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Composting is the aerobic (oxygen using) decomposition and stabilization of solid organic materials by aerobic organisms. This process can be done under controlled conditions (optimal conditions and environmentally safe) but also occurs in almost every environment where solid organic materials are supplied with oxygen, moisture and the right temperature. The created compost should be a stable and hygienic substance which is rich in humus and looks like soil. The word composting originates from the Latin word "componere", which means blend, merge. Actually it is one of the oldest forms of recycling of waste.


The purpose of the composting process is the removal of the biodegradable part of the organic materials, reducing volume, mass, particle size and humidity of the original waste. This process transforms the waste into a valuable soil conditioner that can be used for gardening and agricultural purposes. The compost favors the oxygen concentration in the ground and helps to regulate the water content of the soil due to the better structure of the soil, after adding compost. At rainy days, the drainage of (especially heavy) ground is better when compost is added to the soil, in warm periods (especially with sandy soil) the dehydration slows down when compost is provided. The biodegradable part consists of saccharides (glucose, fructose, lactose, sucrose, starch), proteins and most fats. In the case that there is no oxygen available, the composting process is impossible and an anaerobic digestion takes place. This causes production of several gases (methane, small amounts of hydrogen sulfide and hydrogen gas...), resulting in bad smells.

The conversion process

The active composting process takes place at the surface of the composting particles. Every particle consists of an anaerobic inner core, a partially aerobic layer below the particle surface, an outer aerobic surface layer and an aerobic liquid film surrounding the particle. The microbial community lives in the surrounding liquid layer, so while the composting proceeds, the particles shrink till the original raw materials are discernible. To stay alive, reproduce and regulate itself, every living being needs water, energy sources (light (phototrophs) or chemicals (chemotrophs)) and nutrient sources (macronutrients: C, H, O, N, P, S, P, Mg, S, Ca, Fe and some micronutrients: Co, Cu, Mn, Mo, Ni, Se, W, Zn) and sometimes some growth-factors are necessarily. In this case the energy (in the form of electrons from oxido-reduction-reactions) is coming from the oxidation of organic matter with oxygen (chemo-organotrophic), and the carbon source is the organic matter (heterotrophic), so these organisms are called hetero-chemo-organotrophs. The micro-organisms produce enzymes to do the job, these are proteins acting as a catalyst in the oxidation of the organic waste and in producing microbial biomass. This is the overall conversion during the composting:

C6H12O6 + 6O2 => 6CO2 + 6H2O + energy (2830 kJ mol-1)

Since the release of heat is directly related to the microbial activity, temperature can be used as an important process indicator. During the initial days, the readily degradable compounds are metabolized and the temperature rises fast, depending on the conditions the temperature can rise well above 60°C. This high temperatures causes the weed seeds and pathogens to be killed, but also the desirable composting micro-organisms begin to die or at least slow down. Normally the temperature increases fast in the initial days up to 45-60°C, and stays in this interval for several weeks, this is the active composting phase. When the readily available compounds become depleted, the metabolic rate slows down and the temperature decreases slowly till ambient air temperature.

Recommended conditions for rapid composting
Condition Reasonable range Preferred range
C/N ratio 20/1 - 40/1 25/1 - 30/1
Moisture content 30-60% 40-50%
Oxygen concentration >5% >>5%
Particle size (diameter in cm) 0.3-5 varies
pH 5.0-9.0 6.5-7.5
Temperature (°C) 40-65 50-60

After the active composting phase, there is a maturation period. During this period, the composting process goes on, but at a much slower rate. Due this slower rate, the oxygen consumption decreases, and the temperatures stay lower. While this process continues, the amount of humus increase, and nutrients (like nitrogen) are stored within stable organic compounds. This reduces the immediate availability of nutrients to the plants and allows them to be released at a more gradual rate. This rate usually follows the pattern of the needs of the vegetation: faster in warm periods, slower in colder periods. After the active composting phase, most nitrogen is readily available in the form of ammonium (NH4+). Concentrated amounts of ammonium can cause damage to several plants. Some of the processes occur only at low temperatures or in well-decomposed organic matter, like this conversion (oxydation) of ammonium to nitrate (NO3-) which is less harmful to plants. Because the composting process does not stop at a particular point, the process can go on till only "energy-exhausted" organics and inorganics remain. However, compost becomes relatively stable and useful long before this point. An immature compost however continues to consume oxygen after application to the field and thereby reducing the availability of oxygen in the soil to the roots of plants, and diminishing their growth. An immature compost can also contain high levels of organic acids (low pH) or have a high C/N ratio (carbon/nitrogen). If the C:N ration is greater then 30:1 the compost will tie up nitrogen from the soil, and this can cause competition between the plants that need the nitrogen for growth and the nitrogen needed for the stabilization of the organic matter. Immature compost can thus cause damage to crops and plants. Compost is judged to be stable by characteristics such as C/N ratio, oxygen demand, temperature and odor.

Indicators of compost stability:

  1. Temperature: if the temperature of the compost is more then 8°C above ambient air temperature, the compost is judged as fairly unstable.
  2. Respiration rate: the oxygen uptake indicates the microbial activity in the compost, the higher the oxygen uptake (and carbon dioxide release), the greater the activity in the compost. If the temperature is not limiting the processes in the compost (not to low nor to high temperature), the O2 uptake rate or the CO2 release can be indicative for the stability of the compost:
    • < 5 mg CO2 carbon/g compost carbon/day: the compost is considered stable and suitable for seeds.
    • > 20 mg CO2 carbon/g compost carbon/day: the compost is considered fairly unstable
  3. Length of the composting process: compost made by the aerobic windrow method needs a minimum of 60 days to complete the active process, and another 30-60 days for the maturation period. However, some experts suggest a curing period up to 6 months before using the compost.
  4. Carbon/Nitrogen ratio: during the compost process the C/N-ration will decrease, so we can use this ratio to determine the stability of the compost. A good ration at the beginning of the composting process is 30:1, this compost will be finished if this ration reaches a value between 15:1 and 20:1. This ratio is dependent on the ratio in the beginning process and on the composting materials (the amount of readily available organics or proteins for example).
  5. Olfactoric judgement: the look, the smell and the feel of the finished product gives an indication of the maturity, these techniques are not very reliable, although they can be valuable because they are cheap and very fast to execute.

Factors affecting the composting process

Oxygen and aeration

The major oxygen supply for the microbial population in the compost is the air present in the pore space of the composting mass (the interstices). The amount of pore space depends on the size, size variability, form and hardness of the particles, and the amount of water present. To prevent the formation of bad smells, on should take care of the airiness of the blend.

There are 2 important processes to provide the micro-organisms with oxygen:

Bulk movement: this is the movement of air in the pores due to mechanical forces. This movement can have different causes: density difference between the warm (lighter) air in the pile and the cold (heavier) surrounding air so that the warm air will lift and cold air will move into the pile at the bottom; wind that strikes a composting pile moves air in the pores; an electric blower or turning the pile (forced aeration). The aeration not only provides fresh air to the composting mass, but also removes heat, water vapor and other gases trapped in the composting mass. When the aeration is too excessive, the pile can cool down and eventually dry out when there is more heat and water vapor removed than there is produced by the composting process.

Diffusive movement: diffusion is the movement of atoms or molecules (for example: O2) from higher concentration to lower concentration: the driving force is the gradient (concentration difference) between 2 places.

In composting piles there are 3 oxygen gradients:

  1. between the surrounding air and the air in the pores of the pile
  2. between the air in the pores of the pile and the liquid layer around the composting particles
  3. between the liquid layer around the composting particles and the micro-organisms at the interception between the liquid film and the particle.

Because the oxygen concentration is decreased after each step, only a small concentration of oxygen is available for the micro-organisms. When the aeration is not sufficient, oxygen can be a restricting compound in the composting process.


Water provides the medium for chemical reactions, nutrient transport and allows the micro-organisms to move. Water is thus necessary to support the metabolic processes of the micro-organisms: a moisture content under 40% inhibit the composting process. However, a moisture content above 65% displace much air in the pore space and leads to diminished oxygen supply. During the composting process, a considerable amount of water is produced by the organisms as a result of the degradation of organic matter. Besides the water production, water is also lost due to the aeration and the heat production. Generally the moisture content tends to decrease as the composting proceeds. As a rule of thumb, the materials are too wet if water can be squeezed out a handful and too dry if the handful does not feel moist to the touch (like a wrenched sponge). In rainy climates it can be a good idea to protect the heap against the huge amounts of rain. Sometimes when the composting materials are too wet, one adds some chalk to the composting mass to create a more granular structure with more pore space (this is often a bad technique because this causes a pH rise which lower the availability of some other nutrients). The desirable amount of water in the composting mass depends for a great deal on the materials: it is not really the quantity of water that counts, but the "available" amount of water (often called the water-activity aw).


The heath produced in a composting pile origins from the chemical bindings in the organics that are available for the micro-organisms. The organisms produce enzymes that catalyzes the biochemical reactions. To break a chemical binding, there is energy needed: the chemical bindings-energy (kJ mol−1). When a chemical binding is formed, this energy is released in the form of heath. For example lets take a look at the combustion process of glucose: the bindings that are formed in CO2 and H2O release more energy than the energy needed for the breakdown of the chemical bindings in glucose and O2 (warning: this is a simplification):

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (2830 kJ)

The available organics can be oxidized completely into CO2, H2O and heath, or can be partially oxidized transforming a part of the original organic material in new microbial biomass and only a part into CO2, H2O and heath. Normally the composting pile has insulating properties due the porosity of the material, this causes accumulation of the heath, and a temperature rise. Heath lose is the result of air movement, conduction, radiation (infrared) and the evaporation of water, these can also be used to keep the temperature in the preferred range: insulating, aeration and adjusting water. A small pile has a greater ratio of pile surface area to volume, and therefore the heath loss is greater than in big piles. Enzymes has an optimal temperature, mostly in the range of 40-60°C, so when the temperature rises, the process accelerate and more heath is released, this can push the temperatures well above 60°C. Since microorganisms are not capable to control the temperature in the composting mass, it is the temperature that selects the microorganisms equipped with enzymes that can work optimal at the temperature of the composting mass. Roughly there are 2 kinds of composting types: mesophilic and thermophilic.

Mesophilic composting: the temperature ranges between 10-45°C so the microbial community is very diverse. This allows a relatively slow but effective composting, the resulting compost however is not hygienic: weed seeds and larvae are not killed.

Thermophilic composting: the temperature ranges between 45-75°C, and the microbial community is not so diverse: only thermophiles can survive this heath. This results in a hygienic compost without viable weed seeds or larvae.


under construction


The preferred range for composting is 6.5-8 because most enzymes work optimal near neutral pH (pH = 7), although the composting is relatively insensitive to the pH because of the broad spectrum of microorganisms that are involved. When the composting process starts, some organic acids are formed (acetic acid for example) and this may lower the pH in the early stage of the composting process. Also the CO2, produced when organic materials are mineralized, can lower the pH when dissolved in the water present in the compost:

CO2 + H2O H2CO3 H+ + HCO3-

Sometimes, when the pH is too low, one can add some chalk:

CaCO3 Ca2+ + CO32-

CO32- + 2 H+ H2O + CO2

However, it is a misunderstanding that chalk favors the composting process, this is only the case when the pH of the composting mass is to low. Normally, one should not add extra chalk to the composting mass. Another possibility to higher the pH is to add wood ash. Be careful with this ash because the pH can easily rise to much and lower the availability of some other nutrients.

If the pH is to high (pH > 9), which doesn't occur in normal conditions, some suggest to add sulfur (S):

S(s) + O2(g) SO2(g)

2 SO2(g) + O2(g) 2 SO3(g)

SO3(g) + H2O(l) H2SO4(l)

H2SO4 + H2O H3O+ + HSO4-

HSO4- + H2O H3O+ + SO42-

Raw materials

A very important factor in building a composting heap is the structure of the materials: one should avoid settling and compaction of the mass because this diminishes the amount of pore space, the effective porosity and thus the presence of oxygen. The structural strength is a parameter that describes the capacity of maintaining porosity. Larger and more uniform particles generally increases pore space but have a smaller surface area, and since the micro-organisms lives on the surface, the process will proceeds slower. Smaller and less uniform particles have a greater surface area, but diminishes the pore space and the effective porosity. Good results are obtained when the particle size ranges from 0.3-5 cm.


under construction

Organisms active during composting

under construction

Building a compost heap

Types of compost heap

Raw materials to add

In general anything that has organic origins will be broken down by composting process over time. However different materials compost at different rates and produce different qualities of compost. To generate a good quality compost with a dark, airy and crumbly texture some forethought needs to be given to what materials you select for your heap and in what ratios they are added. In addition some materials may attract animal pests or seed your compost with perennial weeds, fungi or disease.

Compostable materials providing structure to the heap generally contain less nitrogen and hence compost slower. Materials that provide little structure contain great amounts of nitrogen but add little to the porosity of the heap. Both types of material are essential to a balanced heap and to help gardener's understand the contrast between the two it is common to use the labels 'greens' and 'browns'.

Brown materials are those that provide structure and are usually more woody and therefore brown in colour e.g. twigs, bark, plant stems and leaves from woody plants. Green materials are those that provide high quantities of nitrogen and are generally green in colour e.g. grass, soft foliage, fruit and vegetables.

In addition to material from the garden some kitchen and household waste can often be included in a compost heap. Newspaper, card and cardboard, uncooked vegetable waste, tea bags, coffee grounds and egg shells can all safely be added.

It is the composter's aim to balance greens and browns as effectively as possible to ensure fast and complete composting of the ingredients.

Raw materials to avoid

  • Cooked food. All cooked food should be avoided as it encourages rats and other pests to populate your heap. This includes bread, cooked vegetables, sauces and meat
  • Raw meat and fish. Again these ingredients can attract pests, however they can also become health hazards if your heap is not composting at sufficiently high temperatures.
  • Large pieces of wood. Logs, branches and other forms of wood take a very long time to compost completely. While smaller twigs are helpful to promote a good structure even they might need to be cycled more than once to fully compost. Larger pieces of wood however will simply bulk out your heap reducing it's capacity.
  • Too much of either brown or green materials. Too much brown material and the compost will generate very slowly and will contain many uncomposted items. Too much green material and the compost will have little texture and a slimy consistency.
  • Fire ash. Wood ash can be added in small quantities however coal ash contains large amounts of inorganic compounds such as compounds of sulphur for example. These will affect soil pH and can be mildly toxic to plant life.