Raised bed soil mix recipe

Knowing how much soil to buy, covered in the soil volume guide, only answers the total quantity question. What to actually fill a bed with is a separate question, and an unbalanced fill, all topsoil with nothing else, or all compost with nothing else, tends to perform worse than a simple blended mix.

A starting ratio

A commonly used starting point is roughly sixty percent topsoil or a bagged garden soil blend, thirty percent compost, and ten percent a coarse drainage material such as perlite or coarse sand. This is a starting ratio to adjust from based on results, not a fixed formula that fits every climate, water source, and crop selection.

What topsoil contributes

Topsoil provides the bulk mineral structure of the fill, the base that holds the bed's shape and provides most of the volume. Topsoil alone, without organic matter mixed in, tends to compact over a season and drain poorly, which is why it is a majority share of the mix rather than the entire mix.

What compost contributes

Compost adds organic matter and nutrients, and improves how well the mix holds moisture without waterlogging, since decomposed organic matter has more structure than fine mineral soil on its own. A well finished, fully decomposed compost is worth confirming before buying in bulk, since compost that is still actively decomposing can temporarily tie up nitrogen that plant roots would otherwise use.

What the drainage component contributes

Perlite or coarse sand keeps the mix from compacting into a dense, poorly draining mass over time, particularly in a mix with a higher clay content topsoil. This is a small share of the total mix, around ten percent, since too much coarse material reduces the mix's ability to hold water and nutrients between waterings.

Adjusting for what you are growing

Root vegetables such as carrots benefit from a slightly higher drainage share and a finer overall texture, so roots are not obstructed by large clumps. Leafy greens and most vegetables tolerate the base ratio well without adjustment. If a specific crop has an unusual soil preference, that is worth researching separately rather than assuming the base ratio is universal.

Filling in layers versus mixing first

Some gardeners mix the three components together before filling the bed, others layer them in and let watering and the first season's root growth do some of the mixing naturally. A pre-mixed fill gives more even results from the very first planting, while a layered fill is somewhat less labor at fill time; either approach works, and the choice mostly comes down to whether you would rather mix now or let the season do part of the work.

How much of each to buy

Once the total volume is known from the free configurator or the soil volume guide, apply the sixty-thirty-ten split to that total figure to get the amount of each component to buy. A 4x2 bed needing about 6 cubic feet total, for example, would call for roughly 3.6 cubic feet of topsoil, 1.8 cubic feet of compost, and 0.6 cubic feet of a coarse drainage material.