Decomposers:

Majority of decomposers are microorganisms. Macroorganisms such as earthworms, termites and other insects also help break down organic materials.

Composting Parameters

1. Composting materials

a Good quality compost contains high organic matter content and a minimum of non-organic material. Some compostable wastes, particularly from industrial areas, can contain high levels of metals such as copper, lead, nickel and zinc and should therefore be removed. Other non-organic materials such as glass, plastics and artificial fibers should also be removed.

b. Succulent and young plants can be decomposed much faster than old and tough ones because they are high in water and contain relatively more sugars.

c. If possible, use materials that are high in N. such as residues from leguminous plants because they are preferred by microorganisms since they provide both C and N. They are also easier to break down. The insects, worms, bacteria and fungi found in the compost pile do the work of composting.

2. Particle size — The smaller the size of the particles of organic material, the greater surface area available for attack by the microorganisms. If the particle size is very large, the surface area for attack is smaller, and the reaction will then proceed slowly or may stop altogether. It is necessary to chop or shred bulky material to reduce the particle size to a range of 10-50 mm.

3. Moisture— All organisms require water for life. The optimum moisture content of ingredients for composting is 50-60 percent. At too low a moisture content, the biological reactions in a compost heap slow down considerably. Excess water on the other hand, leads to waterlogging of the spaces between the particles of the materials. The maximum practical moisture content depends on the structural wet strength of the materials. For practical purposes the material should be as damp as a squeezed-out sponge.

4. Aeration —An adequate supply of air to all parts of a compost heap is essential in order to supply oxygen for the organisms and to flush out the carbon dioxide produced. Absence of air (anaerobic conditions) will lead to the development of different types of microorganisms causing either acidic preservation or putrefaction of the heap, producing bad odors.

Aeration is achieved through the natural movement of air into the compost heap, by turning the material over regularly.

5. Temperature — When organic material is gathered together for composting, some of the energy released by the breakdown of the material is given off as heat. This causes a rise in temperature. The higher the temperature within certain limits, the faster the activity of microorganisms.

At the beginning of the process the material is at ambient temperature. In the first stage, warming up, the microorganisms present on the materials multiply rapidly and the temperature rises. During this period all the very reactive compounds such as sugars, starches, and fats are broken down. When the temperature reaches 160°F the fungi stop working and the breakdown is continued by actinomycetes and spore-forming strains of bacteria. The breakdown slows and the temperature peak is reached. At this period, the heap is losing as much heat as the microorganisms produce.

When cooling down, the straws and stalks are decomposed, mainly by fungi. This is because as the temperature falls below 160° F the fungi re-invade from the cooler regions of the heap and attack less reactive compounds such as hemicelluloses and cellulose, breaking them down into simpler sugar compounds, which become available for all the other microorganisms. The actinomycetes also help during this period. At the end of the cooling down period most of the available food supply has gone, competition starts among the microorganisms, antibiotics are released, and larger soil organisms, especially worms, move in for a few weeks.

The increase in temperature is one of several factors in the composting process which act against the survival of pathogenic organisms. Table 1 shows that the common pathogens which cause diseases in humans and domestic animals are readily destroyed at temperatures of 55 to 60°C for periods of a few minutes to a few hours under the moist conditions used in composting.

Table 1. Pathogen survival in composting and agricultural application of human wastes

Source: Health Aspects of Excreta and Sillage Management, World Bank, 1980.

6. Acidity (pH)—Compost material becomes slightly acidic at the start of composting due to the simple organic acids produced at the initial phase of decomposition. The heap then turns slightly alkaline after a few days as proteins are attacked and ammonia is released. Highly alkaline conditions will lead to excessive loss of nitrogen as ammonia; accordingly it is wise not to add lime to a heap. Highly acid initial conditions may lead to a failure of the heap to warm up. If careful attention is paid to the mixing of materials, moisture content and aeration, there is no necessity to influence the pH of the process. The amount of ammonia lost from a compost heap can be reduced by adding a little soil, about 1% of the weight of the heap.

7. Nutrients—The composting process depends upon the action of microorganisms which require a source of carbon to provide energy and material for new cells, together with a supply of nitrogen for cell proteins. Nitrogen is the most important nutrient and, in general, if sufficient nitrogen is available in the original organic matter, most other nutrients will also be available in adequate quantities. It is desirable that the ratio of carbon to nitrogen (C/N) is in the range of 30-35/l in the initial mixture. If it is much higher, the process will take a long time before sufficient carbon is oxidized off as carbon dioxide, if it is lower, then nitrogen, which is an important fertilizer component of the final compost, will be given off as ammonia. The simplest method of adjusting the C/N ratio is to mix together different materials of high and low carbon and nitrogen contents. For example, straw materials which have a high C/N ratio can be mixed with materials such as manures which have low C/N ratio.

References:

Bautista, O.K. et al. 1983. Introduction to Tropical Horticulture. pp. 205-206

Cosico, W.C. 1985. Organic Fertilizers: Their Nature, Properties and Use. pp. 39 - 50

Dalzell, H.W. et al. 1987. Soil Management: Compost Production and Use in Tropical and Subtropical Environments. FAO Soils Bull. 56: 22-27,162.