How much compost do you need?

Compost is measured like mulch and used like nothing else in the yard. It is not a surface covering and it is not soil, and the depth that helps a bed will smother a lawn. Here is the arithmetic, the right thickness for each job, and how to tell whether what arrived is actually finished.

The formula

The same one that governs everything sold by depth:

A 200 square foot bed getting a 2 inch amendment is (200 × 2) ÷ 324 = 1.2 cubic yards. Bagged compost is usually 1 cubic foot a bag, so it takes 27 bags to make a cubic yard, which is the highest bag count of any material in the yard and the reason compost is worth buying loose.

One cubic yard covers:

How thick, by job

The range is wider than it is for anything else, because the same material is doing completely different work in each case.

• Topdressing a lawn: a quarter to half an inch. You should still see grass through it after you have brushed it in. An inch of compost on a lawn is a tarp.
• Amending a bed: 2 to 3 inches, forked into the top 6 to 8 inches. Spreading it on the surface and leaving it there is a slower version of the same job.
• Maintaining a bed: half an inch to an inch a year, laid on top. Once a bed is established, digging it over does more harm than the compost does good.
• Filling a raised bed: about a third of the mix, not the whole thing. Pure compost sinks, dries hard at the surface and holds more nutrients than most plants want.

Compost is not mulch

They are bought the same way and they behave nothing alike. Mulch sits on the surface to block light and hold moisture, and it is chosen for being slow to break down. Compost is already broken down, which is the point of it: it goes into the soil to feed it.

Laid on top as a surface layer, compost is a superb seedbed, and the seeds that find it first are weeds. If you want both, put the compost down and the mulch over it. That is the order that works, and it is the reason a bed done properly gets two orders rather than one.

It is not topsoil either. Topsoil is the bulk you build a bed out of; compost is the fraction that makes it worth planting into. Buying compost to fill a volume is an expensive way to end up with a bed that has sunk four inches by spring.

Is it actually finished?

Unfinished compost is still decomposing, and it does that decomposing at the expense of your plants: the microbes breaking it down take nitrogen out of the soil to do it, and give it back months later. Three checks, none of which need equipment:

• Smell. Finished compost smells of woodland floor. Sour, sharp or ammonia means it is still working, and it should go on the pile rather than the bed.
• Look. You should not be able to tell what it was made from. Recognisable leaves, bark chunks or food waste mean it needs longer.
• Heat. A finished pile is at air temperature. If the middle of the heap is warm when you dig into it, it is not done.

Ask what it was made from, too. Mushroom compost is excellent but carries salts that some plants dislike, so it wants a season or a good soaking before anything sensitive goes in. Manure compost needs to be properly aged rather than merely stacked. Municipal green waste varies enormously by facility and is worth one trial load before a large order.

Weight and delivery

A cubic yard of compost runs roughly 1,000 to 1,400 pounds, so it sits between mulch and soil: heavier than bark, and about half what the same volume of topsoil weighs. A half-ton pickup takes a yard, though it will be a damp and unpleasant load.

The crossover against bags comes early, because 27 bags to the yard is a lot of bags. Two or three bags for a container is a bag job. Anything covering a bed is worth pricing loose the moment you clear a third of a yard.

Ordering

• Work out the yardage at the depth for the job, not at mulch depth. Compost at 3 inches on a lawn is a wasted order and a dead lawn.
• Compost does not need a waste allowance the way a counted material does, but round up to the next half yard because that is how it is sold.
• Spread it and work it in the same week. A heap of compost sitting on a driveway in the rain loses nutrients into the gutter.
• If you are amending and mulching, order both and lay them in that order. They are not substitutes for each other.

Work out your beds in the planner

Paint the beds, set compost as a layer under the planting, and the planner counts it at its own depth and its own price alongside everything going in on top of it.