Raised Bed Soil Mix: Ratios, Settling and What to Order

Filling a raised bed is the one time a gardener gets to choose their soil from scratch. That is a real advantage, and a slightly daunting one, because the internet offers a dozen recipes that all claim to be the best.

They are not as different as they look. Every good mix balances the same three things, and once you understand those, you can judge any recipe, including the one your local supplier sells ready-blended.

The three jobs of a soil mix

1. Hold water and nutrients. This is the job of organic matter: compost, peat moss, coir, leaf mold. Organic matter acts like a sponge and a pantry.

2. Provide structure and weight. This is the job of the mineral part: topsoil, which is sand, silt and clay. Mineral soil does not rot away. It gives roots something firm to anchor in and holds the level of the bed from year to year.

3. Let air and water move. Roots need oxygen, and water has to be able to drain. Coarse particles do this: perlite, pumice, vermiculite, coarse sand, or fine bark.

A mix that is all sponge stays wet and shrinks. A mix that is all mineral sets hard and starves plants. A mix with no coarse material drains slowly. Recipes differ in how they share out the three jobs.

The common recipes compared

There is no winner. For a deep bed that you want to fill once and top up lightly, a mix with a good share of mineral soil, such as 60 / 30 / 10, is the practical choice. For a shallow bed on a patio or over poor ground, where weight matters and you want perfect drainage, a soilless mix works well.

What each ingredient does

Topsoil. The mineral backbone. Good topsoil is dark, crumbly and screened to remove stones and roots. Poor topsoil is subsoil with a new name: pale, sticky, and full of clods. If you are buying in bulk, look at the pile before you order. Squeeze a handful. It should hold together loosely and break apart when poked.

Compost. Decomposed organic matter, and the main source of nutrients in the mix. Compost made from several sources, such as leaves, manure and plant waste, has a broader range of nutrients than compost from one. It should smell earthy, not sour, and you should not be able to recognize what it was made from.

Peat moss or coir. Both hold several times their weight in water. Peat is acidic and hard to re-wet once it dries out. Coir, made from coconut husk, is close to neutral and re-wets easily. They do the same job in a mix.

Vermiculite. A mineral that has been heated until it puffs up. It holds both water and air and does not break down. The coarse grade is the one for garden mixes.

Perlite and pumice. Lightweight volcanic materials that create air spaces. They hold little water themselves. Perlite is the white granules in potting mix. It tends to float to the surface over time, and pumice does not.

Coarse sand. Improves drainage if, and only if, it is coarse. Fine sand mixed into clay soil makes something close to concrete.

How much the mix will settle

Every freshly filled bed sinks. There are two causes.

First, bagged and freshly blended materials are fluffed up. Shoveling, pouring and mixing trap air, and that air is squeezed out by watering and gravity over the first few weeks.

Second, the organic part keeps decomposing. Compost is not finished rotting when you buy it. It continues to shrink for years, slowly.

As a guide:

On a bed 12 inches deep, 15% is nearly 2 inches. This is why the raised bed soil calculator adds a settling allowance. Without it, a bed filled level in April is visibly low by July.

You can deal with settling in two ways. Overfill slightly at the start, mounding the mix an inch above where you want it to finish. Or fill to the planned level and top up with compost at the start of each season, which you will want to do anyway to replace nutrients.

From recipe to shopping list

Recipes are given by volume. “60% topsoil” means six buckets out of every ten, whatever those buckets weigh. So the arithmetic is the total volume multiplied by each share.

Example. One bed, 8 feet by 4 feet inside, 12 inches tall, filled to an inch below the rim with a 60 / 30 / 10 mix.

• Depth of soil: 12 − 1 = 11 inches
• Volume: 8 × 4 × 11 ÷ 12 = 29.3 cubic feet
• With 10% for settling: 32.3 cubic feet
• Topsoil, 60%: 19.4 cubic feet
• Compost, 30%: 9.7 cubic feet
• Aeration material, 10%: 3.2 cubic feet

In 1.5 cubic foot bags, that is 13 bags of topsoil, 7 of compost and 3 of perlite or pumice. Each ingredient is rounded up on its own, because bags do not come in fractions.

In bulk, this bed needs about 1.2 cubic yards in total. Most suppliers sell a blended garden or raised bed mix, and a single order of one and a half yards covers it with some left over for pots.

Bags, bulk, or both

For one small bed, bags are simple and there is nothing to shovel off the driveway. For two or more standard beds, bulk is nearly always cheaper and far less handling. Our guide to bagged versus bulk shows how to find the break-even point with your own prices.

A common and sensible compromise is to buy the bulky ingredients loose and the specialist ones in bags. Have blended topsoil and compost delivered, then mix in a few bags of perlite or vermiculite as you fill.

If you buy a ready-blended bulk mix, ask the supplier three questions:

1. What is in it, and in what proportions?
2. Is the compost fully finished? Unfinished compost can rob plants of nitrogen and may still be hot.
3. Has it been screened, and to what size?

Filling a deep bed without buying it all

Vegetable roots do most of their work in the top 12 inches. In a bed 18 inches deep or more, the bottom third does not need to be premium mix.

Common fillers for the bottom layer are logs and thick branches, twigs and prunings, upturned sod, leaves, and rough unfinished compost. They rot slowly, hold moisture, and save a good deal of money. Expect a bed filled this way to sink more over the first two or three years as the wood breaks down, and top it up as it does.

Avoid treated lumber, plywood, anything painted, and black walnut wood, which contains a substance that stunts many vegetables.

When you use fillers, enter only the depth of real soil in the calculator. For a 24 inch bed with 8 inches of logs in the bottom, enter 16 inches as the bed height.

Mixing and filling

• Mix before you fill, if you can. Dump the ingredients onto a tarp, turn them together with a shovel, and then load the bed. Mixing inside the bed with a fork works too, but takes longer to get even.
• Fill in layers and water each one. Dry peat and coir shed water at first. Wetting each 4 inch layer as you go avoids dry pockets deep in the bed.
• Do not tread on it. Firm the mix gently with the back of a rake. Stepping in the bed squeezes out the air you just paid for.
• Leave a gap at the top. An inch or two below the rim keeps water and mulch inside.
• Wait a few days before planting. Water the filled bed well and let it settle. Top up the low spots, and then plant.

Keeping the level up in later years

Each spring, the bed will be a little lower than you left it. Spread an inch or two of compost over the surface and work it lightly into the top few inches. That replaces the nutrients the last crop took out and restores the level at the same time.

To work out how much, use the topsoil and compost calculator with the bed’s area and the depth of your top-up. For a 4 × 8 bed, an inch of compost is 2.7 cubic feet, which is two bags.

Written and checked by Bertha
Last updated October 4, 2026 · How we work out our numbers