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

Raised beds need soil that drains well, holds nutrients, and doesn't compact over time. Garden soil from your yard will compact and drain poorly in a raised bed because it lacks the air pockets that make soil workable. The standard approach is to combine three materials: topsoil as your base, compost for nutrients and structure, and coarse material like perlite or wood chips to keep the mix loose.

A common ratio is 40 percent topsoil, 40 percent compost, and 20 percent coarse material. You can adjust this depending on what you're growing and what you have on hand. Vegetables and herbs do well with this balance. If you're growing plants that need very fast drainage—like succulents or cacti—increase the coarse material to 30 or 40 percent. If you're growing moisture-loving plants like blueberries, reduce the coarse material and add more compost.

Key Takeaways

  • Mix topsoil, compost, and coarse material in roughly equal parts rather than using garden soil straight from your yard.
  • Topsoil provides structure and minerals; compost adds nutrients and helps retain moisture; coarse material keeps the mix from compacting.
  • A 40-40-20 ratio works for most vegetables, but you can shift the percentages based on what you're planting.
  • Avoid peat moss and garden soil alone, both of which compact and drain poorly in raised beds.

What each component does and where to find it

Topsoil is the dark upper layer of soil that contains organic matter and microorganisms. It's the foundation of your mix because it holds minerals plants need and gives the soil structure. Buy it by the cubic yard from landscape suppliers, garden centers, or online delivery services. Check that it's screened (free of rocks and debris) and hasn't been treated with herbicides. A raised bed 4 feet by 8 feet by 12 inches deep needs about 32 cubic feet, or roughly 1.2 cubic yards.

Compost is decomposed organic matter—leaves, grass, food scraps, or wood. It adds nitrogen and other nutrients, improves water retention, and feeds the microbes in your soil. You can make your own by composting kitchen scraps and yard waste, or buy it bagged or in bulk. Homemade compost is free but takes months to break down. Bagged compost from a garden center is ready to use when ready. Bulk compost from a landscape supplier is cheaper per volume than bags but requires delivery or a truck to haul it.

Coarse material keeps the mix from settling and compacting. Perlite (a volcanic glass that's been heated and expanded) is lightweight and doesn't break down, so it lasts for years. Coconut coir (shredded coconut husk) holds some water and breaks down slowly. Wood chips or aged bark work but will decompose over time and need topping up. Avoid fresh wood chips, which tie up nitrogen as they break down and can heat up as they decompose. Perlite is the most reliable choice for long-term drainage.

Why garden soil and peat moss don't work in raised beds

Garden soil is designed to work in the ground, where it's constantly being aerated by roots, worms, and weather. In a raised bed, it compacts under its own weight and the weight of water. Within a season or two, it becomes dense and hard to work, and drainage slows to a crawl. Peat moss was once common in raised bed mixes because it holds water and is cheap, but it compacts the same way and has become expensive and environmentally costly to harvest.

If you already have garden soil on hand, you can use it as part of your mix—up to 40 percent—but don't use it as your only ingredient. Mixing it with compost and coarse material will keep it loose enough to work.

How to calculate how much soil you need

Measure your raised bed in feet: length × width × depth (in feet). That gives you cubic feet. Divide by 27 to convert to cubic yards, which is how soil is usually sold.

A few common sizes: a 4×8×1 bed needs 1.2 cubic yards. A 4×4×1 bed needs 0.6 cubic yards. A 3×6×1.5 bed needs 0.8 cubic yards. Most suppliers have a 1-cubic-yard minimum, so if your bed is smaller than that, buy bagged soil instead. One 2-cubic-foot bag of topsoil or compost covers about 8 square feet at 3 inches deep.

Order slightly more than you calculate—about 10 percent extra—because soil settles after you fill the bed. You'll need that extra to top it up after the first watering and settling.

Filling your bed and what to expect in year two

Pour the topsoil in first, then the compost, then the coarse material. You don't need to layer them perfectly—just mix them together as you fill. Water the bed thoroughly after filling so the soil settles and compacts slightly. The soil level will drop an inch or two over the first few weeks.

In year two and beyond, the compost and coarse material will have broken down and settled. Add 2 to 3 inches of fresh compost to the top of the bed each spring. This replaces what decomposed and keeps the soil loose and nutrient-rich. You don't need to replace the entire bed unless it's been more than five or six years and the soil has become hard and depleted.

Budget options and what to skip

The cheapest approach is to make your own compost and buy topsoil and perlite in bulk. If you have access to free compost from a municipal composting program or a local farm, use it. Some cities give away finished compost free or at cost to residents.

Bagged soil mixes marketed as "raised bed soil" or "garden mix" are convenient but cost two to three times more per volume than buying components separately. They're worth it if your bed is small (under 20 cubic feet) or if you don't have access to bulk suppliers. Read the label to see what's in them—good ones list topsoil, compost, and perlite or coir. Avoid bags that list only peat moss or that don't specify ingredients.

Skip "garden soil in a bag" sold at big-box stores. It's usually heavy clay-based soil that will compact just like garden soil from your yard. Skip potting soil too—it's designed for containers and drains too fast for a raised bed.

Frequently Asked Questions

Can I use only compost in my raised bed?

No. Compost alone will compact and drain poorly over time. It also breaks down quickly, so you'd need to replace it every year. Use compost as part of a mix with topsoil and coarse material for better structure and longer life.

What if I can't find perlite or it's too expensive?

Coconut coir, aged wood chips, or even shredded leaves work as coarse material. They break down faster than perlite, so you'll need to add more compost each year, but they're cheaper or free. Avoid fresh wood chips, which can heat up and tie up nitrogen as they decompose.

Do I need to replace all the soil every year?

No. Add 2 to 3 inches of fresh compost to the top each spring to replace what broke down and to refresh nutrients. Replace the entire bed only if it's been five or more years and the soil has become hard and compacted.

Can I use leaves or grass clippings instead of buying compost?

Yes, if they're fully decomposed. Fresh leaves and grass will heat up and break down in the bed, which can damage roots and tie up nitrogen. Shred leaves and let them sit for several months, or use them as a mulch layer on top of finished compost rather than mixing them in.

What's the difference between topsoil and potting soil?

Topsoil is mineral-based soil meant for outdoor beds and holds nutrients well but can compact. Potting soil is a lightweight mix of peat, coir, or bark designed for containers and drains very fast. Potting soil is too expensive and drains too quickly for a raised bed. Use topsoil as your base instead.