Inches out, cubic yards in
Everything about spreading compost is described in inches and everything about buying it is described in cubic yards, and the conversion is the whole job. Area in square feet times depth in feet gives cubic feet; 27 cubic feet is a cubic yard. One inch over a thousand square feet is 1000 x 0.0833, which is 83.3 cubic feet, or 3.09 cubic yards. Two inches is 6.17 yards. Those are large numbers relative to what people picture when they say "a couple of inches of compost", and they are the reason a bagged order for anything bigger than a single bed becomes absurd — 166.7 cubic feet is 112 bags at 1.5 cubic feet each.
The useful figure to carry around is that an inch over a thousand square feet is a bit over three cubic yards. Scale from there and you can sanity-check any quote in your head.
What the incorporation percentage tells you
Spreading two inches and tilling it into the top six is not the same intervention as spreading two inches and leaving it there, and the number that describes the difference is the amendment fraction of the finished layer. Two inches of compost mixed into six inches of soil produces roughly eight inches of amended material, of which the compost is two, so about 25 percent by volume.
| Fraction by volume | What it is | Typical use |
|---|---|---|
| Under 10% | Maintenance dressing | Established beds, annual top-up |
| 10-25% | Genuine amendment | Improving structure in a working bed |
| 25-40% | Heavy amendment | Building a new bed in poor or compacted ground |
| Over 40% | Effectively a growing medium | Rarely what you want in the ground |
The point of putting a number on it is that "till in some compost" hides an order of magnitude. The same two cubic yards spread over a small bed and over a whole vegetable plot are entirely different treatments, and only one of them will do anything you can see.
Topdressing against tilling
Tilling gets the material into the root zone immediately, which is what you want when establishing a new bed in ground that has never been worked. It also destroys existing soil structure, chops fungal networks and brings a fresh layer of weed seed to the surface, and those costs are paid every time. Topdressing puts the compost on the surface and lets earthworms, rain and roots move it down over a season or two. It is slower and it is undramatic, and on ground that is already growing something it is generally the better trade.
The practical limit on topdressing is depth. More than about two inches over an established planting starts smothering crowns and low foliage, and a thick surface layer of fine compost can cap and shed water rather than absorb it. If you need more than that, put it down over two seasons or till it in and accept the disturbance.
Compost is not fertiliser
They get used interchangeably and they answer different questions. Compost supplies organic matter, improves aggregate structure and water holding, feeds soil biology and releases a modest amount of nutrition slowly and unpredictably. Fertiliser supplies a known quantity of a specific nutrient on a schedule you control. Neither substitutes for the other: a soil with good structure and no nitrogen still grows badly, and a soil with plenty of nitrogen and no organic matter still compacts and dries out. If what you are chasing is a measured nutrient rate rather than a volume of material, that is the job of the fertilizer application calculator, which works in pounds of nutrient per thousand square feet rather than cubic yards.
The other thing worth being blunt about is that compost quality varies enormously and you cannot tell by looking at a photograph on a website. Finished compost smells of soil, not of ammonia or sourness, and it should be cool. Immature material continues to break down after you spread it, which ties up nitrogen while it does so and can leave a bed noticeably hungrier than before you amended it. Buy where you can see the pile. For filling frames rather than amending ground, the raised bed soil calculator handles the blend side, and topsoil is the calculator for the mineral component on its own.
Questions people ask
How many cubic yards of compost do I need for 1,000 square feet?
A little over three cubic yards per inch of depth. The arithmetic is 1,000 square feet times one twelfth of a foot, which is 83.3 cubic feet, divided by 27 to give 3.09 cubic yards. Half an inch is 1.54 yards, two inches is 6.17, three inches is 9.26. Those last figures are delivery-truck quantities rather than car-boot quantities, which is worth knowing before you plan a weekend around it.
How deep should I spread compost?
It depends on whether it is maintenance or construction. An annual topdress on ground that is already in reasonable shape is half an inch to an inch, which is enough to keep organic matter from declining without smothering anything. Building a new bed in poor or compacted soil is two to three inches tilled in, which lands somewhere around a quarter of the amended layer by volume. Going deeper than that on established plantings does more harm than good, because thick fine material caps the surface and holds moisture against crowns.
Should I till compost in or leave it on top?
Till when you are establishing a bed in ground that has not been worked and needs the amendment down where the roots will be immediately. Topdress on anything already growing. Tilling gets the material into the profile in one pass, but it also breaks down existing soil structure, disrupts fungal networks and lifts buried weed seed into germinating position, and those costs recur every time you do it. Surface applications get worked down by earthworms and rain within a season or two on active soil, which is slower and leaves more behind.
Is compost the same as fertiliser?
No, and treating them as interchangeable causes both over- and under-feeding. Compost is a soil conditioner: organic matter, structure, water holding, biology, plus a slow and variable trickle of nutrients that depends on what it was made from. Fertiliser is a measured dose of a specific nutrient available on a known timescale. A bed can have excellent structure and still be nitrogen-hungry, and it can be well fertilised and still be a compacted brick. Use compost for the soil and fertiliser for the crop, and calculate the two separately.
How much does a cubic yard of compost weigh?
Commonly somewhere in the region of 800 to 1,200 pounds, with moisture content doing most of the moving. That range matters if you are collecting it yourself: two cubic yards of wet compost is over a ton, which is more than a half-ton pickup should be carrying, and it will not look like much in the bed. It also matters if a supplier sells by weight rather than volume, because a wet yard and a dry yard are the same volume and very different tickets.