The simple ratio that works
Container and in-ground soil pull in opposite directions. Potting mixes are light and fast-draining for pots that dry quickly; native topsoil is dense and holds water but compacts under its own weight. A raised bed sits between the two, so the best mix blends both ideas: enough mineral body to hold moisture and nutrients, enough organic matter to feed plants, and enough coarse material to keep air in the root zone.
A 50 / 30 / 20 blend by volume hits that balance:
You do not need to be exact. Anywhere from 40 to 60% topsoil and 25 to 35% compost works fine. The mistake to avoid is too much compost: beds that are mostly compost sink dramatically as it breaks down and can hold too much water early in the season.
How to fill a raised bed without overspending
Soil volume adds up fast, and a deep bed can cost more to fill than it cost to build. Two strategies keep it reasonable.
Filling a bed economically
Calculate the volume
Length times width times depth gives cubic feet. Order topsoil and compost in bulk by the cubic yard if you need more than a few bags, since bulk is far cheaper.
Fill the bottom with bulk material
For beds deeper than about 12 inches, the bottom third can be cheaper fill: coarse wood chips, small branches, or a layer of leaves. They break down slowly and reduce how much premium mix you buy.
Reserve the good mix for the root zone
Most vegetable roots live in the top 8 to 12 inches, so put your blended mix there where it matters.
Water and settle before planting
Fill, water deeply, and let it settle a day. Top up the inevitable sinking before you plant.
Bagged options for a small bed
If you are filling one small bed, mixing components by the bag can cost as much as buying a purpose-blended product, and bagged is far less work. A high-quality bagged mix already contains compost, aged forest products, and aeration, so you can fill, water, and plant the same day.
This kind of rich, pre-amended mix is on the light and fertile side, so for a raised bed we suggest cutting it with about a third topsoil to add body and reduce cost. On its own it works, but it dries faster and settles more than a blended bed. For a deeper comparison of bagged products, see our roundup of the best organic potting soil.
If you are still choosing the bed itself, our guide to the best raised garden beds covers metal, wood, and depth tradeoffs.
Keeping raised bed soil healthy over time
A raised bed is not "fill once and forget." Every harvest exports nutrients, and compost breaks down and settles, so the level drops a few inches each year.
Each season, top up with an inch or two of compost and a balanced organic fertilizer worked into the surface. This is also when a slow-release organic feed earns its place, since raised beds drain freely and lose soluble nutrients faster than heavy ground.
Mulch the surface to slow evaporation, and avoid walking on or compacting the soil. One of the main points of a raised bed is that the soil never gets stepped on, which keeps it loose without tilling. That makes raised beds a natural fit for no-dig gardening, where you build fertility by layering compost on top instead of digging it in.
What not to use
Skip pure peat moss as a base: it is acidic, hard to re-wet once dry, and not a sustainable choice. Skip straight builder's sand, which can cement into a hard layer when mixed with fine soil. And be cautious with free "topsoil" from unknown sources, since it can carry weed seeds, herbicide residue, or construction debris. When in doubt, pay for screened topsoil from a landscape supplier.
What is the best soil mix ratio for raised beds?
A widely recommended starting point is about 50% topsoil, 30% finished compost, and 20% aeration material like perlite or pine bark fines, by volume. This balances moisture retention, nutrients, and drainage. Adjust within roughly 40 to 60% topsoil and 25 to 35% compost depending on what you can source.
Can I use potting soil in a raised bed?
You can, but on its own it is light, dries quickly, and costs more than necessary for a large bed. Potting mix is engineered for containers. For a raised bed, cut it with about a third topsoil to add body, or reserve it for small beds where convenience outweighs cost.
How deep should the soil be in a raised bed?
Most vegetables are happy with 8 to 12 inches of good soil, since that is where the bulk of their roots grow. Root crops like carrots and deep feeders like tomatoes prefer the deeper end. You can fill beds deeper than 12 inches with cheaper bulk material at the bottom.
Do I need to replace raised bed soil every year?
No. You refresh it, not replace it. Each season top up with an inch or two of compost plus a balanced organic fertilizer, and add aeration material if the soil feels dense. Full replacement is only needed if the soil becomes contaminated or severely diseased.
Get the blend roughly right at the start, top it up with compost each year, and a raised bed will improve season after season. The ratio is forgiving; the only real mistakes are skimping on compost over time or trying to grow in straight topsoil or straight potting mix.