How does composting work?


Kitchen Composting Bins

Making good compost

To make compost you will need a 2:3 ratio of ‘green’ and ‘brown’ organic material (for more on this, see our How To Compost page).

In the first stage of the composting process much of the work is done by bacteria that use the nitrogen in the greens to break down the carbon in the browns. To do this efficiently the bacteria, and therefore your compost, needs heat, moisture and air.

There are 2 commonly used methods of producing compost:

  • Hot and Quick: Collect materials until there is enough to fill the bin. Place the materials in the bin in even 2"- 4" alternating layers of green and brown and turn every two weeks.
  • Cold and Slow: A continuous process of adding the materials as they are produced with little or no turning. This is a minimum effort approach but can take up to four times as long as the method outlined above. This is the case for the majority of home composters.

The 3 things good compost needs

Carrots grown with home compost

Heat

A healthy compost heap will generate a lot of heat (up to 50°C at the core) and covering it or containing it in a compost bin will help trap this useful warmth. Putting your heap in a sunny spot will also help, but if it is too exposed it may dry out (which can slow down or even stop the composting process).

Moisture

Keeping your heap or bin covered will allow you to control its moisture content but be careful it doesn't get too dry; on the other hand, if your compost becomes waterlogged it will be deprived of air. In both cases the composting process will slow dramatically.

Air

In the initial stages of composting much of the work is done by 'aerobic' bacteria (those that need oxygen). Later in the process, worms will become important. Both need air to live. Some commercial compost bins have holes in them for aeration but these may be counter-productive and let out too much heat. An active heap or bin should create its own flow of air, with the heat it generates rising up and drawing fresh air into the base.

  • Large heaps have to be turned regularly to let air into their centres.
  • Help a smaller heap by not compacting the compost.
  • Introduce a small amount of twiggy material to help keep its structure open.
  • If you have a large amount of compost to deal with, form it into a 'windrow'. A windrow is an elongated heap about 1.5 metres tall, 2.5 metres wide, and as long as is necessary.

The Three Stages in Compost Production

In some heaps the conditions for composting are never quite right and the material breaks down only very slowly. This is not an ideal state of affairs but the heap will eventually produce usable compost. On the other hand, a good heap should compost in three stages.

Heat stage

Most of the work is done by fungi and bacteria. They multiply rapidly and break down organic matter by digesting it. These oxygen-fuelled micro-organisms work quickly and generate a lot of heat - sometimes sufficient to kill weed seeds. Compost can start to dry out at this stage so keep an eye on it.

Worm stage

After the bacteria and fungi have done their work, the heap cools and earthworms and compost worms - also called brandlings or tiger worms - consume what is left, mixing the compost fragments within their guts. At this stage the compost can be used as a mulch or fertiliser for heavy-feeding plants. Under ideal conditions these first two stages can be completed in two weeks. (If compost worms have not yet found their way to your heap you might consider buying some - see 'Wormeries').

Ripening stage

Over six months to a year, a variety of soil organisms gradually ripens the compost, breaking it down further and sifting it, turning it into a crumbly fragrant humus.

Using Your Compost

Once made, compost is best used as a mulch spread over the soil surface. Worms will do the hard work of mixing the soil and compost by taking it down into the soil to eat, releasing the nutrients in a form that plants can use. Home made compost can even be used to make growing mixtures.

Sowing Compost: half leaf mould, half soil

Potting Compost: equal parts garden compost, soil and leaf mould

Container Compost: half garden compost, half soil