Does my Compost Need Turning?

The necessity of turning your compost is a highly debated topic that divides the experts. This short article explores whether turning is required in the HOTBIN and what exactly the pros and cons are of turning compost.

What do we mean by turning compost?  

Usually using a garden fork to dismantle, turn and re-build the heap.  Turning does not mean just giving the new waste on top of the heap a quick mix with the top 5-10 cm of existing compost as a way to get the bacteria mixed in with new waste and breaking it down quickly.

Why do we turn compost? 

The most common answer you will read is to ‘aerate’ (provide oxygen to) the bacteria in the heap and subsequently prevent the waste going anaerobic and smelly. 

There are four core elements that effect how well the aerobic process works: oxygen, warmth (heat), waste (food for bacteria to eat) and water. (Read more).

When these elements are in perfect balance they result in the quick production of beautiful compost, when out of balance however it can lead to a smelly pile of mush. 

Turning affects all four of these composting elements – hence you really need to understand the effect of each condition as well as the overall balance of these effects before deciding if turning is good or bad.

Below discusses the pros and cons of these elements related to the HOTBIN, cold heaps, tumblers etc. by asking;

1) Oxygen - How does turning affect oxygen getting to the bacteria?

Compost bacteria need oxygen - the heap/bin needs to be aerated – i.e. new air (oxygen) needs to be drawn in for the bacteria to use.  Many composting experts advise turning as a means of aerating the heap.

In terms of new air being added, turning has a minor and temporary benefit. The oxygen introduced is small and will be used by bacteria within minutes or hours (ref Haug, Compost Engineering).

Oxygen only diffuses quickly through the waste to the bacteria via the pore spaces between the waste (technically known as the ‘Free Air Space’ - FAS).  As waste decomposes water is released and the waste becomes soft. It begins to compress and water fills the FAS, the oxygen can no longer easily move through the waste and it quickly turns anaerobic. 

Studies prove "passive aeration" (i.e. buoyancy flow using rising hot air to pull cold air in) is sufficient to aerate heaps up to 3m high if the heap maintains  a ‘free air space' of around 15% of volume.

Turing can disturb the soft compressed layers and re-introduce the pores (FAS). However, if the turned material has no ‘self-supporting’ particulate matter, it will quickly re-compress and oxygen flow will stop.  Conversely, if the heap already has a self-supporting particulate structure, it will not need turning!  Such a structure is created by adding ‘bulking material’, i.e. rough coarse pieces of waste such as chopped up woody pruning or wood chips from a shredder. In simple terms: if you add the right materials at the start, you do not need to turn the heap to aerate it.

HOTBIN and Oxygen

The HOTBIN process uses bulking agent to create and maintain pores (FAS). No turning is needed. (Read more on bulking agents).

The taller the compost pile the heavier it will be. Tall heaps compress the compost below and this reduces air spaces and pores. Keeping the heap below 1m avoids excess compression, emptying the HOTBIN every six months also helps.

Tumblers and Oxygen

Tumblers can increase compression and reduce aeration - i.e. the opposite effect of that promised. The weight of material circulating can create a ‘solid’ compressed slug or ball of waste that rises to top of turn then falls to the base 'slamming' the waste together and compressing it. The wetter the waste the more pronounced the problem.

2) Water - How does turning affect the water distribution in the heap?

Bacteria need water to develop. Too much water and the FAS around the waste fills and restricts the oxygen flow (see above). Water is produced via the composting reactions – hence there tends to be too much water in the active part of the heap. Turning a ‘cold’ heap can redistribute water from the wetter middle to the dryer outer layer.

In a hot compost system such as the HOTBIN, a significant quantity of moisture is converted to water vapour, which leaves the top of the bin as steam. The amount of water removed depends on the temperature and FAS, but it does dry away excess water.  There is no advantage turning a hot heap to rebalance water – the system does it for you.

3) Food - How does turning affect the food available to bacteria?

In cold composting the outer layer of the heap decays slower than the central & middle areas, therefore there is some benefit of turning the heap to move the outer layers into the centre where the bacteria are more active.  

HOTBIN and Food

With the insulated walls, the active zone extends to the outer edges - all the waste is composted so there is no need to turn to move the waste into the centre.

4) Core Principal Heat - How does turning affect the temperature of the heap? 

When you turn a cold heap, vast amounts of heat are lost as the hot middle is exposed to the cold ambient air. When new waste is added to the heap the bacteria create some heat as they have new oxygen, however overall the heat benefit is negative. Turning is often used by industrial sites just to reduce temperature (e.g. from 65°c back down to 55°c).

The HOTBIN draws cold air in through the base inlet, this passes through the waste using the free airspaces created by bulking agent. The heat produced by the bacteria breaking down the waste is then contained within the bin due to the thick insulating walls.  

The higher the temperature (up to 70°c) the faster bacteria will compost. This means you can have compost within 30-90 days in a HOTBIN. (Read more on hot composting).

Can I compost without turning? Do I need to turn my compost heap? 

Whether your compost requires turning or not will depend on what kind of compost you wish to achieve, what you wish to compost and essentially which specific composting system you choose.

Can turning fix an anaerobic smelly bin? 

An anaerobic bin arises when no new oxygen can enter because the soft waste has compressed and the water has filled the FAS.

Turning alone will not fix an anaerobic bin.  You will need to fix excess water issues and lack of FAS spaces.. With a HOTBIN, this can be recognised and easily corrected by adding more bulking agent to create FAS and shredded paper to absorb moisture. (Read more on anaerobic bins).