Composting is the partial decomposition of organic materials by aerobic or anaerobic processes. Composting techniques create moist environments that stimulates to growth of decomposer microbes. This has the effect of "stabilising" the nutrients, sanitising pathogens in the organic material and producing a useful soil amendment. There are a wide range of approaches to composting that require different infrastructure, inputs, and produce different outputs.
- organic matter - the larger the pieces the slower the decomposition
- nitrogen - a moderate carbon to nitrogen ratio of around 25:1 is required for hot aerobic composting.
- water - rapid bacterial decomposition requires a moist but not soggy environment.
- Cool compost piles produce insects and worms.
- Well built aerobic composting produces heat.
- At the end of composting you have relatively stable humus.
Purpose of composting
There are two main purposes of composting:
- it is a waste management technique as it reduces the volume of waste materials. In some circumstances it may also have a beneficial sanitising effect on dangerous microbial pathogens in the waste
- creation of compost from the plant matter, which is a very useful compound in farming. It is useful for this as
- it improves soil structure, improves aeration, water-retention, erosion problems, and makes the soil easier to work.
- when used as mulch, it slows the growth of competitive weeds between rows and around plants. See mulching for further details.
- it adds nutrients and organic matter to the soil. As a sponge holds water, so organic matter helps to retain nutrient ions in the soil that the plants need to grow. These nutrients may not have originated in the compost - so soils with higher levels of organic matter can be said to be more fertile than those with much lower levels.
- Oxygen and water: whilst composts can be too dry, it is more of a problem when it is too wet, as the water displaces oxygen within the matrix, which may prevent efficient aerobic microbial activity.
- Easy sources of nitrogen and carbon: limitations in either may lead to reduced growth of beneficial microbes
- Optimal temperatures: 'friendly' compost microbes multiply at higher temperatures.
When conditions are not optimal, the process may be slowed, may not happen at all, or may not destroy harmful microbial pathogens. Care should be taken as it is possible to have areas of poor conditions within a compost heap that is performing well. High or low levels of moisture may also have a impact on the length of time it takes for the organic material to stabilise into compost. Compost is dark in colour (exact colour will depend on the materials used), consistent (in the sense that it is hard to identify what was used to make it), is fairly fine and does not contain many contaminants (such as pieces of plastic, large pieces of wood and pieces of uncomposted wastes). In most places with good management, stable compost can be produced within 8 weeks. However systems which are not ideal (sometimes known as cold composting may take a lot longer. Composting toilets should be left to mature for at least a year after use and may still not be fully stable.
Composting is the cumulative digestion effect of many different kinds of organism and may involve several different steps and various types of organism, which may include worms, microbes and other biota.
- aerobic composting - the careful assembly of materials and moisture into a pile that rapidly heats up either in a windrow or a bin, to produce a high quality compost that can be used in gardens, particularly for clean cultivation.
- anaeroboic composting - Also known as the Indore method, where materials are kept moist in a sealed container in the absence of oxygen.
- sheet mulching - where a nitrogen rich layer is placed under a carbon rich layer across an area of ground, often used with a paper weed barrier layer to convert existing vegetation to a mulch garden
- mulching - where organic matter is placed in layers on the ground surface.
- brush piles - coarse material is piled up and breaks down very slowly while providing some habitat in the interim
- humanure - methods used to treat human feces to kill pathogens and parasites so it can be used in the garden.
- worm boxes - use manure worms to digest food waste and produce worm castings.
It is important to understand the carbon/nitrogen (C/N) ratio as this will have a dramatic effect on the success of a composting system.
Sludges, animal wastes and fresh green materials are high in nitrogen. Straws, browned plants and woody materials are high in carbon. If using woody materials, chopping will be necessary as the microbes require a high surface area to get to the carbon in the wood, which is often stored in a way that is difficult for them to access. On the other hand, having larger pieces of material within the compost may assist the process as it will aid air circulation. It may be necessary to remove these towards the end of the process.
Successful composting requires a mix of organic materials as large amounts of high N (nitrogen) material will lead to high losses of N and other undesirable processes. As a rule of thumb, a target ratio of carbon: nitrogen should ideally be 30:1 or lower. Materials which are high in N, such as animal manures, human sewage sludge, food wastes etc should therefore be mixed with large amounts of materials which are high in carbon, such as straw, wood, sawdust etc. Compostable materials is an introduction to different types of materials that can be effectively composted.
In ideal conditions, the rapid respiration of microbes within the compost will lead to high temperatures and so this is sometimes refered to as "hot composting". Higher temperatures are highly desirable as it will denature pathogenic microbes (who are usually most active at around body temperature) and seeds of many species of weed.
Cold composting is composting in sub optimal conditions. Materials may be left in heaps without attention, mixing or protection from rain. Composting may be slow, may not take place at all or the conditions may even encourage undesirable processes to occur. Cold composting should therefore be avoided and the products may need additional processing or composting to be safely used.
Composting at different scales
Many different types of composting are conducted in different places around the world and at different scales. These range from bin composting at a household scale all the way to full industrial scale windrow composting.
Pile or bin composting
In fairly small scale composting, bins, piles and heaps are used. Bins may be produced from various different kinds of materials and can help to produce good compost if they assist with the flow of air through the heap. The disadvantage of bins is that the compost can be more difficult to turn and aerate. A heap on the ground can be seen as an easier option, however care may be needed to prevent the material from speading and to ensure it is adequately aerated. This page contains more information about home composting
Regarding the types of bin, plastic bins such as the geobin system have been used to produce compost of reasonable quality, but bins made of wood or other locally available materials can be used just as effectively. Sometimes bins have been designed with special equipment to aggitate and aerate the compost, which can also be a good way to ensure that the compost is properly aired without the user having to manhandle the compost.
See this technical brief about manufacturing bins.
This is a nice video by British composting expert Nicky Scott on home composting
Industrial scale composting
Windrows are large compost heaps which can be hundreds of metres long. They are usually this shape because specialist turning equipment is used to move along the whole length to turn and mix the compost. Arcata Marsh is an example of a large industrial scale composting plant.
Farm waste composting
In many farm systems, animal extreta from animal houses is mixed with urine and bedding and stored in bays beneath the buildings, to be excavated and removed at a later date. The effectiveness of this as a composting system will depend on the animals and the bedding materials used, but farmyard manure can often be used as a valuable soil amendment. In many areas it makes more sense to remove and store the waste away from the animal houses due to problems with dust and odors. Care should be taken when removing animal wastes from underground storage pits as there may be hazards from airborne pathogens and the material may not be adequately sanitized.
- Regularly turning compost to encourage the movement of air is always a good idea
- Increase surface area of materials by chopping them up.
- If your compost is smelly it is not getting enough oxygen (going anaerobic).
- If space is an issue (ie if you are living in a house or apartment with not much of a yard), you could try vermiculture (i.e. using a worm bin).
- Animals (ie bears, raccoons, rats, and other animals) may become a problem if there is something in a compost pile they would like to eat. Access to compost must be restricted (ie by using a high, fully closed bin); avoiding the scraps that attract them too can help, especially meat, fish.
This is another video about troubleshooting home compost
- Composting human feces
- Home composting (Practical Action Brief)
- Compost bin manufacture (Practical Action Technical_Brief)
- High fiber composting
- chickens - can be used to prepare materials for composting
- International Compost Awareness Week
- low nitrogen compost piles - can be used to grow vining tubers like potatoes
- Composting, vegan permaculture, native plants - a podcast by Paul Wheaton and Helen Atthowe (very informative on composting)
- Territorial Seed
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