How much soil a raised bed actually needs, and why the bags surprise you

The lumber for a raised bed is the part people budget for. The soil is the part that surprises them, usually in the car park of a garden centre with a trolley that will not hold enough bags.

So here is the arithmetic, done properly, for four standard beds.

How the number is calculated

Two things trip people up. The first is that the volume you need is the interior volume, not the outside dimensions — the boards themselves take up an inch and a half on each side. A bed that is 48 inches across on the outside is 45 inches across on the inside.

The second is freeboard. You do not fill a bed to the brim, because the first time you water it, some of the mix goes over the side. Leave an inch. So an 11-inch bed — which is two 2×6 boards, a 2×6 being an actual 5½ inches — holds ten inches of soil, not eleven.

Interior length × interior width × soil depth, divided by 1,728 for cubic feet, divided by 27 for cubic yards.

The number that catches people is Bed B, the ordinary 4×8 at two courses high. It is the most common raised bed in America and it takes 0.90 cubic yards of mix. That is seventeen bags at 1.5 cubic feet each, which is more than fits in most cars and considerably more than most people expect to carry.

Bed D — the same footprint at three courses, for carrots and parsnips and anything else that wants depth — takes 1.39 cubic yards. Twenty-six bags.

Where bulk stops being optional

Somewhere around half a cubic yard, bagged mix stops making sense. Bulk soil is usually sold by the half yard or yard and is dramatically cheaper per unit volume; you are paying for the bag, the pallet and the handling every time you buy small.

Price both before you decide. The comparison is not purely money — bulk means a delivery, a pile on your driveway, and an afternoon with a wheelbarrow, which is a genuine cost if you have a bad back or a tight schedule. But for a 4×8 bed the difference is usually large enough to be worth the wheelbarrow.

What to put in it, which is less settled than you would think

I went looking for a definitive soil recipe and found three different ones, all from university extension services, none of them wrong.

NC State's Clay County office gives 40% compost, 40% topsoil and 20% aeration material — coarse sand, pine bark fines or perlite. Penn State says 70% soil to 30% compost. The University of Minnesota gives a range: one-half to two-thirds topsoil, one-third to one-half plant-based compost.

These differ mostly in an assumption they do not state, which is how much structure your topsoil already has. If you are buying screened topsoil that is largely silt, you want the aeration material. If your bulk supplier's “garden mix” is already open and friable, you do not.

The one thing all three agree on is worth more than the disagreement: do not fill a bed with compost alone. NC State's reason is that compost “breaks down over time, can lead to disease problems.” There is a practical reason too — a bed filled with pure compost will drop several inches in the first year as it decomposes, and you will be topping it up forever.

A note on aeration material, given what I wrote last time

If you read my piece on why gravel at the bottom of a bed is a mistake, there is an apparent contradiction here: how can perlite and pine bark fines be good when gravel is bad?

Because they are doing a completely different job. Mixed through the soil, coarse particles hold the structure open and increase the number of large pores throughout the whole column. Layered underneath the soil, coarse material creates a texture boundary that water will not cross until the soil above is saturated.

Mixed in, good. Layered under, bad. Same material, opposite effect, and the difference is entirely about whether you created a boundary.