Raised bed soil, split by ingredient

About 29 cubic feet if the bed is 12 inches high, built from 2x lumber and filled to within 2 inches of the rim. The arithmetic that gets skipped is at both ends: the inside is 3 ft 9 in by 7 ft 9 in rather than 4 by 8, and the fill is 10 inches rather than 12. Work from the name on the plan and you get 32 cubic feet, so the order is about ten percent generous — which is not a disaster on a bed, and matters when it is four beds and a delivery minimum.

Outside, virtually always, and the soil goes inside. A bed sold or described as 4 by 8 is 4 by 8 across the outer faces of the frame, because that is what the timber cut list produces. Nominal 2x lumber is 1.5 inches thick under the PS 20 sizing standard, so two boards take three inches off each direction. On a 4 by 8 bed that removes nine percent of the area; on a 2 by 4 bed it removes eighteen percent, which is well past the point where it changes how many bags go in the cart.

Something in the region of 60 percent mineral soil, 30 percent compost and 10 percent coarse aeration by volume, and the proportions matter more than the brand names. The mineral fraction is the body of the bed: it holds the structure up, retains nutrients rather than letting them wash through, and does not disappear over time. Compost supplies the fertility and the biology. The aeration fraction — perlite, pumice, coarse bark, even sharp sand — keeps pore space open in a container that has no subsoil under it to drain into. Mixes built from only the second and third ingredients are common online and they slump within a season.

Yes, in a bed tall enough that roots never reach the layer — call it eighteen inches or more, with a solid foot of real mix above the fill. Logs, branches, coarse chip and leaves all work, and the practice is old enough to have a German name. Two things to know before doing it: the layer rots, so the surface will drop further and for longer than a normal bed, and vegetables with any depth to them will find the boundary in year one if you are optimistic about how much soil is left above it. The bottom fill field takes that depth off the soil volume.

Two separate things happen and they are worth telling apart. The mix settles in the first weeks as water closes the air pockets left by handling, which is a one-off drop of roughly a sixth of the loose depth. Then the compost fraction is slowly consumed by soil life, and most of what leaves the bed leaves as carbon dioxide — that one continues for years and is the reason a bed with a high compost share keeps sinking long after it has been watered in. Neither is a fault. Plan on a bag or two per bed each spring rather than over-filling in year one.

Not usually, and the crossover is around a single bed. Pre-blended raised bed mix in 1.5 or 2 cubic foot bags is convenient and costs several times what the same volume of bulk topsoil and compost costs delivered; for one small bed the delivery minimum swamps that difference and bags win. For three or more beds — anything past a cubic yard or so of total volume — buying the ingredients separately is both cheaper and better, because you control the ratio instead of trusting a bag that does not print one.

Fill to the boards, but plan the soil surface an inch or two below the rim. That gap is what stops a watering can or a hose from floating the fine fraction over the edge, and it gives you somewhere to put mulch without burying stems. It is set as freeboard here rather than assumed, because a bed that is going to be mulched wants more of it and a bed being direct-sown wants very little. Filling proud of the rim looks generous on day one and is on the path two weeks later.