The basic formula: soil, compost, and drainage

A raised bed works best when you fill it with a mix of materials, not just plain garden soil. The standard approach is to build layers from bottom to top: drainage material at the base, then a mix of compost and topsoil that makes up most of the depth. This layered approach lets water drain away from plant roots while keeping nutrients and moisture where plants can reach them.

The exact ratio depends on what you're growing and what materials you have on hand, but a common starting point is equal parts compost and topsoil, with an optional drainage layer underneath. You don't need to buy expensive "raised bed soil" mixes—you can assemble the same thing for less by buying components separately and mixing them yourself.

Key Takeaways

  • Start with 2 to 4 inches of coarse drainage material (wood chips, gravel, or cardboard) at the bottom to prevent waterlogging.
  • Fill the rest with a mix of topsoil and compost in roughly equal amounts, aiming for a total depth of 10 to 12 inches for most vegetables.
  • Compost breaks down over time, so plan to add 1 to 2 inches of fresh compost each spring to maintain soil quality.
  • Avoid using pure compost alone, which can compact and hold too much water; topsoil provides structure and drainage.
  • If you're starting with poor native soil underneath, a drainage layer keeps roots from sinking into clay or compacted ground.

Drainage layer: what goes on the bottom and why

The bottom layer prevents water from pooling around plant roots, which causes rot and fungal disease. This layer doesn't need to be thick—2 to 4 inches is enough. You have several options depending on what's available and affordable in your area.

Wood chips or coarse bark are common choices because they're often free or cheap from tree services and they break down slowly. Avoid treated wood or wood from diseased trees. Gravel or pebbles work well and last indefinitely, though they're heavier to move and don't add any organic matter as they decompose. Cardboard (flattened boxes, no tape or ink) is free if you have it on hand and serves double duty: it blocks weeds from below while it breaks down over a season or two. Some gardeners skip the drainage layer entirely if their raised bed sits on well-draining ground, but it's cheap insurance if you're unsure about the soil underneath.

Topsoil: the structural base of your mix

Topsoil is the dark, crumbly layer from the top few inches of undisturbed ground. It contains some organic matter and microorganisms, and it holds its structure well—it won't compact into a brick-hard mass the way pure compost can. For a raised bed, you want topsoil that's free of clay clumps and weed seeds. Bagged topsoil from a garden center is screened and usually reliable; bulk topsoil from a landscape supplier is cheaper per cubic yard but quality varies, so ask to see a sample or read reviews from local gardeners first.

Topsoil alone isn't rich enough in nutrients for vegetables, which is why you mix it with compost. A 50-50 blend of topsoil and compost gives you the structure of soil with the nutrient density of compost. If your topsoil looks very sandy or very clay-heavy, adjust the ratio—add more compost to sandy soil to help it hold moisture, or add more topsoil to very rich compost to improve drainage.

Compost: the nutrient engine

Compost is decomposed organic matter—leaves, grass clippings, kitchen scraps, and plant waste broken down by microbes and time. It's dark, crumbly, and rich in the nutrients plants need. You can buy bagged compost at any garden center, or buy it in bulk from a landscape supplier. Homemade compost is free if you have a compost bin or pile, though it takes 6 to 12 months to finish.

Don't use pure compost as your entire fill. Compost compacts over time and can hold too much water, which suffocates roots. Mixed with topsoil at a 1:1 ratio, it provides nutrients without those drawbacks. As your bed settles over the first season, the compost breaks down and the level drops—this is normal. Each spring, add 1 to 2 inches of fresh compost on top to replenish nutrients and bring the level back up.

How much material you need: calculating cubic feet

Raised beds are usually 4 feet by 8 feet or 4 feet by 4 feet, and 10 to 12 inches deep is a good working depth for most vegetables. To find how much soil to buy, multiply length × width × depth (in feet) to get cubic feet, then divide by 27 to convert to cubic yards if the supplier sells by the yard.

For example, a 4-by-8-foot bed that's 1 foot deep needs 32 cubic feet, or about 1.2 cubic yards. If you're using a drainage layer, subtract that depth first—so if you're putting down 3 inches of wood chips, you're really filling 9 inches of soil mix, which is 24 cubic feet or 0.9 cubic yards. Most bagged soil comes in 2-cubic-foot bags, so you'd need about 12 bags for that bed without the drainage layer. Bulk suppliers usually have a 5-cubic-yard minimum, so if you're filling multiple beds, bulk is cheaper; if you're filling one bed, bags may be more practical.

Amendments for specific growing situations

If you're growing acid-loving plants like blueberries, mix in peat moss or sulfur to lower the pH. If your water drains very slowly and you suspect clay underneath, add extra compost or perlite to the mix to improve drainage. If you're growing in a very hot, dry climate, add more compost to help the soil hold moisture longer.

For most vegetable gardens in most climates, the basic topsoil-and-compost mix needs no amendments. Avoid adding perlite, vermiculite, or other commercial soil conditioners unless you have a specific reason—they're expensive and the compost does the same job. If you're concerned about heavy metals in urban soil, buying bagged topsoil and compost from a reputable supplier is safer than using native soil.

What not to use and why

Don't fill a raised bed with native garden soil from your yard unless it's already dark, crumbly, and free of clay. Compacted yard soil will compact further in a raised bed and won't drain well. Don't use sawdust, which can tie up nitrogen as it breaks down and can contain chemicals if it's from treated wood. Don't use mushroom compost if you're growing acid-loving plants—it's alkaline. Don't use manure unless it's fully aged (at least 6 months old); fresh manure can burn plant roots and may contain pathogens.

Avoid "garden soil" products sold in bags at big-box stores—they're often heavy clay mixes that compact quickly. Bagged "potting soil" or "seed-starting mix" is too light and expensive for a raised bed; it's meant for containers and pots. If you see a mix labeled "raised bed soil," check the ingredients—it's usually just topsoil and compost at a markup.

Settling and maintenance over time

Your new raised bed will settle by 10 to 20 percent in the first year as the compost breaks down and the soil compacts slightly. This is normal. Fill the bed to the top when you first build it, and expect it to drop a few inches by the end of the growing season. Don't overfill to compensate—you'll end up with soil spilling over the edges.

Each spring, add a 1- to 2-inch layer of fresh compost on top. This replaces what broke down the previous year and keeps nutrient levels high. After 3 to 5 years, you may want to replace the bottom third of the soil if it's become very compacted or depleted, though many gardeners just keep adding compost on top indefinitely. If you notice drainage problems developing—water pooling on the surface or roots rotting—it's time to refresh the mix or check whether the drainage layer has become clogged.

Frequently Asked Questions

Can I use just compost without topsoil?

You can, but it's not ideal. Pure compost compacts over time and can hold too much water, which leads to root rot and fungal problems. A 50-50 mix with topsoil gives you better drainage and structure while keeping the nutrients. If you use pure compost, plan to replace it more often.

Do I need a drainage layer if my raised bed is on concrete or a patio?

Yes, especially on concrete. Water has nowhere to go, so a drainage layer prevents it from pooling around roots. Use gravel or wood chips so water can move through it and drain out the sides of the bed.

What's the difference between topsoil and garden soil?

Topsoil is the natural top layer of undisturbed ground and contains some organic matter. Garden soil is usually topsoil mixed with compost or other amendments, making it richer but also more expensive. For a raised bed, buy topsoil and compost separately and mix them yourself—it's cheaper and you control the ratio.

How deep should a raised bed be?

Ten to 12 inches is ideal for most vegetables. Shallow-rooted plants like lettuce and radishes can grow in 6 to 8 inches, but root vegetables like carrots and parsnips need at least 12 inches. Deeper is fine—it just means more soil to buy and more weight on the frame.

Can I reuse soil from last year's raised bed?

Yes. Remove the top 2 to 3 inches (which may be compacted or depleted), then mix the rest with fresh compost at a 1:1 ratio. This saves money and is fine as long as you didn't have serious disease problems the previous year. If you had root rot or fungal issues, replace the soil entirely or let it sit unused for a season.