How to Fill a Raised Garden Bed: What Soil to Buy, What Goes on the Bottom & Common Mistakes

What is the best soil to fill a raised vegetable garden bed?

A blend of topsoil and compost. Maryland’s extension recommends compost and topsoil at a 1:2 or 1:1 ratio¹; Minnesota’s puts topsoil at half to two-thirds of the volume with plant-based compost making up the rest². Oregon State describes the target rather than a ratio: a mix with high organic matter and good nutrient- and water-holding capacity⁶, and its first rule for framed beds is “don’t fill raised beds with compost alone”⁴.

If you’d rather not blend it yourself, bags and bulk products labelled “raised bed mix” are the same idea in one product; Minnesota suggests looking for that label when buying bags². What matters is the composition, not the name, so read the bag or ask the yard what’s in it.

Topsoil, garden soil, raised-bed mix, potting soil, compost: what’s the difference?

The labels describe different things, and two of them are routinely bought for the wrong job. Minnesota’s guidance is blunt about bags: “garden soil” is for planting in the ground and, like bagged topsoil, is too heavy for pots or elevated garden boxes; potting mix is a soilless blend for containers³. Maryland adds that bags labelled “organic soil” typically contain no topsoil at all, just wood waste, bark fines, compost and peat¹.

Summarized from Maryland and Minnesota extension guidance¹³²⁴. Product names vary by brand and region.

What ratio of topsoil to compost should I use?

Anywhere from about a quarter to a half compost by volume, with the rest mineral soil. That range covers every extension recommendation we found: 1:2 to 1:1 compost-to-topsoil in Maryland¹, a third to a half compost in Minnesota², and in Oregon either up to 25% compost mixed into soil you dug on site, or a “three-way mix” of equal parts topsoil, sand and compost⁴. The spread is the point: there’s no magic number, and the real mistake is going past half.

Two adjustments. If the topsoil is heavy with clay, Minnesota suggests adding some sand for a loamier texture². And go easy on manure-based composts, which carry a lot of phosphorus; Minnesota’s advice is to use manure sparingly and only when a soil test shows a need².

Once you’ve picked a ratio, get the total volume from the calculator and split it. A 4 × 8 ft bed filled 12 in deep holds 32 cubic feet, so a one-third compost blend is about 11 cubic feet of compost and 21 of topsoil.

Can I fill the bottom of a raised bed with logs, branches, leaves or wood chips?

Yes, with your eyes open. Oregon State’s soil scientists actually suggest it for tall beds: fill the bottom with carbon-based materials such as wood chips, limbs or straw and top with a mix of loamy soil and compost, partly because it stops people filling the whole bed with compost⁵. Three things to accept first.

It settles. Washington State’s review of hügelkultur, the mounded version of this idea, found the mounds decompose and settle over a period of months, have a lifespan of about five to six years, and then have to be rebuilt⁷. In a framed bed that shows up as a surface that sinks and needs topping up; Illinois extension calls settling “simply a fact of raised bed gardening”¹³.

It isn’t root zone. Whatever you bury is volume your plants can’t use this season. Georgia’s extension puts the minimum at 10 inches of soil for most garden crops¹², so lower fill belongs in beds with room to spare above that, not in a 12-inch bed.

The evidence is thin. The same Washington State publication notes there are no peer-reviewed studies of hügelkultur⁷. What is well established is that organic matter decomposes and needs replacing, and that nitrogen should be added as the season demands⁶.

The calculator’s layered-filling option shows exactly how much growing mix a chosen top depth needs and how much lower fill is left, without recommending a material or a ratio.

Can I use 100% compost or manure to fill a raised bed?

No. Compost lacks the mineral component of soil and has to be combined with native soil or a soil mix to support plant growth⁴. The consequences aren’t theoretical: when Oregon State sampled 27 urban gardens, beds averaged 13% organic matter against a recommended 3–5%, and one compost-only bed hit 30%, where the pepper plants burned and died by the end of the season⁵.

Manure is a narrower problem. Composted manure runs high in phosphorus, and heavy repeated applications of any compost can build up phosphorus and soluble salts that burn seedling roots². Compost is the amendment; it is not the bed.

Should a shallow bed and a tall bed be filled differently?

Yes. In a shallow bed every inch counts, so fill it entirely with growing mix and make sure the ground beneath is usable: Oregon State’s method is to dig or rototill at least 6 inches down and mix the native soil into the fill⁶, and Georgia advises tilling beneath beds that can’t hold 10 inches on their own¹². On a hard surface, where roots can’t go anywhere, Maryland sets minimum depths of 8 inches for leafy greens, beans and cucumbers and 12–24 inches for peppers, tomatoes and squash¹.

In a tall bed, 16 inches and up, you have choices: all growing mix for simplicity, or lower fill with a full growing depth on top. The depth you enter in the calculator should be the depth you’ll actually fill with soil, which is often less than the frame height.

What should I put at the bottom of a raised bed: cardboard, landscape fabric, hardware cloth or nothing?

Usually nothing. Minnesota’s guidance is that in most cases you don’t need a barrier between the bed and the soil beneath, because a barrier is likely to stunt root growth². Roots that can reach the ground below are a feature of the bed, not a problem.

Cardboard is a temporary smother for lawn, not a permanent layer. Oregon State’s sheet-mulching guidance: plain brown corrugated cardboard with tape and staples removed, sheets overlapped by at least 8 inches, and it breaks down over several months⁹. While it’s intact it limits the flow of water and air into the soil⁹, which is why Minnesota takes the stricter line and says to remove tarps or cardboard before placing a bed on soil². If you use it, use it to kill turf, and don’t expect roots through it this season.

Landscape fabric only if you need a liner to keep soil in the frame, and then a permeable one; Minnesota says to choose a material that lets water through rather than non-permeable plastic².

Hardware cloth is for burrowing animals. The University of California’s pest program says to lay hardware cloth or ¾-inch-mesh poultry wire under raised beds before planting, using galvanized or stainless wire so it lasts⁸; Minnesota notes the same barrier deters mice and chipmunks². It goes in before the soil, so decide now.

Can I use the soil already in my yard?

Often, yes, and both Oregon and Maryland’s methods assume you will mix native soil into the fill⁶¹. The condition is a soil test. Maryland’s position is that all vegetable garden soils should be tested for lead: above 400 ppm, don’t grow vegetables in that soil at all, and between 401 and 1,000 ppm, grow leafy and root crops in raised beds filled with clean topsoil and compost to at least 8 inches¹¹. Lead comes from old paint, demolition, leaded fuel and former orchards, so older urban lots are the ones to check¹¹.

If the test is clean, treat the yard soil as your topsoil: Maryland’s ratio for a bed on existing ground is compost to garden soil at 1:2 or 1:1¹. If it’s compacted clay, loosen the top 6 inches and blend rather than building a sealed box on top of it⁶.

How can I tell whether bulk soil from a landscape supplier is good quality?

Go and look at it. Maryland’s checklist for bulk topsoil: it should be dark and crumbly with an earthy smell; avoid soil that is very high in sand or clay, foul-smelling, or has grayish mottling or a chalky, sticky or rough texture¹. Then ask for three things: soil test results, where the soil came from, and how it is mixed and stored on site¹. Topsoil sales aren’t regulated in Maryland and many other states, so the test results are your only specification.

• Look: dark, crumbly, earthy smell. No gray mottling, no sour smell.
• Ask for recent test results, not a verbal assurance.
• Ask where the soil came from and whether it’s screened.
• Ask what the blend actually contains and in what proportions.
• Ask how they manage herbicide risk and whether they test for it.
• Ask about delivery fees, minimum orders and where the pile can go.

The herbicide question matters more than it sounds. Two herbicides used on pasture and hay, clopyralid and aminopyralid, persist through composting and reach gardens in manure, hay and grass clippings; they stunt and curl the new growth of beans, peas, tomatoes, peppers and potatoes, and ordinary lab tests may not detect them¹⁰. Oregon State’s advice is to ask the vendor how they handle the risk, take a sample, grow peas in it alongside peas in clean potting mix, and compare after three to four weeks, making sure the sample came from the batch you’re buying¹⁰.

Should I buy bagged or bulk soil?

Bulk by the cubic yard is generally much cheaper than bags², but it isn’t automatically the better buy. Delivery fees and minimum orders can erase the saving on a small bed, you need somewhere to drop a pile, and you’re trusting the yard’s blend rather than a labelled product. Bags cost more per cubic foot and come in exact, inspectable quantities.

Enter the prices you’ve actually been quoted into the calculator’s cost comparison and let it tell you which comes out lower for your beds, including the delivery fee.

How can I improve drainage without making the bed dry out too quickly?

Through the mix, not through layers. Drainage and water-holding both come from texture: Minnesota says to look for loam or sandy-loam topsoil and avoid very sandy or clay soil, adding sand only if the topsoil is clay-heavy², and Oregon State’s target is a mix with good water-holding capacity, not just fast drainage⁶. The compost fraction is doing the water-holding; the mineral fraction is doing the draining.

The two ways to get it wrong are a soilless potting mix in an open bed, which dries out too quickly², and a barrier under the bed that stops water and roots moving into the ground²⁹. A bed on pavement is the exception: Minnesota suggests raising it a foot or two to reduce heat, and Maryland fills those beds with compost and soilless mix in equal parts, adding topsoil only up to 20% and only in beds at least 16 inches deep²¹.

How do I refresh the soil in an existing raised bed each spring?

Expect it to have dropped. Settling is normal, and Illinois extension’s routine is to top-dress each year with a few inches of compost or topsoil¹³; Oregon State puts the annual compost layer on vegetable beds at a quarter inch to an inch⁴ and reminds you that organic matter disappears as it decomposes, so it has to be added often⁶.

Don’t just keep adding, though. Every 2–3 years, take a soil test². If organic matter is already in the 3–5% range, Oregon State’s advice is to hold off on compost and retest in a year or two⁵; that is the point at which more compost is building phosphorus and salts rather than fertility².

For the top-up quantity, enter the bed at the depth you’re adding. A 4 × 8 ft bed topped up 2 inches needs about 5.3 cubic feet, or three 2-cubic-foot bags.