Composting is the controlled partial decomposition of organic materials (those with plant and animal origins) by aerobic or anaerobic biodegradation. It creates moist environments that stimulate the growth of decomposer microbes, although larger creatures such as earthworms and ants contribute to the process. This has the effect of providing and "stabilising" the nutrients, sanitising pathogens in the organic material, increasing the amount of organic matter and producing a useful soil amendment by improving the soil structure. There are a wide range of approaches to composting that require different infrastructure, inputs, and produce different outputs.

Compost is a common name for humus, which is the result of the decomposition of organic matter. Decomposition occurs naturally in all but the most hostile environments (such as buried in landfills or in extremely arid deserts, which prevent the microbes and other decomposers from thriving).

Purpose[edit | edit source]

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.

Inputs[edit | edit source]

Rather than allowing nature to take its slow course, composting provides an optimal environment in which decomposers can thrive. To provide a healthy environment (and nutrition) for the most active microbes (rapid decomposers, bacteria), the compost pile needs the proper mix of the following ingredients:

  • Organic matter - the larger the pieces the slower the decomposition
  • 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. Therefore, rapid bacterial decomposition requires a moist but not soggy environment.
  • Easy sources of carbon and nitrogen: a moderate carbon to nitrogen ratio of around 25:1 is required for hot aerobic composting. Limitations in either may lead to reduced growth of beneficial microbes
  • Optimal temperatures: 'friendly' compost microbes multiply at higher temperatures.

Decomposition happens even in the absence of some of these ingredients, but not nearly as quickly and not nearly as pleasantly (for example, the plastic bag of vegetables in your refrigerator is decomposed by microbes, but the absence of air encourages the growth of anaerobic microbes that produce disagreeable odors).

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.

Outputs[edit | edit source]

Old fan cases make excellent sieves for separating small and large bits of compost.
  • Cool compost piles produce insects and worms.
  • Well built aerobic composting produces heat.
  • At the end of composting you have relatively stable humus.

Decomposers[edit | edit source]

All guidelines for building compost piles have the goal of creating the proper environment for a decomposing ecosystem. The ecosystem in a compost pile is a microcosm of larger ecosystems. The correct environment must be maintained for a healthy and vigorous community of decomposers. In addition to the decomposers that work directly on the organic content of the pile, compost piles provide habitat for those that prey upon direct decomposers. Their waste also becomes part of the compost material.

The most effective decomposers are bacteria and other microorganisms. Also important are fungi, protozoa, and actinobacteria (or actinomycetes, bacteria that are often seen as white filaments in decomposing organic matter). At a macroscopic level, earthworms, ants, snails, slugs, millipedes, sow bugs, springtails, and others work on consuming and breaking down the organic matter. Centipedes and other predators feed upon these decomposers.

Compostable materials[edit | edit source]

Compostable materials are those which biodegrade completely into substances which are healthy or at least harmless for soil and plants. In general, anything which was once living can be composted, although some materials are more convenient to process than others. To be on the safe side, always wear gloves when handling compost, and try to avoid allowing compost to have any contact with eyes, mouth, or breaks in skin.

Kitchen and household waste can be reduced by composting as much of the organic refuse as possible. However, not all household and kitchen waste can be composted, so it's helpful to know what can be and what can't.

Plant materials[edit | edit source]

In general, plant-based materials are easy to handle and pose fewer health risks to humans.

  • Coffee grounds
  • Some tea bags (a staple may be ok, but many bags sold in the US contain a small amount of plastic. This is not very harmful, but is not perfectly compostable - you can tear off the top and throw in the regular garbage, and just compost the tea leaves & lower portion of the bag.)
  • Green waste from the garden, dead tomato plants after the autumn frost, weeds, etc.
  • Yard waste (lawn and hedge clippings). Woody waste will degrade faster if you shred it before adding it to the pile. Large intact limbs may take years to fully decay. If you cannot reduce the size of large wood chunks, place them toward the bottom of the pile so they are subject to the most possible heat and moisture.
  • Leaves - if you live around deciduous trees, you can collect huge amounts of leaves for your compost pile. If your neighbors place their leaves and yard waste at the curb for collection, you can rescue it for your compost pile, thus saving the fuel cost of hauling it to a central processing facility. To reduce the volume of fresh leaves as quickly as possible, shovel old compost over them as you add them to the pile, and wet them down (preferably with graywater or rainwater rather than potable water). If no old compost is available, an alternative is to throw dirt on the pile, which will innoculate the leaves with microbes and break them down faster.
  • Old mushroom compost
  • Tree bark. Pine bark and some other forms of bark need to be composted for a period of some months to break down harmful components.
  • Vegetables
  • Fruit
  • Christmas trees - lop off the branches, add them to your pile, and save the trunks to use as very sturdy tomato stakes in your garden. You can set the trunks in the ground with a post-hole digger. After they rot and fall down you can compost them too. You can collect trees from all your neighbors when they discard them after Christmas. The branches take a long time to compost, unless you shred them first. If you can't shred them, put them directly on the ground, shovel old compost over them, and wait a year.
  • Kitchen scraps containing no animal products - putrescible wastes (i.e., that stink) attract flies and larger scavengers. Follow the same precautions as for kitchen scraps containing animal products.
  • Nuts and seeds
  • Crumbs
  • House plants
  • Natural fabrics (cotton, wool)
  • Wood shavings and sawdust

Animal materials[edit | edit source]

  • Egg shells - straightforward to compost, add calcium to the final product
  • Humanure and composting human feces (but take proper health precautions)
  • Urine - easier to collect and pour on the pile than faeces, poses far less health risk, and provides most of the usable nitrogen from human waste. Nitrogen from urine will feed the microbes that break down brown (carbon) materials.
  • Manure and liquid manure
  • Food scraps containing meat or bones - beware of pests. Meat scraps are often not advised as they can attract animals, but it depends on the context, the amount of meat, and whether the compost is securely covered. To eliminate odors and flies, shovel a thick layer of old compost or brown waste such as leaves over any fresh food scraps you add to the pile. It may help to arrange two piles, one that you add new material to, and one that you use as a source of old material to cover each addition of new material. While scavengers such as raccoons have extremely sensitive noses, there is a limit to how deeply they will dig into a compost pile. You can compost almost anything without attracting pests if you bury it deeply enough in unpalatable compost material. Materials such as meats, skin, etc. will become unpalatable in just a few days if the interior of the pile is active enough (i.e., hot and wet), but animal bones can remain attractive to pests for weeks. In cold weather, if the pile is dormant i.e. cold through the interior, food scraps may remain attractive to scavengers until the weather warms and decay resumes.
  • Hair trimmings, nail trimmings, fur from people or pets.

Man-made materials[edit | edit source]

  • Compostable packaging: paper, compostable plastic and other fully biodegradable materials. Ensure that any "plastic" is actually compostable, and not just "biodegradable" (i.e. breaking down into tiny pieces of plastic). All packaging for food sold at the Sydney 2000 Olympic GamesW was compostable, making for greener waste management for this large-scale event.
  • Teabags
  • Paper towels
  • Cardboard

Other[edit | edit source]

Some like to put special materials and activators into their compost. A light dusting of agricultural lime (not on the animal manure layers) can curb excessive acidity that can slow down the fermentation. Seaweed meal can provide a ready source of trace elements. Finely pulverized rock (Rock dust - Rock flour) can also provide needed minerals, as opposed to clay (which is trace mineral-poor and/or leached rock dust).

The animal manure part of compost source materials can be collected by composting toilets (in this case, human feces). However, such compost is usually not used as a fertilizer for plants that are directly edible (e.g., salad crops) but is instead be used on trees, bush fruits or else on the ornamental garden. Most composting toilets do not allow for the thermophilic activity needed to completely kill off the pathogens and bacteria. However, there is research that shows that if these high temperatures are reached, there is no danger of contamination, and the resulting compost can be safely used on food crops.

Non-compostable materials[edit | edit source]

  • Meat and bones (can be composted but it is not advisable in home composting)
  • Cheese
  • Dairy products
  • Cooking oils
  • Plastics
  • Foil
  • Glossy magazines
  • Disposable feminine products
  • Stickers, sticky tape

Carbon/nitrogen ratio[edit | edit source]

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. The most rapid composting occurs with the ideal ratio (by dry chemical weight) of carbon to nitrogen, from 25-to-1 to 30-to-1 or lower. In other words, the ingredients placed in the pile should contain 30 times as much carbon as nitrogen. For example, grass clippings average about 19-to-1 and dry autumn leaves average about 55-to-1. Mixing equal parts by volume approximates the ideal range. Commercial-grade composting operations pay strict attention to this ratio. For backyard composters, however, the charts of carbon and nitrogen ratios in various ingredients and the calculations required to get the ideal mixture can be intimidating, so many rules of thumb exist to guide composters in approximating this mixture. (For more information check Compostable materials, an introduction to different types of materials that can be effectively composted)

High-carbon sources provide the cellulose needed by the composting bacteria for conversion to sugars and heat.

High-nitrogen sources provide the most concentrated protein, which allow the compost bacteria to thrive.

Some ingredients with higher carbon content:

  • Dry, straw-type material, such as cereal straws
  • Autumn leaves, browned plants
  • Sawdust and wood chips
  • Some paper and cardboard (such as corrugated cardboard or newsprint with soy-based inks)
  • Woody materials. 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.

Some ingredients with higher nitrogen content:

  • Food wastes
    • Greasy food waste and wastes from meat, dairy products, and eggs should not be used in compost because they tend to attract unwanted insects and other animals. Eggshells, however, are a good source of nutrients for the compost pile and the soil although they typically take more than one year to decompose.
  • Fresh green plant material (fresh or wilted) such as crop residues, hay, grass clippings, weeds
  • Animal manures (from vegetarian animals, not carnivores)
    • Poultry manure provides lots of nitrogen but little carbon. Horse manure provides both. Sheep and cattle manure don't drive the compost heap to as high a temperature as poultry or horse manure, so the heap takes longer to produce the finished product.
  • Fruit and vegetable trimmings
  • Seaweeds
  • Used Coffee grounds
  • Sludges, human sewage sludge

In an attempt to judge the proper mix of materials, different rules of thumb are available. Some prefer to add one basket full of nitrogen source followed by one basket of carbon source. Mixing the materials as they are added increases the rate of decomposition, but some people prefer to place the materials in alternating layers, approximately 15 cm (6 in) thick, to help estimate the quantities. Keeping carbon and nitrogen sources separated in the pile can slow down the process, but decomposition will occur in any event.

Systems and approaches[edit | edit source]

Aerobic vs anaerobic[edit | edit source]

  • 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.
  • Anaerobic composting - Also known as the Indore method, where materials are kept moist in a sealed container in the absence of oxygen.

Aerobic[edit | edit source]

There are two primary methods of aerobic composting:

  • Active (or hot) composting: 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 allow the most effective decomposing bacteria to thrive, denature pathogenic microbes (who are usually most active at around body temperature) and seeds of many species of weed and rapidly produces usable compost.
  • Passive (or cold) composting: Letting nature take its course in a more leisurely manner and leaves many pathogens and seeds dormant in the pile. 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.

Most commercial and industrial composting operations use active composting techniques. This ensures a higher quality product and produces results in the shortest time (see compost windrow turner).

Home composters use a range of techniques varying from extremely passive composting (throw everything in a pile in a corner and leave it alone for a year or two) to extremely active (monitoring the temperature, turning the pile regularly, and adjusting the ingredients over time) and combinations of both.

Some composters use mineral powders to absorb smells, although a well-maintained pile seldom has bad odors.

Microbes and heating the pile[edit | edit source]

An effective compost pile is kept about as damp as a well wrung-out sponge. This provides the moisture that all life needs to survive; in a compost pile, it provides an environment in which microbes can begin to do their work. Bacteria and other microorganisms fall into a variety of groups in terms of what their ideal temperature is and how much heat they generate as they do their work. Mesophilic bacteria enjoy midrange temperatures, from about 20 to 40 °C (70 to 110 °F). As they decompose the organic matter, they generate heat, and the inner part of a compost pile heats up the most.

The heap should be about 1 m (3 ft) wide, 1 m (3 ft) tall, and as long as is practicable – the advantage to making the heap at least 1 m³ (1 yd³) is that it provides suitable insulating mass to allow a good heat build-up as the material decays. The ideal temperature range hovers around 60 °C (140 °F), which kills most pathogens and weed seeds and also provides a suitable environment for thermophilic (heat-loving) bacteria, which are the fastest acting decomposers. The centre of the heap should get quite warm, possibly hot enough to burn a bare hand. If this fails to happen, common reasons include the following:

  • The heap is too wet, thus excluding the oxygen required by the compost bacteria
  • The heap is too dry, so that the bacteria do not have the moisture needed to survive and reproduce
  • There is insufficient protein (nitrogen-rich material)

The solution is to add material, if necessary, and/or to turn the pile to aerate it.

Depending on how quickly the compost is required, the heap can be turned one or more times to bring the outer layers to the inside of the heap and vice versa, as well as to aerate the mixture. Adding water at this time keeps the pile as damp as a wrung-out sponge. One guideline is to turn the pile when the high temperature has begun to drop, indicating that the food source for the fastest-acting bacteria (in the center of the pile) has been largely consumed. After the temperature stops rising after the pile has been turned, there is no further advantage in turning the pile. When all the material has become barely recognisable from the original ingredients, turning into dark brown or nearly black crumbly matter, it's ready to use. Some practitioners like to leave the compost to mature further for up to a year as this seems to make the benefits of compost last longer.

Composting at different scales[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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.

Pigs do well turning compost when left with it in a confined area (see Pigs turning compost)

Troubleshooting[edit | edit source]

  • 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


See also[edit | edit source]

External links[edit | edit source]

Projects for Schools

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