The basic formula: soil, compost, and drainage material

A raised bed works best when filled with a layered mix rather than garden soil alone. The standard approach is to start with a drainage layer at the bottom, add a base of coarser material, then top it with a blend of topsoil and compost. This layering lets water drain away from plant roots while keeping nutrients and moisture where plants can use them.

The exact ratio depends on what you're growing and what materials you have on hand, but a common starting point is 40% topsoil, 40% compost, and 20% other amendments like peat moss, coconut coir, or aged bark. You don't need to hit those numbers exactly—the goal is a mix that drains well, holds some moisture, and feeds your plants as they grow.

Key Takeaways

  • Start with 2 to 4 inches of drainage material (gravel, wood chips, or cardboard) at the bottom to prevent waterlogging.
  • Fill the rest with a blend of topsoil and compost, aiming for roughly equal parts of each, adjusted based on what you're planting.
  • Avoid pure garden soil or clay, which compacts and drains poorly in a raised bed.
  • You can layer materials as you build the bed or mix them together before filling; both methods work.
  • Plan to add 1 to 2 inches of fresh compost each spring to replace what settles and what plants consume.

Drainage layer: what goes on the bottom

The bottom layer prevents water from pooling around roots and keeps the bed from becoming a swamp after rain. Use materials that won't break down quickly: gravel, pea gravel, river rock, or coarse wood chips work well. Aim for 2 to 4 inches depending on how wet your site tends to be—wetter sites need more drainage material.

Cardboard or newspaper also works as a bottom layer, especially if you're building the bed directly on grass or weeds. Lay it down first, then add your drainage material on top. The cardboard blocks weeds from growing up into the bed and breaks down over time, adding organic matter to the soil below.

If your raised bed sits on a hard surface like concrete or compacted clay, you can skip the drainage layer entirely—water will run off the bottom anyway. If the bed is on bare ground, the drainage layer is worth the effort.

Topsoil: the foundation of your growing mix

Topsoil is the dark, nutrient-rich layer from the top of the ground. It holds moisture and provides structure for roots to grow into. Buy it by the cubic yard from a landscape supply company or garden center, or ask if they deliver—bags are expensive for the volume you need to fill a bed.

Look for topsoil that's dark and crumbly, not compacted or clay-heavy. Ask the supplier what it contains: good topsoil is mostly soil with some organic matter mixed in, not pure clay or sand. If you're filling a 4-by-8-foot bed that's 12 inches deep, you'll need roughly 32 cubic feet, or about 1.2 cubic yards.

Garden soil from your yard can work in a pinch, but it often compacts in a raised bed and drains poorly. If you use it, mix it with compost and other amendments to loosen it up. Pure clay or heavy soil will frustrate you—spend the money on decent topsoil instead.

Compost: the nutrient engine

Compost is decomposed plant and animal matter. It feeds your plants, improves drainage, and helps soil hold moisture. Use finished compost—dark, crumbly, and earthy-smelling—not fresh or partially decomposed material. Finished compost is available bagged at any garden center or by the cubic yard from landscape suppliers.

You can use any type: mushroom compost, leaf compost, kitchen-scrap compost, or blended compost. Each has slightly different nutrients and texture, but all work. If you make your own compost, use it when it's fully broken down and no longer hot to the touch.

Mix compost with topsoil rather than using it alone. A bed filled with pure compost will settle significantly over the first year and drain too quickly. A 50-50 blend of topsoil and compost is a solid starting point; adjust toward more compost if your soil is heavy clay, or more topsoil if you want better structure.

Optional amendments: peat moss, coconut coir, and bark

These materials improve drainage and water retention without adding heavy nutrients. Peat moss is harvested from bogs and holds water well but is slow to rewet once it dries out. Coconut coir (coconut husk fiber) does the same job and rewets more easily, though it costs more. Aged bark or wood chips improve drainage and add structure as they break down.

Use these as 10 to 20% of your mix, not the main ingredient. They're useful if your topsoil is heavy or if you're growing plants that like loose, well-draining soil (like herbs or root vegetables). For most vegetable beds, a topsoil-and-compost blend is enough without them.

Avoid fresh wood chips or bark—they tie up nitrogen as they decompose and can slow plant growth. Use aged material that's been sitting for at least a year, or buy bagged products labeled as aged or composted.

What not to use: materials that cause problems

Don't fill a raised bed with pure garden soil, clay, or subsoil (the pale layer below topsoil). These compact in a raised bed and drain poorly, leaving roots sitting in water. Don't use fresh manure, which can burn plants and harbor pathogens—use aged manure (at least 6 months old) mixed into compost instead.

Avoid treated wood chips or sawdust, which may contain pesticides or herbicides. Don't use diseased plant material or weeds with seeds—they'll spread problems through your bed. Skip landscape fabric or plastic liners at the bottom; they trap water and prevent drainage.

Don't buy "garden mix" or "potting soil" by the cubic yard to fill a raised bed—it's expensive and designed for containers, not ground beds. Stick with topsoil, compost, and amendments from a landscape supplier.

Layering versus mixing: which method to use

You can build a raised bed two ways. The layering method means adding materials in distinct bands: drainage layer, then coarser material, then topsoil, then compost on top. This works well if you're building the bed in stages or want to see what you're using. The materials will gradually mix as you water and as roots grow through them.

The mixing method means blending topsoil and compost together (and any amendments) before filling the bed. This takes more work upfront but gives you a uniform growing medium from day one. If you're filling a large bed, mixing in a pile or a wheelbarrow before filling saves time and effort.

Either method works. Choose based on what's practical for your situation. If materials are delivered separately, layering makes sense. If you can blend them on-site, mixing gives faster, more consistent results.

How much to buy and how to calculate volume

Measure your bed's length, width, and depth in feet, then multiply them together to get cubic feet. Divide by 27 to convert to cubic yards. A 4-by-8-foot bed that's 12 inches (1 foot) deep is 32 cubic feet, or about 1.2 cubic yards. A 4-by-4-foot bed that's 12 inches deep is 16 cubic feet, or about 0.6 cubic yards.

Order slightly more than you calculate—material settles, and you'll want extra to top up the bed in future years. For a 1.2-cubic-yard bed, order 1.5 cubic yards. Suppliers often have minimums (usually 0.5 to 1 cubic yard), so small beds may be cheaper to fill with bags.

Budget for replacement: a raised bed loses 1 to 2 inches of height each year as material breaks down and compacts. Plan to add 1 to 2 inches of fresh compost each spring to keep the bed at working depth.

Frequently Asked Questions

Can I reuse soil from an old raised bed?

Yes. Sift out any debris, mix in fresh compost (about 25% of the volume), and refill. Old soil loses nutrients and structure over time, so adding compost restores it. If the old bed had disease problems, replace at least the top 6 inches with fresh material.

What's the difference between topsoil and garden soil?

Topsoil is the natural top layer of earth, usually darker and richer. Garden soil is topsoil mixed with compost and amendments, sold ready to use. Both work in raised beds, but garden soil is more expensive and often unnecessary if you're already adding compost.

Do I need to add fertilizer if I'm using compost?

Not at planting time. Compost releases nutrients slowly as it breaks down. After the first season, add 1 to 2 inches of fresh compost each spring, or use a balanced fertilizer mid-season if plants look pale or slow-growing. Heavy feeders like tomatoes may need extra nitrogen.

How deep should a raised bed be?

Twelve inches is the minimum for most vegetables and herbs. Sixteen to 18 inches is better for root crops like carrots and potatoes. Shallow beds (6 to 8 inches) work for lettuce, spinach, and other greens. Deeper beds hold more moisture and give roots more room, so go deeper if you can.

Can I use leaves or grass clippings in a raised bed?

Shredded leaves work well mixed into compost or as a mulch layer on top. Fresh grass clippings can mat down and smell bad—use them only if they're from an untreated lawn, and mix them with other material. Aged or composted grass clippings are safer and break down faster.