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close this bookOrganic and Compost-based Growing Media for Tree Seedling Nurseries (WB, 1995, 90 p.)
View the document(introduction...)
View the documentForeword
View the documentAbstract
View the documentPreface
View the documentAcknowledgments
View the documentIntroduction
View the documentObjectives and layout
Open this folder and view contentsPotting media characteristics & components
Open this folder and view contentsCompost
Open this folder and view contentsCompost production
View the documentConclusion
View the documentAnnex I - Organic and inorganic potting media components
View the documentAnnex II - Chemistry and processes of composting
View the documentAnnex III - Construction of compost heaps
View the documentAnnex IV - Compost and growing media mixes
View the documentAnnex V - Bark and sawdust
View the documentAnnex VI - Vermicomposting
View the documentGlossary of terms
View the documentReferences
View the documentDistributors of world bank publications

Annex II - Chemistry and processes of composting

The process of composting can be either aerobic or anaerobic and the microorganisms involved may require one or other of these conditions to function or may be able to function under either condition. These latter organisms are normally more efficient under one or other condition. Under normal aerobic conditions organic matter breaks down into a mixture of CO2 and nitrate nitrogen and under anaerobic conditions the product is a mixture of CO2 and methane with ammonia and nitrogen In both cases these products are next processed into proteins, carbohydrates and fats (Gotaas, 19563. A feature of composting is that the life processes of one group of microorganisms will create an environment suited to others (Goluele, 1992). To efficiently make compost these processes must be manipulated to favor microorganisms which will give results appropriate to production needs. Virtually all modern literature favor the aerobic process and this will be given preference in the following section.

Aerobic decomposition (composting) of organic matter begins when the oxygen and moisture content is favorable for growth and reproduction of microorganisms (bacteria, fungi, and actinomycetes) found commonly in organic material The initial activity (at temperatures below 40 °C) is called the mesophilic stage. As composting proceeds heat is generated by the activity of the microorganisms and the temperature rises. Within usually four to six days a thermophilic stage is attained when temperatures rise above 40 °C and stabilize near 60 °C (temperatures above 70 °C kill microorganisms). In the 50°-60 °C stage aggressive microbial activity results in the highest rate of decomposition. A compost management system is needed to prevent temperatures reaching 70 °C and if this is done after several weeks the temperature starts to fall into another mesophilic stage. At this time the organic matter has undergone significant change.

Carbon and nitrogen are needed by the microorganisms which compost organic material. Carbon is an energy source essential for cell growth. Nitrogen is the major source for proteins needed by microorganisms. A material's carbon and nitrogen content is expressed as the carbon/nitrogen ratio (C/N) on a weight basis. To balance the C/N ratio for composting, a mixture of materials which have high and low ratios are used. Fertilizers may also be used to balance the C/N ratio. The C/N ratio decreases during composting as carbon is lost in the form of carbon dioxide. Table 6 summarizes the optimal conditions for composting.

For materials (such as rice straw) with very high C/N ratios, the rate of decomposition can be increased through ammoniation. In this process, a source of nitrogen is added to a material with a high C/N ratio in order to lower the ratio and enhance decomposition. Sources of nitrogen include NaOH, NH3 (ammonia), urea, or urine). One method recommends that 3.5 kg NH3/100 kg straw be used to break down lignins within four weeks (Jackson, l 978). The reaction should take place in a contained environment such as a covering of plastic sheeting or in a pit below ground. In rural areas it is recommended that urea be used as a source of nitrogen because it is not pressurized nor a particularly strong solution (Barreveld, 1989). It is commonly available and easy to transport and handle. Its use requires that the environment be relatively hot and moist so that the urea can decompose into ammonia and that urease (an enzyme found in many materials which can be substituted with soybean meal). Generally urea should be added in solution (about 5% and 1:1 straw:solution) and again left in a sealed environment. Use of urine for this process requires a ratio of 1:1 urine straw.

Table 6 - Optimal composting parameters



C/N ratio

25 to 35/1

Particle size

10 mm for agitated and forced aeration

50 mm for windrows and natural aeration

Moisture content

50-60% (higher values possible when using bulking agents)

Air flow

0.6-1.8 m³ air/day/kg volatile solids during thermophilic stage, or maintain oxygen level at 10-18%


55-60° C held for 3 days


No agitation to periodic turning in simple systems; short burst of vigorous agitation in mechanized systems

pH control

Normally none necessary

Heap size

Any length, 1.5 m high and 2.5 m wide for heaps using natural aeration; with forced aeration, heap size depends on need to avoid overheating

Source: Dalzell et. al., 1987

A final stage of ripening or maturing (during which the C/N ratio changes little) may take several months. This is necessary to break down the lignin and cellulose in materials which are most resistant to decomposition. The final C/N ratio should range between 10 and 12 and the time it takes to reach that point depends on the initial C/N ratio of the materials the initial particle size, moisture content, and porosity of the mass during composting. Once the process is completed, compost can be stored in piles (under a protective cover) with no increase in internal temperature and without anaerobic activity. The final product should have a dark earthy color; an earthy, musty smell; and a light, fluffy texture.

Organisms which decompose organic material need oxygen to function. If the pile lacks oxygen it becomes anaerobic. This decreases the rate of decomposition, the pile starts to ferment, and generate foul, putrid odors. To retain aerobic conditions, compost must be turned (aerated) several times during composting to incorporate fresh air into the pile. This also controls the temperature and helps retain its loose structure and preventing the heap from becoming compacted. The more frequently compost is turned, the higher the rate of decomposition and frequency of turning is normally related to temperature control, as it nears 60 °C turning will be needed.

Moisture content for successful composting should range from 50-65% (by weight) (Hoitink and Poole, 1980). Below 40%, decomposition is extremely slow because it is too dry for microorganisms to function. Above 60% the material is too wet, air spaces are reduced and the heap becomes anaerobic. A higher moisture content may be maintained if the pile is turned frequently or has a loose structure (maintained with lots of straw and fibrous materials) to enhance aeration (Gotaas, 1956).

The pH during composting is rarely a concern. Most plant materials added to a compost pile have a pH between 5-7 which is desired for microbial activity. Once decomposition starts, the pH drops slightly as organic acids are generated and then increases during the thermophilic stage to the optimal range of between 6. 5 and 8.5 (Hoitink and Poole, 1980). If the pile becomes anaerobic, the pH drops further as acids are generated from fermentation. Restoring aerobic conditions by turning will reverse this trend. It is not recommended to buffer or add lime to the pile because it results in ammonia generation and a loss of nitrogen. Liming is best added to finished compost or to the growing media if needed.

Most pathogens and weed seeds are destroyed during thermophilic decomposition. Pathogenic destruction is a most important aspect of this stage, not only the accelerated rates of decomposition. This is of particular importance if compost contains nightsoil or sewage sludge. In order to make sure all pathogens and seeds are exposed to temperatures and microbes capable of killing them, it is necessary to turn the pile at least three times and mix the outside of the pile with the inside.

Fly breeding can be an unwanted nuisance in a composting operation. If wastes are infested with eggs and/or larvae at the time of collection it must be processed and composted immediately. Frequent turning or insulation with a manure plaster is an effective method of controlling flies because eggs and larva are exposed to fatally high interior temperatures. Burning of dry material piled on the outside of the heap is a method used in India to kill larva which have migrated to the cooler outer layer of the pile (Gotaas, 1956). To prevent larva from migrating into the cooler surrounding soil it may be necessary to locate the heap on a surface which is covered or sealed (e.g. a paved or compacted area). Allowing chickens and fowl to eat the larva also helps control fly populations, however, their presence often needs control as the birds tend to scratch at and scatter piles.