The basic fill is topsoil, compost, and drainage material in layers

A raised bed needs three things: something to drain water away from plant roots, something to hold nutrients, and something to anchor it all. Most gardeners use a layering system that starts with coarse material at the bottom, moves to a mix of topsoil and compost in the middle, and finishes with finished compost or topsoil on top. You do not need to buy expensive "raised bed soil" mixes—you can build the same thing for less by buying materials separately and stacking them yourself.

The exact ratio depends on what you are growing and what you have on hand. Vegetables and herbs want a richer mix with more compost. Perennials and shrubs tolerate leaner soil. If you are filling a bed that is 4 feet by 8 feet by 12 inches deep, you are working with about 32 cubic feet of space, which translates to roughly 1.2 cubic yards—a useful number to know when you are pricing materials at a garden center or landscape supplier.

Key Takeaways

  • Layer coarse material (wood chips, gravel, or straw) at the bottom for drainage, then add a mix of topsoil and compost in the middle, and finish with 2 to 4 inches of finished compost or quality topsoil on top.
  • Topsoil and compost are not the same thing—topsoil is mineral soil that holds water and nutrients, while compost is decomposed organic matter that improves structure and feeds plants slowly.
  • Buying materials separately and mixing them yourself costs less than pre-made raised bed soil mixes, though you will need to calculate volume in cubic feet or cubic yards before ordering.
  • Wood chips, straw, and shredded leaves all work as bottom drainage layers, but avoid treated wood chips or anything that has been sprayed with herbicide.
  • If you are filling a very deep bed or want to save money, you can use cardboard, newspaper, or old leaves as the bottom layer instead of buying drainage material.

Bottom layer: drainage material keeps roots from sitting in water

The bottom 2 to 4 inches should be something chunky that lets water pass through without compacting. Wood chips work well—use untreated chips from a tree service or landscape supplier, not the dyed mulch sold for decoration. Coarse gravel, perlite, or even crumpled newspaper also work. The goal is to create air space so water drains down and roots do not rot.

If cost is tight, you can skip this layer entirely and use cardboard or several layers of newspaper instead. Lay it flat on the ground under your bed frame. It will eventually decompose, but it buys you a season or two of drainage while the cardboard breaks down. This is especially useful if you are filling a bed that is deeper than 12 inches, because the bottom layer does not need to be soil at all—it just needs to not be clay or compacted earth.

Do not use treated wood chips, pressure-treated sawdust, or anything that has been sprayed with herbicide. These can leach chemicals into your soil. If you are unsure whether chips are treated, ask the supplier directly or choose a different material.

Middle layer: topsoil and compost mixed together

Once drainage material is down, fill the middle 6 to 8 inches with a blend of topsoil and compost. A common ratio is 60 percent topsoil and 40 percent compost, though you can adjust this based on what you are growing. Vegetables and herbs want more compost (up to 50 percent). Shrubs and perennials do fine with less (down to 25 percent).

Topsoil is mineral soil—sand, silt, and clay—that holds water and nutrients. It is the dark brown material you see in a healthy garden. Compost is decomposed organic matter (leaves, grass, food scraps) that improves soil structure, feeds plants slowly, and helps soil hold water without becoming waterlogged. They do different jobs, so mixing them gives you both benefits.

When you order topsoil and compost, ask the supplier whether it is screened (free of rocks and debris) and whether it has been tested for contaminants. Screened material is easier to work with and less likely to clog drip lines. If you are buying from a landscape supplier rather than a garden center, you will usually pay less per cubic yard, though you may need to order a minimum amount (often 3 to 5 cubic yards).

Top layer: finished compost or quality topsoil for planting

The top 2 to 4 inches should be your finest material—finished compost or premium topsoil that is free of sticks, stones, and weed seeds. This is where your plants' roots will spend the most time, so this layer matters more than the middle one. Finished compost is darker, crumbly, and smells earthy. If your compost still smells like ammonia or looks wet and matted, it is not finished yet and will rob nitrogen from your plants as it continues to break down.

You can use homemade compost here if you have it, or buy bagged finished compost from a garden center. Bagged compost is more expensive per cubic yard than bulk compost, but it is convenient for small beds and you can see what you are getting. If you are filling multiple beds, buying bulk finished compost from a landscape supplier and having it delivered is usually cheaper overall.

Cost comparison: bulk versus bagged materials

Bulk topsoil and compost from a landscape supplier typically cost $30 to $60 per cubic yard, depending on your region and whether the material is screened. A 4-by-8-by-12-inch bed needs about 1.2 cubic yards, so bulk materials run roughly $40 to $70 total. Bagged topsoil and compost cost $3 to $8 per bag, and a typical bag is 0.5 to 0.75 cubic feet, so filling the same bed with bagged materials costs $150 to $300.

The trade-off is convenience and minimum orders. Bulk suppliers often require a 3 to 5 cubic yard minimum and charge delivery fees ($50 to $150 depending on distance). If you are filling only one or two small beds, bagged materials may be cheaper even at the higher per-unit cost. If you are filling three or more beds, or one very large bed, bulk is almost always cheaper.

Some garden centers will sell topsoil and compost by the cubic yard without a minimum if you pick it up yourself. Call ahead and ask whether they offer this option—it can split the difference between bulk and bagged pricing.

Alternative fills: what works if you cannot get standard materials

If topsoil and compost are unavailable or too expensive in your area, you have other options. Aged manure (horse, cow, or chicken) mixed with leaves or grass clippings works well for vegetables, though you should let it sit for at least a month before planting to avoid burning roots. Coconut coir (shredded coconut husk) holds water well and is becoming more available, though it is more expensive than compost and does not add nutrients the way compost does.

Shredded leaves alone can fill a bed if you have access to them—collect leaves in fall, run a mower over them to shred them, and pile them in the bed. They will compress and decompose over the season, so fill higher than you think you need to. This is free if you have trees on your property, and it recycles material you would otherwise bag up.

Straw (not hay, which has seeds) mixed with compost or aged manure also works, especially for root vegetables. Avoid peat moss—it is expensive, depletes wetlands, and does not improve soil structure the way compost does.

Settling and topping off in the first season

Expect your bed to settle by 1 to 3 inches in the first few months as organic material breaks down and the whole mass compacts. This is normal. In spring, before you plant, top off the bed with another inch or two of finished compost or topsoil. After the first season, you will only need to add 1 to 2 inches of compost each year to replace what has decomposed and been harvested.

If you are planting in fall, do the settling and topping off in early spring. If you are planting in spring, fill the bed in fall so it has all winter to settle, or fill it in early spring and wait two weeks before planting to let it compact.

Frequently Asked Questions

Can I use garden soil instead of topsoil?

Garden soil is topsoil that has been amended with compost and other materials, so it is richer and more expensive. For a raised bed, plain topsoil mixed with compost is cheaper and gives you the same result. Save garden soil for in-ground beds where you are amending existing soil.

How deep does a raised bed need to be filled?

Most vegetables and herbs need 10 to 12 inches of soil. Root vegetables like carrots and parsnips want 12 to 18 inches. Shallow-rooted plants like lettuce and spinach do fine in 6 to 8 inches. If your bed is deeper than 12 inches, you can use cheaper material (wood chips, cardboard, or leaves) for the bottom half and save topsoil and compost for the top half.

Should I mix in perlite or vermiculite?

Perlite improves drainage in heavy clay soil; vermiculite improves water retention in sandy soil. If you are mixing topsoil and compost in a 60-40 ratio, you probably do not need either. Add perlite only if your topsoil is very heavy and clay-like, or vermiculite only if it is very sandy and drains too fast.

What if I have old soil from a previous bed?

You can reuse it, but refresh it first. Remove the top 2 inches (which may be compacted or depleted), then mix the remaining soil with 25 to 50 percent new compost before refilling the bed. This restores nutrients and structure without starting from scratch.

Do I need to line the bottom of the bed with landscape fabric?

No. Landscape fabric blocks drainage and roots cannot penetrate it. If you are worried about weeds coming up from below, use cardboard or newspaper instead—it blocks weeds while it decomposes and does not interfere with drainage or root growth.