Hot composting is a process that rapidly decomposes and transforms raw materials into usable compost. It’s hot because it needs to reach at least 65 degrees Celsius (150 degrees F) for at least three days. Any part of the pile that doesn’t each those temperatures for a sustained period of time, won’t be effectively treated.
Hot composting can transform pathogenic raw material into useable compost. It can kill viruses and cook weed seeds. Once our hot compost is finished, we can add it to the garden or use it to transform dirt into soil. My blog on creating healthy soil discusses that process.
Ingredients in a hot compost
To reach the desired temperatures, hot composting needs at least one cubic metre of raw material. That’s a big ask for most people and most gardeners. Overall we need a raw ingredient ratio of 1:3, nitrogen:carbon.
Nitrogen based ingredients are the green materials like fresh lawn clippings and green leaves from shrubs and trees. Dry leaves, dry grasses and dry mulches like sugarcane mulch make up the carbon ingredient and brown matter component. The smaller the pieces the faster they break down.
So if we add dried leaves for example, perhaps run a mower over them and break them into smaller pieces. Sticks and woody materials are high in carbon but they take a long time to break down. I don’t use these in hot composts. They go into the cold compost pile.
We want three times the amount of carbon materials to green materials.
We also want manure because the manure will help the composting process by adding vital microbes in sufficient quantities and diversity. I like to use horse manure mainly because it’s readily available to me however, you can use any type of manure you like. If I had chickens I’d use that for my edible gardens. I wouldn’t use it for my native plants as it is high in phosphorus. If you have an abundance of horse manure, you can build a horse manure compost and my blog on that discusses the process.
I like to have between 10% and 25% of my hot compost composed of manure. For hot composting, it’s better to use fresh manure.
Hot composting method
The better the compost ingredients are mixed, the more effective the process. So in the absence of a mixer, we need to make layers… the more layers the better. Always keeping in mind the ratio of green to brown as 1:3.
On the ground place a layer (10cm thick) of carbon materials (sugarcane mulch, dried leaves), and water it. Then we add the layer of manure (1-4cm thick) and water it. Follow this with a layer (2-5cm thick) of the green nitrogen rich layer (lawn clippings, fresh leaves) and then back to brown carbon and then manure and nitrogen green.
I like to put the manure directly on top of the carbon matter so that microbes in the manure have direct access to the carbon. As water seeps down through the pile, microbes will wash through this carbon layer and this tends to speed up the spread the microbes through the pile.
Once we’ve built the pile cover the entire pile with a layer of the brown carbon material. I like to use sugarcane mulch for this rather than leaves. Cover the pile top and sides. This helps to hold the heat in increasing the likelihood that any weed seeds will cook and it also reduces nitrogen from vaporising into the atmosphere. Wet the outside layer of carbon material with water.
Note that if we add a tarpaulin or black plastic or something like that we are effectively suffocating the pile. Effective composting requires air and oxygen. Without air the pile will soon turn pathogenic.
Maintaining a hot compost
Effective hot composting does not require sunlight or trapping the heat. The hotness of compost comes from microbial activity. In particular, thermophile microbes – largely bacteria – thrive in environments hotter than 65 degrees Celsius (150 degrees F). These microbes kick start the decomposition of our raw materials.
Generally, pathogens and viruses in raw compost materials need 3 days above 65 degrees Celsius to cook away. To maintain that heat, we need to supply vital conditions to the thermophilic microbes.
Thermophilic microbes need a lot of oxygen. All their activity, generating all that heat burns through the available oxygen in the pile. That’s why we need to turn the pile regularly and supply them with fresh oxygen.
We can turn the pile and add water if the pile is drying out. The water will cool the pile for a short period of time, but without the moisture the microbes will die… even with fresh oxygen.
I tend to turn my hot compost every second, maybe third day. But this depends on the pile. If it’s getting too hot or if there are a lot of lawn clippings, I’d turn it more often.
Turn it over and try to get the outside raw materials into the middle so that they can cook. As we turn it we’ll notice the heat and steam.
It’s incredibly hot work, particular in the middle of summer. However, if we neglect it, a hot compost can turn anaerobic and pathogenic very quickly. If we fail to turn it in time we may find ourselves with a pile of stinky, ammonic, pathogenic dung! We can’t put that on the garden…
After a fairly short period of time the thermophilic microbes will be replaced with the cooler temperature microbes. This may be a week or it could be three weeks. It all depends on the ingredients, our management of the pile and other factors. Once the pile cools naturally, we don’t need to turn it.
Once it cools we can let it rest. At this point worms and other creatures will start to enter the compost system. They’ll finish what the thermophiles started.
What the hot compost final product looks and smells like
If we’ve done it correctly, our final product looks dark brown or black in colour. There will be no or very few recognisable raw ingredients. It’ll feel soft in our hands and full of worms.
It’ll smell earthy. It’s a smell of black gold. When it’s got these characteristics it’s ready to add to the garden.