Composting is
produced as the result of decomposition of organic matter by microorganisms in
the presence of oxygen under controlled conditions. Compost has become a
part of sustainable agriculture due to its ability to improve soil properties
as well as yield.
Sources: Biodegradable organics such as
kitchen waste, garden pawn, organic fraction of municipal solid waste (MSW). Proper
C: N ration must be maintained to enhance the microbial activities. Ideal C: N
ratio for composting is 25:1. Carbon and nitrogen rich materials are mixed
together to balance the C: N ratio .Hay, wood ashes, saw dust and dried leaves
are added as the carbon sources. Weeds,
green manure such as Grilicidia sepium, fresh
hay, and cow dung and fish emulsions can be added as nitrogen sources.
Shredding or branding materials:
Lager particles can be shredded into small particles which can be easily
handled. It increases the available surface area for microorganisms. So
duration of compost processing is reduced.
Air can move easily through the pile. It helps to distribute the heat
uniformly throughout the pile. It reduces land requirement for compost pile. It
can be more easily turned for aeration.
Compost
processing is done through three steps such as mesophilic, thermophilic and cooling
and maturation phase. These three steps are dominated by different microbial
communities. The temperature of the compost pile is related to the activity of
the microorganisms.
Mesophilic: This is the initial step of
composting. Rapidly degradable organic matter I degraded by mesophilic
bacteria. Their activity is inhibited above 40◦C. Human and plant pathogens are
destroyed above 55◦C. many microbes cannot survive above 60◦C. Composting also
slow down at higher temperature.
Thermophilic: The second step of composting takes
place at higher temperature. These high temperature acts as the accelerating
factor for decomposing high strength organics such as protein, cellulose and
lignocellulose.
Cooling and maturation phase: When organic matter is becoming
limited, temperature of the compost file is gradually reduced. Mesophilic
bacteria break down the remaining organic matter.
Physiochemical factor affecting the
efficiency of the composting: Oxygen is
consumed in large quantity particularly in the initial stage by microorganisms.
Oxygen can be provided through aeration. Turning of material or embedded
propagated pipes provides oxygen to the composting pile.The frequency of
turning and level of aeration is governed by the type of material and present
moisture content. Turning helps to bring material of outside the pile into
center and decomposes the all of the materials in the pile. Greater aeration accelerates the initial stage
of composting and reduces the duration of compost processing. If suddenly
oxygen supply of the pile is vanished, pile is turned into anaerobic. It
produced odiferous ammonia.
Temperature is another important factor
that regulates the composting. Composting has two distinct temperature ranges.
Mesophilic is up to 40◦C and thermophilic above 40◦C. Turning and heating
prevents the overheating of the pile due to the microbial activities.
Microorganisms
need N, P and K as their primary
nutrients. But in the composting C and N are taken into consideration.
Ideal C: N ratio is 25:1. Low C: N ratio slows down the composting process.
High C: N ratio cause unpleasant odor due to the release of excess nitrogen as
ammonia. Therefore C: N ratio must be maintained as earlier discussed.
pH of the composting pile varies with
decomposition byproducts at each step. As result of acid accumulation, pH of
mesophilic stage is reduced. Decomposition of acids release ammonia which
increase the pH of composting. pH become neutral at final stage of composting.
Moisture is important for the metabolic
activities of microorganisms. 40-65% of
moisture content must be maintained. Above 65% causes anaerobic conditions
because water displaces the gas in the pores of the pile.
Microorganisms: Aerobic respiratory bacteria are mainly employed in composting.
Millions of them can be found in 1g of compost. They are diverse nutritionally group.
They oxidized organic matter to ATP synthesis. These oxidation releases larges
amount of heat. They are active during mesophilic stage. Most are died above
60◦ C and most of remaining forms endospores. They need oxygen level above 5%. Actinomycetes degrade the high strength
organic matter such as cellulose, hemicellulose, lignin, starch and protein. They
inhabit in the areas with moderate temperature in the compost pile. Fungi also are capable of breakdown of
high strength organic matter as actionmycetes. They can be found outside of
compost pile.
Macroorganisms: In addition to microorganisms,
large animals also support to compost processing. Larger animals including
mites, millipedes, spider, flies and ants involve in the last stage of the
composting. They help to mixing, chewing and digesting organic matter. They
also shred the organic matter into small particles. Thereby organic matter can
be easily digested by microorganisms. Their extra provides the nutrients for
microorganisms in the compost pile. Protozoan can be found in the water droplet
in the compost pile. They act as predatory organisms on bacteria.
Benefits of composts:
Farmers tends to apply compost as fertilizer, soil conditioner and
mulch. Compost as fertilizer adds
nutrients to soil. Slow decomposition of humus releases nutrient into soil.
Quality and quantity of nutrient varies greatly with compost according to their
initial sources. Compost as mulch is applied to the surface of soil. It
protects the moisture content without allowing them to evaporation. It also
suppresses the growth of weed which affects the agricultural crops. Compost as conditioner
improves physical, chemical and biological properties of soil.
Psychical properties: Organic matter in the humus helps to bind the
soil particles into aggregates. It increases the soil porosity which enhances
the soil aeration. So plant root have ability take enough oxygen from the
soil. High porosity of soil increases
infiltration. Thereby the surface runoff is reduced. It reduces soil bulk
density. As soil particles binding together, it reduces the soil erosion due to
the rain. Due to the dark color of the
humus, soil can absorbed more heat and maintain soil temperature and avoid
temperature fluctuations. Thereby stable soil temperature encourages the seed
germination.
Chemical properties: Compost acts as a buffer agent and neutralizes
variation in pH and salinity. Microorganisms in the humus provide the plant
growth promoting substance such as vitamins and hormones. In addition to macronutrients
such as N, P and K compost provides micronutrients. Compost provides more ion
exchange sites to the soil in the purpose of negatively charged nutrient biding.
Thereby it enhances the ion exchange capacity of soil.
Biological properties: high temperature that is generated by humus is able to kill pathogenic
microorganisms. Humus encourages the growth of earth worms and other beneficial
soil microorganisms. Predatory microorganisms such as protozoan are killed by
microorganisms inhabit in the humus.
Environmental benefits: Compost is produced from organic
matter which is disposed as waste in most cases. So as a waste organic matter
cause serve environmental problems. Unregulated releasing of organic waste
cause aquatic pollution, eutrophication and degrading biodiversity. So local
governments tend to collect the solid waste from their responsible area and
separate the organic waste and process them into compost. So compost protects
the environmental quality. It reduces the land requirement for disposal solid
waste. It prevents the odor due to the anaerobic digestion, pathogens and pest
problems. Quality of groundwater and surface water is preserved.
Economic benefits: Compost as organic fertilizer, it
has a good price in the market. Rural people can process the composting plant
individually in their home garden. They can apply compost for their
agricultural crops while excess can be sold. Even in urban areas, compost can
be produced by using compost bins. The vegetable and fruits that are grown using
organic fertilizer are sold at higher prices compare to the vegetable and
fruits grown using chemical fertilizers.
Disadvantages of composting:
1) Composting is slow releasing
fertilizer of nutrients into the soil. So it is less suitable for agricultural
lands that are growing annual crops. Humus releases nutrients even after cut
off harvesting within fallow period.
2) Quality and quantity of nutrients
varies greatly with compost. So it cannot be ensured that compost lonely act as
perfect fertilizer and fulfill the all of the nutrient requirements. Chemical
fertilizers release large quantity of nutrients that can be directly and
readily absorbed by plants. But humus is needed to be further decomposed to
release nutrients.
3) In most countries, organic fraction
of municipal solid waste is used to produce compost. But even the organic waste
contains loads of toxic materials which are unable to remove before or after
processing like large materials such as polythene, Aluminum cans and broken
glasses. Therefore addition of toxic substances including heavy metals into agricultural soil cannot
be avoided in the application of compost that is produced from MSW.
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