Use a mix of topsoil, compost, and coarse materials rather than garden soil alone

The soil you put in a raised bed should drain well, hold nutrients, and give plant roots room to grow down. Garden soil from your yard compacts too much and holds water in a way that rots roots. Instead, fill your bed with a blend: roughly 40% topsoil, 40% compost, and 20% coarse material like perlite, vermiculite, or shredded bark. This combination drains fast enough that water doesn't pool, retains enough moisture that you're not watering constantly, and breaks down over time to feed your plants.

The exact ratio matters less than the principle: you want something lighter and airier than straight garden soil, with enough organic matter to hold nutrients and feed microbes. A bed filled this way will settle over the first season—expect it to drop 2 to 4 inches—so fill it slightly higher than the rim when you first build it.

Key Takeaways

  • A 40-40-20 blend of topsoil, compost, and coarse material drains well and holds nutrients better than garden soil alone.
  • Garden soil compacts and holds water too much for raised beds; coarse materials like perlite or bark keep the mix loose.
  • Compost breaks down over time and feeds your plants, so you'll need to add more each year as it settles.
  • Fill your bed slightly higher than the rim because soil will settle 2 to 4 inches in the first season.
  • Bagged mixes labeled "raised bed soil" or "garden mix" can work if you check the label for compost and coarse material content.

Why topsoil, compost, and coarse material work together

Topsoil is the dark, nutrient-rich layer from the top few inches of earth. It holds some water and nutrients without compacting as much as subsoil. Buy it by the cubic yard from a landscape supplier or in bags from a garden center. Topsoil alone is too dense for a raised bed, but it forms the base that gives your plants something to anchor into.

Compost is decomposed plant matter—leaves, grass clippings, food scraps, wood chips—that has broken down into dark, crumbly material. It holds water and nutrients, feeds the microbes that make soil alive, and continues to break down as your plants grow. You can buy bagged compost or get it in bulk from a landscape supplier. The more compost you use, the richer your soil will be for the first season, but it will also settle faster.

Coarse material keeps the whole mix from compacting. Perlite is a volcanic glass that's been heated until it puffs up—it's light, holds air pockets, and doesn't break down. Vermiculite is similar but holds more water, so use it if your climate is dry. Shredded bark or wood chips work too and add a little organic matter as they decay. Without this layer, water sits in the soil instead of draining through.

How to calculate how much soil you need

Measure your raised bed's length, width, and depth in feet. Multiply all three numbers together to get cubic feet. Then divide by 27 to convert to cubic yards—that's the unit landscape suppliers use. For example, a bed that is 4 feet long, 2 feet wide, and 12 inches deep (1 foot) is 8 cubic feet, or roughly 0.3 cubic yards.

Most garden centers sell soil in bags labeled by volume—usually 2 cubic feet or 3 cubic feet per bag. A 4-by-8-foot bed that is 12 inches deep needs about 32 cubic feet of soil, or roughly 16 bags of 2-cubic-foot soil. Buying in bulk from a landscape supplier is cheaper per cubic foot if you need more than a few beds, and they can deliver it.

When you order, ask the supplier to deliver a mix or order the three materials separately and blend them yourself in a pile. If you blend it yourself, spread each layer in the bed and mix it with a shovel or garden fork before planting.

Bagged mixes and when they make sense

Bags labeled "raised bed soil" or "garden mix" are pre-blended and ready to use. They cost more per cubic foot than buying materials separately, but they save time and the mess of mixing. Read the label to see what's inside—look for topsoil, compost, and something coarse like perlite or bark. If the label lists only topsoil and compost with no coarse material, the mix will compact over time.

Bagged mixes work well for small beds or if you're filling just one or two. For larger projects, bulk soil is cheaper. Some garden centers will let you return unused bags if you keep the receipt, so ask before you buy more than you think you need.

What to avoid and why

Do not use pure garden soil or clay from your yard. It compacts under the weight of water and plant roots, squeezes out air, and creates a hard pan that roots can't penetrate. Do not use potting soil—it's designed for containers and drains so fast that it dries out in a raised bed. Do not use sand unless you're mixing it with compost and topsoil; sand alone drains too fast and holds no nutrients.

Avoid "topsoil" that looks like pure clay or is very heavy and dense. Good topsoil should crumble in your hand and have visible bits of organic matter. If you're buying in bulk, ask the supplier what the material looks like or visit the yard to see it before you order.

Refreshing and maintaining your soil over time

After the first season, your soil will have settled and the compost will have broken down. Add 2 to 3 inches of fresh compost on top each spring before you plant. This replaces what settled and decomposed, feeds your plants for the season, and keeps the soil loose. You don't need to replace the whole bed every year—just top it off.

If your bed is more than three or four years old and the soil feels hard or compacted, dig out the top half, mix it with fresh compost, and put it back. This is cheaper and faster than starting over. Over time, the organic matter in your soil will keep improving as long as you add compost each year.

Frequently Asked Questions

Can I use only compost in a raised bed?

Compost alone will compact and hold too much water. Mix it with topsoil for structure and coarse material for drainage. A bed of pure compost will also settle much faster and need refilling sooner.

What if I can't find perlite or vermiculite?

Shredded bark, wood chips, or even straw work as coarse material. They break down over time and add organic matter, so you'll need to replace them more often than perlite, but they do the job. Avoid treated wood chips or bark from diseased trees.

Should I put cardboard or landscape fabric on the bottom of my bed?

Cardboard blocks weeds from below and breaks down over a season or two, adding organic matter. Landscape fabric lasts longer but doesn't decompose. Either one works; cardboard is cheaper and better for soil health, but fabric is easier if you're on a slope or dealing with aggressive weeds.

Can I reuse soil from last year's bed?

Yes. Mix it with fresh compost and coarse material to loosen it up and add nutrients. Reused soil that's been in a bed for several years will be darker and richer than new soil, so you may need less compost the second time around.

How deep should the soil be?

Most vegetables need 12 inches of soil. Root crops like carrots and potatoes need 14 to 18 inches. Shallow-rooted plants like lettuce and herbs can grow in 8 inches. Build your bed to match what you plan to grow, or go with 12 inches as a middle ground.