Hot composting is a way of composting that uses various techniques to achieve higher temperatures than the traditional method of composting. As a result, you can add a wider variety of food waste and compost will be produced faster.    

A compost bin isn’t absolutely necessary for hot composting – you can hot compost in a large pile – but using a container makes the process more manageable and less messy.

Hot composters are designed to nurture heat-generating micro-organisms and to retain the heat they create as they break down organic waste and multiply.

For hot composting, you need the right ingredients in the correct proportions mixed properly.

What are the advantages?

Hot composting takes more effort than traditional composting but most people find it’s worth it for the benefits, such as:

• A wider range of foods can be composted that are not usually recommended for composting, such as cooked food and meat.
• Pests are deterred because food scraps break down faster at higher temperatures.   
• Pieces of natural clothing, such as wool, cotton and linen, can also be added. These usually take a long time to break down in traditional composting (usually called cold composting).
• Weeds can be killed at high temperatures.
• Faster compost is produced in around 4-6 months. Cold composting takes between 6- 18 months.

Materials:

The eco system in a compost bin thrives on a balanced diet of nitrogen and carbon. Nitrogen is needed by the micro-organisms for growth and reproduction, while carbon provides them with energy.  

Nitrogen-rich materials are wetter and faster to break down. Carbon-rich materials are dry and slower to break down. In composting terms nitrogen materials are often referred to as Greens and carbon as Browns.

Nitrogen (Greens)

Food waste

Coffee grounds

Fresh green garden waste, such as grass mowings and green leaves

Wilted flowers

Seaweed/kelp

Tealeaves, tea bags (non-plastic)

Natural fabrics

Manure from herbivore animals

Carbon (Browns)

Dead leaves

Paper

 Cardboard

 Woody garden waste, such as twigs, branches and stalks

Wood chips from untreated wood

Sawdust from untreated wood

Pine needles/ cones

Straw

Balance

Ingredients are added in proportions referred to as the Carbon: Nitrogen ratio (C:N ratio). The ideal C:N ratio is considered to be around 30:1 but it would be difficult to calculate this exactly as every item has a different ratio. Most plant materials are a mixture of both, but more carbon than nitrogen as they get older and tougher.  You don’t have to be too precise with this or worry about it too much.  

A good guide is to simply add equal amounts of carbon to nitrogen (followed by a thin covering layer of carbon to prevent smells), then observe the bin’s contents and make adjustments if your compost lets you know something is wrong.

For example, if your bin starts to smell unpleasant, it’s likely that there’s too much nitrogen and not enough air. If breakdown is very slow, there’s probably too much carbon.

Adding waste

Hot composting can be achieved using different methods; our focus here is on active home composting where householders use one or two bins in an ‘add as you go’ system, adding food waste as it occurs every couple of days, mixed with carbon content.

Aeration

Good airflow is essential for hot composting. The micro-organisms that create heat are aerobic so need oxygen to survive. This is achieved by regularly aerating the bin’s contents, stirring and turning the waste materials with an aerator stick.  

In the immediate aftermath of turning there will be a dip in temperature but ultimately it will result in a higher bin temperature for a longer time.

At the outset, covering the bin’s base with a layer of 15-20cms of twigs will help to create airflow through the contents.

Aerating compost:

• Helps all materials to make their way to the heat in the middle and to decompose at an even rate. 

• Prolongs the time that the bin stays at higher temperatures.
• Helps to distribute water and nutrients to all areas. 

Moisture

Moisture is essential for efficient composting. The contents should be moist but not saturated, with a moisture content of roughly 50 per cent.

The consistency of compost should be damp like a wrung-out sponge.  You can check this by doing a squeeze test – take a few handfuls of compost from different parts of the bin and squeeze – only one or two beads of liquid should be visible.

If compost becomes too wet the water will deplete the amount of oxygen in the bin. Then microbes that thrive in anaerobic conditions will take over, causing slow breakdown and an unpleasant smell. In this case, you should add more carbon content and aerate well.

As micro-organisms become more active, they use up water, causing the compost to dry. In dry compost, microbes will not be active or reproduce.

 If you are regularly adding food waste, there will usually be enough water content in the bin. If you do need to add water, ideally use stored rainwater from a water butt added to a small watering can with a fine rose spray.

Chop or shred

For hot composting, it’s best to chop or shred the materials before adding them. This increases the surface area in contact with microbes in the pile. Smaller pieces – ideally 2 to 5 cm – also make turning the pile much easier.

If you have access to a shredder, you can shred Greens and Browns at the same time, blending materials as you shred. This creates a fine-textured mix that heats up quickly.

Alternatively, you can chop materials using a pair of hedge clippers and a board.

Large pieces will still break down but will take longer. You can get away with larger pieces if you also have other ingredients with a much smaller surface area, such as grass or sawdust.

 If items are too small, they can clump together, reducing the ability of air to circulate through the bin.

What happens?

When there’s enough carbon to fuel these billions of micro-organisms, enough nitrogen and water to help them grow, and enough oxygen to let aerobic bacteria thrive, composting magic happens.  

Heat builds in the bin and heat-loving bacteria multiply, breaking down the materials.

As the temperature increases in the bin, different populations of micro-organisms rise and fall, either going dormant, or becoming fodder for the next wave of bacteria that thrive in even hotter temperatures.

When temperatures are between 20-40 degrees Celsius, this is called the mesophilic phase; from 40-70 degrees Celsius is the thermophilic stage when fast, hot composting takes place.  If temperatures go above 72 degrees Celsius this is too hot for the aerobic micro-organisms to survive. This will cause the process to slow down or stall.

The power of observation

Composting depends on a blend of factors: contents, moisture, size of materials, size of bin, the composter’s efforts, climate and soil conditions.  Even neighbouring households will experience different outcomes due to a difference in diet.  

 Ultimately the best guide is your own observations – how your bin looks and smells. With experience you develop a feel for your materials and can tell what’s needed just by looking.

 Most people find the fact that they can influence the quality and speed of their compost a rewarding aspect of hot composting.  

Why a Green Johanna works well for hot composting

• The Johanna has been designed so that vents take air in at the bottom to send it upwards through the bin. Incoming air goes up past the four ventilation plates on the inside of the base plate, past the maturing compost layer up into the decomposing compost where it provides oxygen for the micro-organisms.
• Ventilation in the lid helps control airflow depending on conditions. Switch to minimum in cold weather to keep heat in the bin.
• Use of the Insulating Jacket helps to control temperature.
• The conical shape encourages compost to fall back into the centre and not stick to the sides.
• The generous capacity of 330 litres means the mass of materials acts as an insulating factor.
• The Green Johanna is recommended as a Best Buy by Gardeners’ World magazine and Which? consumer group.

For more information about composting in general we recommend A Gardener’s Guide to Composting Techniques by master composter Rod Weston.