Filling a Raised Bed With Soil and Compost

The Box Went Together in an Afternoon. The Filling Arrives on a Lorry

Three courses of 200 by 100 sleepers, sixteen coach screws and a level put the thing in the garden on Saturday. It measures 2.4 metres by 1.2 inside the timber, the top course stands 600 millimetres above the grass, and it is empty. Everything that goes in it now costs more than the timber did, arrives in one movement or not at all, and cannot be adjusted afterwards with a saw.

The asymmetry is worth naming before the yard is phoned. The volume itself is not in doubt — a rectangular void with three known dimensions is the least ambiguous measurement in the trade, with none of the offsetting and averaging that an irregular border demands. All of the uncertainty sits on the other side of the tape: how far a loose blend consolidates once it is watered, how much of it burns off over a growing season, and what fraction of the load should be compost rather than mineral soil. Get those wrong and the bed is still exactly the right size. It is simply half empty by June, or it is a peat bog, or it has to be topped up twice in a year nobody priced for.

One delivery is the goal because most yards apply a minimum bulk order and a delivery charge that does not scale down. Coming up 200 litres short does not save the price of 200 litres; it costs a second drop, or a trip with bags in the back of a car. So the number that goes through on the phone has to carry the settlement inside it, and the compost has to be decided at the same moment, because whether it comes pre-blended or as two separate heaps changes what is actually being ordered.

Measure the Void, Not the Timber

The dimension that matters is the inside face to inside face, and on a bed built from anything substantial the difference is not trivial. A 2.6 by 1.4 metre box built from 100 millimetre sleepers encloses 2.4 by 1.2, which is 2.88 square metres rather than 3.64 — a fifth of the order gone in one careless tape reading. Corner posts, cross-ties and a bracing batten across a long side each take their own slice; on a bed with a central tie the honest thing is to measure the two compartments separately and add them. A liner tacked to the walls takes almost nothing, but it does move the inside face inwards by its own thickness plus the batten holding it.

Depth is where the reading is most often optimistic. Fill depth is not wall height: the medium should finish 25 to 50 millimetres below the top course so that a hose or a heavy shower does not float the surface over the edge and down the timber. On ground that falls — and a lawn that looks flat rarely is — the bed is level and the ground is not, so the fill depth at the downhill end is greater than at the top. Measure both ends from the inside face of the timber down to the soil and work from the average, not from the end you happened to be standing at.

Depth Is a Crop Decision, Then a Structural One, Then a Volume

Salad, herbs and annual bedding are content in 150 millimetres. Most vegetables want 300. Carrots, parsnips, leeks and anything perennial want 450 or more, and the reason is only partly root length. A shallow bed is a container: it holds a small volume of water against a large evaporating surface, and in a dry fortnight the difference between 150 and 300 millimetres is the difference between watering every evening and watering twice a week. Depth buys reserve more than it buys room.

What sits underneath decides whether that is the whole story. Over open ground that has been forked through, the bed is a deep topsoil layer and roots carry on down past the timber, so the fill depth is a head start rather than a hard limit. Over a patio, a deck or compacted hardcore, the bed is a planter in every sense that matters — the entire root zone has to be inside the box, water can only leave through the base or the joints between boards, and both the depth and the coarseness of the medium have to be chosen accordingly.

Depth also loads the timber, and it does so faster than people expect. Retained fill pushes sideways in proportion to depth, and the resultant force on the wall grows with the square of it: a moist blend at roughly 1,100 kilograms per cubic metre, taken at the textbook at-rest condition, puts on the order of 25 kilograms per metre of wall against a 300 millimetre bed and something near 100 against a 600 millimetre one. Doubling the depth quadruples the push. That is why a long, tall bed made from dry-stacked sleepers bows outward at mid-span after its first winter, and why the cross-tie is not decoration.

The table below is the same 2.4 by 1.2 metre bed at each of the usual depths, with the finished volume and the volume actually ordered once a 20 per cent settlement allowance is carried. Read down the last column before choosing: the step from 300 to 450 millimetres adds two thirds of a cubic yard, which on most price lists is the difference between a comfortable order and one that wants thinking about.

What Actually Goes in the Box, Bottom to Top

Turf underneath a bed 300 millimetres deep or more does not need lifting. It dies in the dark, rots in place and leaves the soil beneath it better than it found it, which is more than can be said for the compaction caused by skimming it off with a spade in wet weather. What the base does need is loosening: a fork driven in and rocked, on a grid of a foot or so, over the whole footprint. That single pass is what turns the bed from a container standing on a lid into a deep profile roots and water can both leave through. Where voles or gophers are a known problem, a sheet of galvanised weld mesh goes down before anything else, run up the inside of the timber by 50 millimetres and stapled.

In a deep bed, a coarse core in the bottom third is a legitimate way to spend less on imported medium — logs, prunings, turves stacked grass-side down, the woody half of last year's clearance. It is also the single biggest cause of a bed that sinks alarmingly in year two, because that material loses most of its bulk as it decays. Treat it as a deliberate trade: cheaper now, a substantial top-up later, and never within 300 millimetres of the surface where the seedbed has to be stable. In a 300 millimetre bed there is no room for it at all.

What does not belong at the bottom is a drainage layer of gravel. Water does not cross from a fine-pored medium into a coarse one until the fine material immediately above the interface is close to saturated, so a stone layer under soil does not drain the bed — it holds a wet zone at the base and raises it. That is a capillary break, the same effect deliberately engineered into a landfill capping layer to stop water moving downward, and it works exactly as well in reverse when nobody wants it. If a bed on a hard surface needs to drain, the answer is gaps or holes through the base and a medium coarse enough to shed, not a course of shingle.

The imported material has to be worth importing. Subsoil out of an extension trench is not topsoil, and the distinction has a document behind it in both directions: BS 3882, Specification for topsoil, and BS 8601, Specification for subsoil and requirements for use, describe two different products with different tests, and ASTM D5268, Standard Specification for Topsoil Used for Landscaping Purposes, covers the same ground in North America. Screened is worth paying for on a bed you will hand-weed; unscreened topsoil arrives with couch and bindweed rhizomes in it, and a raised bed is the worst possible place to discover that, since the whole point of the box is that the weed seed bank inside it starts at nothing. Where the garden has an industrial or old-paint history, the reason for importing is the contamination itself, and the guidance to work to is the regulator's — in the United States, the Technical Review Workgroup recommendations issued by the EPA on gardening and reducing exposure to lead-contaminated soils — which turns on a clean imported profile and a barrier, not on a number this page could publish.

Getting a Loose Heap Through a Side Gate

The delivery is one drop in one place, and every metre between that place and the bed is barrow work. Two cubic yards of moist blend is somewhere between one and a half and one and three-quarter tonnes depending on how much of it is compost and how wet the yard's bay was, which is the better part of twenty loads in an 85 litre builder's barrow. Decide before the lorry arrives where the heap can stand, put boards down if that place is block paving or a lawn, and have a tarpaulin ready — a heap left uncovered through a wet night gains weight, sheds fines into the gully, and becomes markedly less pleasant to shovel.

Filling is not tipping. Barrow in, spread with a rake rather than the barrow, and resist walking on the surface: the entire point of a raised bed is that its medium never gets compacted, and a boot print in the first lift is still there in three years. Work in lifts, water each one, and leave the freeboard deliberately short at the top rather than filling to the timber and hoping. If the load runs out slightly high, that is what the last lift is for; if it runs out slightly low, the settlement allowance has already done its job and the bed is at the level it was designed for.

1. Fork the base on a grid across the whole footprint, and lay weld mesh first if burrowing animals are a known problem.
2. Stack any coarse core in the bottom third only, packed rather than piled, and keep it 300 millimetres clear of the finished surface.
3. Barrow in a lift of roughly 150 millimetres, forking the compost share through it if the load did not arrive pre-mixed, and rake it level — taking the barrow no further than the edge of the timber and turning the blend with a fork rather than a rotavator.
4. Water that lift until it is damp through, then leave it while the next barrows are loaded.
5. Repeat until the surface sits 25 to 50 millimetres below the top course, checking the depth against a marked stick rather than by eye.
6. Water the whole bed heavily once more and leave it a week before planting, so the mechanical settlement happens before anything is in it.
7. Cover the heap or clear the drop point the same day, because bulk material left on a hard surface stains it and washes into drains.

What the Bed Tells You in March

A drop of 40 or 50 millimetres over the first winter is consolidation and organic matter doing what they do, and it is corrected with a dressing. A drop of 100 or more, with a surface that has gone dark, fluffy and quick to dry out, is a diagnosis: the compost fraction was too high, and the bed is losing its bulk because there was never enough mineral soil in it to hold a level. The fix is counter-intuitive to anyone who associates compost with generosity — it is a screened topsoil top-up, not more compost, and the following year's dressing should go back to 25 millimetres.

Beyond that the bed only needs the annual layer, laid on the surface and left for the worms rather than dug in. Over several seasons the level creeps back up towards the rim, which is the sign the system is working, and the freeboard quietly comes back as something to manage rather than something to chase. The one number worth writing on the underside of the timber before the first planting is the ordered volume, because in five years' time, when the bed is rebuilt or a second one is added alongside it, that figure is the only part of this afternoon anyone will need again.