What is the best soil mix for a raised garden bed?

A widely recommended blend is roughly 60 percent topsoil or a quality raised-bed soil, 30 percent compost, and 10 percent aeration material like perlite or coarse sand. This gives drainage, fertility, and structure at once. The often-cited Mel's Mix from square-foot gardening uses equal thirds of compost, peat moss or coir, and vermiculite. Either works; the key is not filling a bed with pure compost, which compacts and stays too wet, or pure topsoil, which is heavy and drains poorly. Blend for balance.

Can you fill a raised bed with just topsoil?

Not on its own. Plain topsoil, especially bagged fill or native clay, compacts in a raised bed, drains poorly, and holds little nutrition, so plants struggle. Topsoil should be one component of the mix, not the whole thing. Combine it with compost for fertility and an aeration amendment like perlite or coarse sand for drainage. If all you have is topsoil, work in a generous amount of compost — at least a quarter to a third by volume — before planting.

How do you fill a raised bed cheaply?

For deep beds, fill the bottom third to half with free or low-cost bulky organic matter — logs, branches, leaves, straw, grass clippings, or cardboard — then top it with purchased soil and compost only where the roots grow. This technique, sometimes called hugelkultur or simply bottom-layering, can cut the volume of bought soil by a third or more. Buying soil in bulk by the cubic yard rather than in bags is also far cheaper for anything more than one small bed.

Should you put anything at the bottom of a raised bed?

It depends on depth. For beds deeper than about 12 to 18 inches, filling the bottom with bulky organic matter saves soil and adds long-term fertility as it breaks down. For shallow beds, fill the whole thing with good soil mix, since roots need it. If your bed sits on grass or weeds, lay cardboard on the bottom first to smother them. Over open ground, leave the bottom open so roots and worms can move between the bed and the earth below; add hardware cloth only if burrowing pests are a problem.

How much soil do you need to fill a raised bed?

Multiply length by width by depth in feet to get cubic feet, then divide by 27 to get cubic yards. A 4-by-8-foot bed that is 12 inches deep needs 32 cubic feet, about 1.2 cubic yards. Bagged soil is usually sold in 1.5 or 2 cubic-foot bags, so that bed would take roughly 16 to 21 bags — which is why bulk delivery is much cheaper for larger beds. Buy about 10 percent extra to allow for settling after the first few waterings.

How to Fill a Raised Garden Bed (Cheaply and Correctly)

Written & researched by Elena Marsh Gardening & Landscaping Editor

Updated February 2026

Wide environmental view of a person's hands pressing seedlings into dark soil in a timber raised bed, in bright midday sun

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To fill a raised garden bed, use a balanced soil blend where the roots grow, and save money on deep beds by layering bulky organic matter underneath. A reliable mix is roughly 60 percent topsoil or raised-bed soil, 30 percent compost, and 10 percent aeration material like perlite or coarse sand. For beds deeper than about 12 to 18 inches, fill the bottom third to half with logs, branches, leaves, or straw, then cap it with that soil mix. Avoid the two classic mistakes: filling with pure compost (too soggy) or pure topsoil (too dense).

Getting the fill right is the biggest single factor in how a raised bed performs, and it is where most of the cost sits. Below are the ratios, a cheap deep-fill method, and how to calculate exactly how much you need. Our product rankings draw on aggregated owner reviews and expert consensus, not first-hand testing — see how we rank.

Start with depth: it changes everything

How you fill a bed depends first on how deep it is, because depth decides whether you can economize at the bottom.

Bed depth	Approach
6 to 11 inches (shallow)	Fill entirely with quality soil mix; roots need all of it
12 to 18 inches (standard)	Bottom layer of bulky organics optional, top with soil mix
18+ inches (deep)	Bottom third to half bulky organics, top with soil mix

Most vegetables root in the top 6 to 12 inches, so a deep bed does not need premium soil all the way down. That is the key to filling a tall bed affordably. For guidance on how deep to build in the first place, see how deep should a raised bed be.

Overhead view of a person's hands pressing seedlings into dark soil in a timber raised bed, in soft overcast light

The soil mix that works

The growing layer is where you should not cut corners. A good raised-bed mix drains freely, holds moisture, and carries enough nutrition for a season of growth. Two proven recipes:

The 60/30/10 blend: 60 percent topsoil or bagged raised-bed soil, 30 percent compost, 10 percent aeration (perlite, coarse sand, or fine bark). Balanced and forgiving.
Mel's Mix (square-foot gardening): one-third compost, one-third peat moss or coconut coir, one-third coarse vermiculite. Light, excellent for shallow beds, but pricier and needs more feeding over time.

Whichever you choose, blend the components before or as you fill rather than leaving them in layers, so drainage and fertility are even throughout the root zone. For the full breakdown of ratios and why each ingredient matters, see the best soil mix ratio for a raised bed, and for ready-to-use options, best soil for a raised garden bed.

What not to fill it with

The wrong fill causes more raised-bed failures than pests or watering, so it is worth being clear about what to avoid.

Pure compost. Too rich and too water-retentive, it slumps into a dense, soggy mass that suffocates roots and can burn seedlings with excess nutrients.
Pure topsoil or fill dirt. Compacts hard, drains badly, and offers little nutrition. Bagged "topsoil" is often low quality.
Native clay alone. Turns to a brick in a contained bed; if you must use it, cut it heavily with compost and aeration.
Potting mix throughout a large bed. Fine for containers, but expensive and too light to fill a big bed by itself.

The goal is a middle ground: rich but well-drained, moisture-holding but never waterlogged.

Low-angle view of a person's hands pressing seedlings into dark soil in a timber raised bed, in bright midday sun

Filling a deep bed cheaply

Deep beds are expensive to fill with bought soil, but you rarely need soil at the bottom. Bulky organic matter fills the void, costs little or nothing, and slowly breaks down into rich material.

Suppress weeds first. If the bed sits on grass or weeds, lay plain cardboard on the ground to smother them.
Add coarse woody material. Logs, branches, and untreated wood chips go in the bottom, filling the lowest third to half of a deep bed (the hugelkultur approach).
Layer in lighter organics. Fall leaves, straw, grass clippings, and small twigs fill gaps around the wood.
Add a transition layer of compost or aged manure over the organics.
Cap with the soil mix. Fill the top 8 to 12 inches with your 60/30/10 blend, where the roots will actually grow.

Expect the bed to settle over the first season as the organic base decomposes; top it up with more soil and compost each spring. Other budget strategies are collected in the cheapest way to fill a raised bed.

Calculating how much you need

Buying the right amount avoids both wasted money and a half-filled bed. The math is simple.

Volume in cubic feet = length (ft) x width (ft) x depth (ft).
Cubic yards = cubic feet divided by 27 (bulk soil is sold by the yard).
Add about 10 percent for settling after watering.

Bed size	Depth	Volume needed
4 x 4 ft	12 in	16 cu ft (~0.6 cu yd)
4 x 8 ft	12 in	32 cu ft (~1.2 cu yd)
4 x 8 ft	18 in	48 cu ft (~1.8 cu yd)

For anything larger than one small bed, buying soil in bulk by the cubic yard is dramatically cheaper than bagged product, which typically comes in 1.5 or 2 cubic-foot bags.

Over-the-shoulder view of a person's hands pressing seedlings into dark soil in a timber raised bed, in soft early-morning light

Finishing and maintaining the bed

Once filled, a few steps set the bed up for the season and keep it productive year after year.

Water it in. A thorough first watering settles the mix and reveals low spots to top up.
Mulch the surface. A couple of inches of straw or shredded leaves conserves moisture and suppresses weeds — see should you mulch a raised garden bed.
Replenish each season. Beds lose volume as organic matter breaks down, so add an inch or two of compost every spring.
Feed as needed. Even a rich mix depletes over a season; side-dress hungry crops with compost or an organic fertilizer.

The bottom line

Fill the root zone with a balanced blend — about 60 percent topsoil, 30 percent compost, and 10 percent aeration — and save money on deep beds by layering free bulky organics underneath. Avoid the extremes of pure compost or pure topsoil, calculate your volume before buying, and top the bed up with compost each spring. Get the fill right and a raised bed rewards you with years of productive, low-maintenance growing.

Explore more at the raised beds and planters hub, including best soil for a raised garden bed and the best soil mix ratio.

How to Fill a Raised Garden Bed