Compost moisture and aeration: mixing, turning, and how often?

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Should you turn your compost? And if so, how often? Advice varies from one book to the next, from “every time you add scraps” to “never,” which is enough to discourage any beginner. In reality, it all comes down to two very practical needs of microorganisms: air and water, in the right amounts.

Here's how to check moisture in a few seconds, how often to mix depending on your equipment, and how to rescue compost that is too dry or too wet.

Why air is essential

Composting is an “aerobic” process: the bacteria and fungi that break down your scraps breathe oxygen. When air runs short, other microorganisms take over. These work without oxygen, more slowly, and produce methane and foul-smelling gases. That's where the notorious rotten-egg smell comes from.

Air circulates through the gaps left between pieces of material. Over time, and under its own weight, the material settles, especially if it is very wet or rich in fine matter such as grass clippings. Mixing therefore recreates these air pockets, blends fresh additions with material that is already breaking down, and lets you check moisture at the same time.

Water is just as vital: microorganisms live in the film of moisture surrounding each piece. Too dry, and they stop working. Too wet, and the water drives out the air, bringing back the bad smells.

The squeeze test

You can check the moisture level in ten seconds, wearing a glove. Take a handful of compost from the center of the bin and squeeze hard:

• a few drops form between your fingers: the moisture is perfect;
• water runs out freely: the compost is too wet;
• nothing comes out and the handful crumbles: it is too dry.

This test is described in most composting guides. The image to keep in mind is a wrung-out sponge: damp to the touch, but not dripping. In the diagram: too dry (1), just right (2), and too wet (3).

Key takeaways

Healthy compost is as damp as a wrung-out sponge, has air between its pieces, and doesn't smell bad. If any of these three points slips, it's a sign that you need to step in.

Mix with every addition or turn the whole pile?

There are two very different actions to tell apart.

Light mixing, with every addition

For a bin composter, mix the new scraps into the existing material to a depth of about 8 inches (20 cm). This quick step keeps peels from forming a compact layer on the surface, limits fruit flies, and spreads the moisture evenly.

Thorough mixing, from time to time

This time the aim is to aerate the whole volume. Here, recommendations vary between guides: at least once a month for some, every one to two months (a little more often at the start) or once or twice a month for others, and only “several times a year” for the least demanding.

Our take: a light mix with every addition and a thorough mix about once a month in the warmer months, and hardly anything in winter. If you mix less, the compost will still form, just more slowly: a well-balanced mix will eventually break down without any intervention.

How often, depending on your equipment?

• Bin composter (wood or plastic): mix to a depth of 7.9 in (20 cm) with every addition, and mix more deeply about once a month with an aerator or a fork.
• Compost pile: turn it completely with a fork, moving the pile over to one side, about once a month in the warmer months. It takes more effort, but the result is very even.
• Tumbler composter: a few turns of the drum each time you add scraps. That's the whole point of this system: aeration takes no effort.

To choose between these systems, see our guides Compost pile or compost bin and Tumbler composter.

The right tools

Turning a full bin with a fork is tedious and awkward, since the sides get in the way. That's where aerators come in: a rod fitted with small wings that you push into the material, then twist and pull back up. The Brass'Compost compost aerator works on this principle: it brings material up from the bottom and lets air into the heart of the bin, without having to empty it.

For a pile, a spading fork or a manure fork is still the most effective tool. On the Maze 245L tumbler composter, which gets heavy once it's well filled, the EASY-TURN handle makes rotating the drum easier.

Willy's tip

Use your monthly mix to do the squeeze test in the center of the bin, not just on the surface: the top layer dries out quickly in summer, while the bottom can stay waterlogged. It's often in the center that you discover the true state of your compost.

Fixing compost that is too dry or too wet

Too dry

The compost is dusty, fibrous, and has stopped working. Water it while mixing, ideally with rainwater, until it's back to the wrung-out sponge stage. Add green materials (peels, fresh grass clippings), which bring moisture. In summer, cover the bin and place it in partial shade if possible.

Too wet

The compost is sticky, compact, oozes liquid, and smells bad. Add dry, coarse material (wood chips, leaves, torn cardboard) while mixing well, cut back on grass clippings, and cover the bin if rain gets in. You can also mix in a little dry compost or soil, or spread the material out in the sun for a few hours.

If the smell persists despite these steps, our guide Smelly, too wet, or too dry compost: troubleshooting reviews all the possible causes.

Safety: gloves and mask

Compost is a living environment, rich in bacteria and fungal spores. Health risks are rare, but a few simple precautions are a must:

• wear gloves when handling and mixing compost;
• a dust mask is recommended when turning dry compost, which kicks up dust;
• wash your hands after each handling, and wash fruits and vegetables from your vegetable garden before eating them.

To compare with the worm composter, which hardly needs turning at all since the worms aerate the material themselves, read our guide Should you stir your worm bin?

In summary

Good compost needs air and the moisture of a wrung-out sponge, which you can check with the squeeze test. Lightly mix each addition to about 8 inches (20 cm), mix the whole thing about once a month with an aerator or a fork, and leave the pile alone in winter. Too dry: water it and add greens. Too wet: add coarse browns and mix. And don't forget your gloves.

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