The basic fill: soil, compost, and drainage

A raised bed needs three things to work: something to drain water through the bottom, a middle layer that holds nutrients, and a top layer where plants actually root. The simplest approach is to fill the whole bed with a mixture—usually topsoil blended with compost—but many gardeners layer different materials to save money and improve drainage.

The depth matters. Most vegetables and flowers need 8 to 12 inches of soil to root properly. If your bed is deeper than that, you can use cheaper filler material underneath—wood chips, straw, or even crumpled cardboard—and reserve the good soil for the top 8 to 12 inches where roots actually go.

Key Takeaways

  • A raised bed needs drainage at the bottom, a nutrient-rich middle, and 8 to 12 inches of rooting soil at the top, so you can layer cheaper materials underneath if your bed is deep.
  • A 50-50 mix of topsoil and compost works for most vegetables and flowers, though you can adjust the ratio depending on what you are growing and what your soil naturally contains.
  • Wood chips, straw, or cardboard in the bottom 6 to 8 inches saves money and improves drainage without harming plant growth, as long as the top layer is proper soil.
  • Avoid filling with only compost, only peat moss, or only wood chips—each alone drains too fast, holds too much water, or breaks down too quickly to support a full season.
  • The cost per cubic foot varies widely by region and supplier, so getting a quote from a local landscape supply yard is faster than calculating volume yourself.

Topsoil and compost: the standard mix

The most common fill is a blend of topsoil and compost in roughly equal parts. Topsoil is the dark, crumbly layer from the top of the ground—it holds nutrients and water but can compact over time. Compost is decomposed plant material that improves drainage, feeds soil organisms, and breaks down slowly to release nutrients through the season.

A 50-50 topsoil-to-compost ratio works for most vegetables, herbs, and ornamental flowers. If you are growing root crops like carrots or parsnips, you can use more topsoil and less compost because those plants need looser, less nutrient-dense soil. If you are growing heavy feeders like tomatoes or squash, increase the compost to 60 percent and topsoil to 40 percent.

You can buy both by the bag or by the cubic yard. A cubic yard covers a 4-by-8-foot bed 3 inches deep, or a 3-by-3-foot bed 12 inches deep. Most landscape supply yards will deliver, and the cost per cubic yard varies by region—ask for a quote rather than guessing.

Layering to save money and improve drainage

If your raised bed is 18 inches or deeper, layering different materials lets you use less expensive filler in the bottom and reserve good soil for where roots actually grow. Start with 6 to 8 inches of coarse drainage material—wood chips, straw, or even crumpled cardboard—then top it with 8 to 12 inches of your topsoil-compost mix.

Wood chips break down slowly and create air pockets that improve drainage. Straw (not hay, which contains seeds) does the same and adds organic matter as it decomposes. Cardboard works too—it blocks weeds from below and eventually becomes soil. Avoid treated wood chips or anything that has been chemically treated, as those can leach into your soil.

This layering approach works because plant roots do not go deeper than about 12 inches for most vegetables and flowers. The bottom layer is mainly there to prevent the bed from sitting in water and to give the soil something to rest on. As the bottom layer breaks down over two to three years, it becomes part of your soil, so you will need to add fresh topsoil and compost to the top each spring.

What not to use alone

Compost by itself drains too fast and can be too rich for young seedlings—it also settles heavily as it breaks down, leaving you with a half-empty bed by midsummer. Peat moss holds water too aggressively and is expensive; it is better mixed into compost than used as a main fill. Wood chips alone do not hold enough water or nutrients for most plants, and they tie up nitrogen as they decompose, which can starve plants early in the season.

Sawdust compacts into a dense, airless layer and should never be used as a main fill. Mulch (the shredded bark sold for landscaping) is meant to sit on top of soil, not fill a bed—it breaks down too fast and does not provide the structure soil needs. Sand alone drains so fast that water runs straight through without plants being able to use it.

Filling a new bed: the step-by-step process

Start by laying cardboard or newspaper on the ground inside the bed frame to block weeds from below. Wet it so it stays in place. If you are layering, spread your coarse material (wood chips, straw, or more cardboard) 6 to 8 inches deep on top of that.

Pour in your topsoil-compost mix until the bed is full, or until you have 8 to 12 inches of good soil if you are layering. Rake it level and water it thoroughly—new soil is often dry and will settle as it absorbs water. Let it sit for a day or two, then add more soil if needed to bring it back to the top of the frame.

Do not plant when ready if you used fresh compost or fresh manure. Let the bed sit for at least a week so the soil can settle and the microbial activity can calm down. If you are planting seeds rather than transplants, wait two weeks.

Refreshing soil year to year

After the first season, your soil will have settled and broken down. Each spring, add 2 to 3 inches of fresh compost or a topsoil-compost blend to the top. This replaces what has decomposed and adds back nutrients that plants used. After three to four years, you may need to replace the entire top layer if the soil has become compacted or depleted.

If you layered the bed with wood chips or straw underneath, that material will gradually become soil as it breaks down. By year three or four, you can straightforward add fresh topsoil-compost to the top without worrying about the layers—they will have merged into a single, darker, richer layer.

Frequently Asked Questions

How much soil do I need for my raised bed?

Measure the length, width, and depth of your bed in feet, then multiply them together to get cubic feet. Divide by 27 to convert to cubic yards. For example, a 4-by-8-foot bed that is 12 inches deep is 32 cubic feet, or about 1.2 cubic yards. Most suppliers sell by the cubic yard, so round up.

Can I use garden soil from my yard instead of buying topsoil?

Yes, if your yard soil is dark, crumbly, and not compacted. If it is clay-heavy, sandy, or full of rocks, mix it 50-50 with compost to improve it. Avoid soil from areas where you have had disease or heavy pest problems, as those can carry over into the new bed.

What if I want to use only compost?

You can, but it will drain too fast and settle heavily by mid-season, leaving you with a half-full bed. Mix it at least 50-50 with topsoil for better results. Pure compost works better as a top dressing or amendment than as a main fill.

Do I need to add fertilizer to the soil mix?

Not right away. Good compost and topsoil contain enough nutrients for the first season. After that, add compost each spring or use a balanced fertilizer during the growing season depending on what you are growing. Heavy feeders like tomatoes may need extra nitrogen mid-season.

How long does it take for layered materials to break down?

Wood chips take two to three years to fully decompose, straw takes one to two years, and cardboard takes six months to a year. As they break down, they become part of your soil, so you will see the bed settle and darken over time. This is normal and expected.