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 waste” to “never”, which is enough to put any beginner off. In reality, it all comes down to two very practical needs of micro-organisms: air and water, in the right amounts.

Here is 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 waste breathe oxygen. When air runs short, other micro-organisms take over. These work without oxygen, more slowly, and produce methane and foul-smelling gases. That is 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: micro-organisms 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

The right moisture level can be checked in ten seconds, wearing a glove. Take a handful of compost from the centre 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 points

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

Mix with every addition or turn the whole heap?

There are two very different actions to distinguish.

Light mixing, with every addition

For a bin composter, mix the new waste into the existing material to a depth of about twenty centimetres. This quick step stops peelings 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 view: 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 20 cm with every addition, and mix more deeply about once a month with an aerator or a fork.
• Compost heap: turn it completely with a fork, moving the heap alongside, about once a month in the warmer months. It is more physical, but the result is very even.
• Tumbler composter: a few turns of the drum each time you add waste. That is the whole point of this system: aeration takes no effort.

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

The right tools

Turning a full bin with a fork is tedious and awkward, as the sides get in the way. Hence the value of aerators: 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 heap, a digging fork or a manure fork remains the most effective tool. On the Maze 245L tumbler composter, which becomes heavy once 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 centre of the bin, not just on the surface: the top layer dries out quickly in summer while the bottom can stay waterlogged. It is often in the centre 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 is back to the wrung-out sponge stage. Add green materials (peelings, 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 (shredded prunings, 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 remain rare, but a few simple precautions are a must:

• wear gloves when handling and mixing compost;
• a dust mask is advisable when turning dry compost, which raises dust;
• wash your hands after each handling, and wash fruit and vegetables from the garden before eating them.

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

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 twenty centimetres, mix the whole lot about once a month with an aerator or a fork, and leave the heap alone in winter. Too dry: water it and add greens. Too wet: add coarse browns and mix. And remember your gloves.

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