Regular composting, also known as “cold composting”, involves placing a variety of organic materials in a compost bin, enclosure, or even just in a large heap, and leaving it there until it breaks down several months later. It’s a very slow process and typically takes 6 to 12 months. It can be sped up by turning the compost, that is, moving around the material at the bottom of the heap to the top and vice versa to mix it up and get more oxygen in there, but it’s still a long wait.
The other approach to composting is “hot composting”, which produces compost in a much shorter time. It has the benefits of killing weed seeds and pathogens (diseases), and breaking down the material into very fine compost. In contrast, cold composting does not destroy seeds, so if you cold compost weeds, any weed seeds will grow when you put the compost into the garden.
Cold composting does not destroy pathogens either, so if you put diseased plants into your cold compost, the diseases may spread into the garden, hence the common advice not to (cold) compost diseased plants. The other issue with cold composting is that you end up with lots of large pieces left over in the compost when the process is completed, whereas hot compost looks like fine black humus (soil).
One hot composting method, the Berkeley method, developed by the University of California, Berkley, is a fast, efficient, high-temperature, composting technique which will produce high quality compost in 18 days.
The requirements for hot composting using the Berkley method are as follows:
- Compost temperature is maintained between 55-65 degrees Celsius
- The C:N (carbon:nitrogen) balance in the composting materials is approximately 25-30:1
- The compost heap needs to be roughly 1.5m high
- If composting material is high in carbon, such as tree branches, they need to be broken up, such as with a mulcher
- Compost is turned from outside to inside and vice versa to mix it thoroughly
With the 18 day Berkley method, the procedure is quite straightforward:
- Build compost heap
- 4days – no turning
- Then turn every 2nd day for 14 days
Composting Materials and the Carbon-Nitrogen Balance
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.
- Materials that are high in carbon are typically dry, “brown” materials, such as sawdust, cardboard, dried leaves, straw, branches and other woody or fibrous materials that rot down very slowly.
- Materials that are high in nitrogen are typically moist, “green” materials, such as lawn/grass clippings, fruit and vegetable scraps, animal manure and green leafy materials that rot down very quickly.
If 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.
Many ingredients used for composting do not have the ideal ratio of 25-30:1. When using hot composting methods, you need to make an assessment of how quickly materials decompose, and then use a blend of things that rot quickly and things that rot slowly.
- Composting materials with a very low C:N ratio of 7:1 would rot very quickly, because they are high in nitrogen, eg. fish, this decomposes very quickly
- Composting materials with a very high C:N ratio of 500:1 would take a long time to decompose, because they are low in nitrogen, and need to be broken up, eg. tree branches
For example, if the C:N ratio is too high, you can lower it by adding manure or grass clippings. If the C:N ratio is too low, you can raise it by adding cardboard, dry leaves, sawdust or wood chips.
In trying to understand what C:N ratios are about, it may help to point out that all plants have more carbon than nitrogen (remember, they get their carbon from the carbon dioxide CO2 in the air) so that is why the C:N ratios are always greater than always above 1:0.
Below are the average C:N ratios for some common organic materials used for composting.
- The materials containing high amounts of carbon, but low in nitrogen are considered “browns“
- The materials containing higher amounts of nitrogen are considered “greens.”
Here are some C:N ratios of some common composting materials:
|Browns = High Carbon||C:N|
|Greens = High Nitrogen||C:N|
Anything that was once living can be hot composted – and I really do mean anything. All manner of things, including strange additions such as wool and cotton clothing, bones, leather boots, even things like “roadkill”, ie. dead animals, but these have to go in the very centre of the heap to break down properly. There’s no trace of the original ingredients when the process is complete! Remember, the greater the variety of ingredients, the better the compost, because it will have a wider range of nutrients in the final product.
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 1 metre square and a bit taller than that. There’s no real need to get caught up in the mathematics of precise C:N ratios. It’s more a matter of trying it our, though I can’t stress how easy it all is.
To illustrate the point, a friend with a small with only a courtyard (in a rental property) wanted to attempt hot composting, and I helped him out with the project. He gathered a wheelie bin full of fallen leaves from his local street, one wheelie bin full of weeds from his garden, purchased a small straw bale for the sake of it. I helped him collect a few garbage bags of cow manure from an urban farm. It took us under an hour to pile it all up in reasonably thin layers (under 5cm) of each ingredient to get a good mix.It was his first attempt at hot composting, and in around 18 days, he had over 1 cubic metre of rich, dark, compost to use in his garden. You couldn’t distinguish any of the original ingredients in the final product either, and it had a very fine consistency. Best of all, it cost him next to nothing – the straw bale was just a $17 luxury, it would have worked just as well without it, and without it it would have cost absolutely nothing.
Just think that 1 cubic metre is 1,000 litres, and if you think how much you pay for a 30 litre bag of potting mix (over $10) here in Australia, you realise what value this entails.
Hot Composting, Step By Step
Some important points to note:
- Locate your compost heap in an area protected from too much sun or heavy rain, to prevent the compost from drying out or becoming water-logged and slowing down the composting process.
- Space required for for your heap should be about 1.5 x 1.5 metres, and enough space in front of it to stand when turning the compost.
- Water each layer until it is moist as you build the heap. After three or four days, give the compost air by mixing and turning it over, then turn every three days until the compost is ready, usually in 14-21 days. Remember, frequent turning and aeration is the secret of successful composting.
- Turn the compost using a garden fork, or even better, a long-handled pitchfork.
- In cold or wet weather, you can cover the compost heap with a tarp or plastic sheet, to prevent the rain cooling it down, since the water will penetrate into the core of the compost pile. Even though cold outside air will cool the surface, but not the core of the compost heap, by covering it, this prevents some heat loss from the surface to cooler outside air, and retains the heat within the compost heap better.