establish, and plants spend most of their feeding time. Use your best mix here — either Mel’s Mix or the budget-friendly formula above. This is what plants are actually growing in, so it’s worth spending more on the top layer even if you economize on the layers below.

How Much Compost to Add Each Year

Raised bed soil depletes over time. Plants pull nutrients out of the mix, organic matter breaks down, and the mix slowly compacts and settles. Annual compost additions keep the bed productive season after season.

General rule: Add 1–3 inches of compost to the top of the bed each spring before planting. Work it lightly into the top few inches or leave it as a topdressing and let rain and worms incorporate it naturally.

For a standard 4×8 raised bed:

1 inch of compost = about 2.7 cubic feet (roughly 3 bags of 1-cubic-foot compost)
2 inches of compost = about 5.3 cubic feet (roughly 5–6 bags)

If your bed produced well last season and the soil still looks loose and dark, 1 inch is enough. If the mix looks pale, compacted, or the bed didn’t perform well, go heavier — 2–3 inches and consider mixing in a handful of balanced granular fertilizer.

Some gardeners also add a thin layer of compost mid-season after heavy-feeding crops like tomatoes and squash have been going for a few months.

What to Avoid

Getting the mix right is partly about what you don’t put in. These common mistakes either compact the bed, starve plants, or create drainage problems.

Straight topsoil. Plain topsoil is too dense for a raised bed. It compacts under irrigation, restricts drainage, and limits root growth. If you use topsoil, dilute it with compost and perlite as described in the budget mix above.

Straight potting mix. Potting mix is designed for containers, which drain much faster than a raised bed. It’s also expensive, usually too light for raised beds (it dries out very fast in hot weather), and breaks down quickly. It’s fine blended in, but using it exclusively wastes money and leads to beds that dry out between waterings.

Heavy clay soil. Never fill a raised bed with clay soil from your yard. Clay compacts in a raised bed even faster than in the ground. It holds too much water, restricts oxygen to roots, and is nearly impossible to amend once it’s inside a bed.

Builder’s sand or fine sand. The instinct to add sand for drainage is understandable, but fine sand mixes with clay or topsoil to create something closer to concrete. If you want to add sand, use coarse horticultural sand or perlite instead — they actually improve drainage.

Uncomposted wood chips or fresh manure. Raw organic material pulls nitrogen out of the soil as it breaks down, starving plants in the process. Use composted materials only in your active planting layers. Fresh materials are fine in the bottom drainage layer, where they have time to break down before roots reach them.

Soil Mix Ratio Quick Reference

Mix	Compost	Organic Base	Drainage Amendment	Best For
Mel’s Mix	1/3	1/3 peat/coco coir	1/3 vermiculite	All raised beds; best performance
Budget mix	30%	60% screened topsoil	10% perlite	Larger beds; good value
Annual refresh	1–3 in topdressing	—	Optional perlite	Spring soil renewal

How to Calculate How Much Soil You Need

Before you buy anything, calculate the volume of your bed. Getting this number wrong means either an expensive shortage mid-project or paying to store leftover material.

The basic formula

Multiply the length × width × depth of your bed (all in feet) to get cubic feet. Divide by 27 to convert to cubic yards.

Example: A 4 ft × 8 ft bed filled to 12 inches:

4 × 8 × 1 (12 inches = 1 ft) = 32 cubic feet
32 ÷ 27 = 1.2 cubic yards

For Mel’s Mix, that 1.2 cubic yards breaks down into roughly 0.4 cubic yards of each ingredient. Most bagged ingredients are sold by the cubic foot, so 0.4 cubic yards × 27 = about 10–11 cubic feet of each.

Use the calculator

For multiple beds, irregular shapes, or when you want bag counts alongside cubic yards, our raised bed soil calculator handles all the conversions. Enter your bed dimensions and it returns total volume, ingredient breakdowns for common mix ratios, and estimated bag counts.

Choosing Between Topsoil and Potting Soil

If you’re weighing the difference between topsoil-based mixes and potting-soil-based mixes for your raised bed, see our guide on raised bed soil vs potting soil — it breaks down when each makes sense and what to watch for on product labels.

Tips Before You Fill

Source compost from multiple types. Compost made from a single source (only leaf compost, only food scraps) tends to be less balanced nutritionally than a blend. If you’re buying bags, mixing two or three different types — mushroom compost, leaf compost, aged manure — gives you a more complete nutrient profile.

Wet the mix as you go. Dry peat moss and coco coir are hydrophobic when first added. Moisten the ingredients as you mix or as you fill the bed so the mix is workable and won’t repel water on first watering.

Don’t fill right to the top. Leave 1–2 inches below the top of the bed wall. This creates a berm that keeps soil and mulch from washing over the edge during heavy rain and gives you room to add the annual compost layer without overflow.

Plan for settling. Fresh mix settles after the first watering and again through the first season. Filling slightly higher than your target depth — or keeping a few bags in reserve — means you won’t end up with a bed that looks sunken.

Use our raised bed soil calculator to get your quantities dialed in before your next trip to the garden center.