Cold vs Hot Composting

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.

Cold composting does not destroy pathogens (diseases) or seeds.  So, if you put diseased plants or weeds into your cold compost, the weed seeds will grow and the diseases may spread when you compost your garden. Hence the common advice not to (cold) compost diseased plants or weeds. 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.

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, and breaking down the material into very 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:

  1. Compost temperature is maintained between 55-65 degrees Celsius
  2. The C:N (carbon: nitrogen) balance in the composting materials is approximately 25-30:1
  3. The compost heap needs to be roughly 1.5m high
  4. If composting material is high in carbon, such as tree branches, they need to be broken up, such as with a mulcher
  5. 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:

  1. Build compost heap
  2. 4days – no turning
  3. 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.
  • Compost 1Compost Browns
  • 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.
  • Compost greensCompost greens

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.

Compost process

 

 

 

 

 

 

 

 

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.
The materials containing high amounts of carbon, but low in nitrogen are considered “browns

  • The materials containing higher amounts of nitrogen are considered “greens.”

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”, i.e. dead animals, but these have to go in the very center 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.

NOTE: If ratios seem too complicated or confusing, you can work with volumes of ingredients instead to simplify things, aim to use 1/3 ‘greens’ (nitrogen containing) materials with 2/3 ‘browns’ (dry carbon materials). In other words, add one bucket of nitrogen-rich material to every two buckets of dry carbon-containing material.

Basically, if you want 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 meter 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 out, though I can’t stress how easy it all is.

Happy Composting