There are two primary types of composting: Cold composting and hot composting.
The most common form of composting most of us will see at the household or small community level will be a cold composting process that resembles what we find in nature. It is a passive system of composting where organic matter is simply piled together in a heap or bin and left to decompose. It might be turned occasionally, checked for moisture or just left to break down naturally. This system can take a very long time to complete the composting process, 6 to 12 months depending on your local environment and seasons. Cold composting has the most opportunity to turn anaerobic and begin to smell, which is usually the only time it’s turned.
The act of turning the compost disperses the moisture in the compost more evenly and introduces oxygen to the pile (aeration). This aeration process kick-starts the slow decomposition process in the cold compost pile and it will begin to warm. Left alone, the decomposition process will begin to slow back down and the temperature will drop.
So, why does compost heat up?
Compost heat is produced as a by-product of the microbial breakdown of organic material. If the pile has enough active microbial content and is turned often enough (every two or three days) to keep the decomposition process peaked at high enough levels, the overall heat of the pile goes way up and the composting process speeds up considerably. This is the primary difference between cold (passive-anaerobic) composting and hot (active-aerobic) composting.
There are other factors that play into what separates hot and cold composting, but most of them are based on managing the decomposition rate. One hot composting method developed by the University of California, Berkley, has the potential to create finished compost in under 20 days. The process, called The Berkley Method, focuses on balancing the C:N ratio and aerating the compost pile on a regular schedule to maintain a temperature between 130-150 degrees Fahrenheit (55-65 degrees Celsius).
The Berkley Method
- Compost temperature is maintained between 130-150 degrees Fahrenheit (55-65 degrees Celsius)
- The C:N (carbon:nitrogen) balance in the composting materials is approximately 25-30:1
- Build the pile by alternating layers of greens and browns.
- Water each layer until it is moist as you build the heap
- The compost heap needs to be roughly 4-6 feet high (1.5m)
- If composting material is high in carbon, such as tree branches, they need to be broken up with a mulcher
Compost is turned from outside to inside and vice versa to mix it thoroughlyWith the Berkley Method, the procedure is pretty simple:
- Build compost heap (4-6 feet high)
- 4 days – no turning (leave the pile alone for a while to let the initial decomposition process start)
- Then turn every two days for 14 days
Importance of C:N Ratio in Hot Composting
In the hot composting method, the ratio of carbon to nitrogen in the compost materials needs to be between 25 to 30 parts carbon to one-part nitrogen by weight.
This is because the bacteria responsible for the composting process require these two elements, in these proportions, as nutrients to construct their bodies as they reproduce and multiply. If the ratio of C:N is right in this technique of fast, aerobic (uses oxygen), hot composting, the compost will break down to the same volume. This is in contrast to slow, anaerobic (without oxygen) composting that happens in a compost bin, which drastically reduces in volume as it rots down.
Don’t get too caught up in the math of managing carbon and nitrogen ratios. It can be fairly simple to rough out. Basically, if you want to to get started in a hurry, aim to use 1/3 Manure and 2/3 dry carbon materials. It will work. Just pile alternating thin layers of greens and browns until you end up with a compost heap that is about 3 ft square and a bit taller than that. It’s really a matter of trying it out. You can use a long soil thermometer to monitor your internal heat.