From: FAO Corporate
Document Repository as produced by Natural Resources Management and Environment
Deparment
Composting is the
natural process of 'rotting' or decomposition of organic matter by
microorganisms under controlled conditions. Raw organic materials such as crop
residues, animal wastes, food garbage, some municipal wastes and suitable
industrial wastes, enhance their suitability for application to the soil as a
fertilizing resource, after having undergone composting.
Compost is a rich source
of organic matter. Soil organic matter plays an important role in sustaining
soil fertility, and hence in sustainable agricultural production. In addition
to being a source of plant nutrient, it improves the physico-chemical and
biological properties of the soil. As a result of these improvements, the soil:
(i) becomes more resistant to stresses such as drought, diseases and toxicity;
(ii) helps the crop in improved uptake of plant nutrients; and (iii) possesses
an active nutrient cycling capacity because of vigorous microbial activity.
These advantages manifest themselves in reduced cropping risks, higher yields
and lower outlays on inorganic fertilizers for farmers.
Types of composting
Composting may be
divided into two categories by the nature of the decomposition process. In
anaerobic composting, decomposition occurs where oxygen (O) is absent or in
limited supply. Under this method, anaerobic micro-organisms dominate and
develop intermediate compounds including methane, organic acids, hydrogen
sulphide and other substances. In the absence of O, these compounds accumulate
and are not metabolized further. Many of these compounds have strong odours and
some present phytotoxicity. As anaerobic composting is a low-temperature
process, it leaves weed seeds and pathogens intact. Moreover, the process
usually takes longer than aerobic composting. These drawbacks often offset the
merits of this process, viz. little work involved and fewer nutrients lost
during the process.
Aerobic composting takes
place in the presence of ample O. In this process, aerobic microorganisms break
down organic matter and produce carbon dioxide (CO2), ammonia,
water, heat and humus, the relatively stable organic end product. Although
aerobic composting may produce intermediate compounds such as organic acids,
aerobic micro-organisms decompose them further. The resultant compost, with its
relatively unstable form of organic matter, has little risk of phytotoxicity.
The heat generated accelerates the breakdown of proteins, fats and complex
carbohydrates such as cellulose and hemi-cellulose. Hence, the processing time
is shorter. Moreover, this process destroys many micro-organisms that are human
or plant pathogens, as well as weed seeds, provided it undergoes sufficiently
high temperature. Although more nutrients are lost from the materials by
aerobic composting, it is considered more efficient and useful than anaerobic
composting for agricultural production. Most of this publication focuses on
aerobic composting.
Composting objectives
may also be achieved through the enzymatic degradation of organic materials as
they pass through the digestive system of earthworms. This process is termed
vermicomposting.
The aerobic composting process
The aerobic composting
process starts with the formation of the pile. In many cases, the temperature
rises rapidly to 70-80 °C within the first couple of days. First, mesophilic
organisms (optimum growth temperature range = 20-45 °C) multiply rapidly on the
readily available sugars and amino acids (Figure 1). They generate heat by
their own metabolism and raise the temperature to a point where their own
activities become suppressed. Then a few thermophilic fungi and several
thermophilic bacteria (optimum growth temperature range = 50-70 °C or more)
continue the process, raising the temperature of the material to 65 °C or
higher. This peak heating phase is important for the quality of the compost as
the heat kills pathogens and weed seeds.
The active composting
stage is followed by a curing stage, and the pile temperature decreases
gradually. The start of this phase is identified when turning no longer reheats
the pile. At this stage, another group of thermophilic fungi starts to grow.
These fungi bring about a major phase of decomposition of plant cell-wall
materials such as cellulose and hemi-cellulose. Curing of the compost provides
a safety net against the risks of using immature compost such as nitrogen (N)
hunger, O deficiency, and toxic effects of organic acids on plants.
Eventually, the
temperature declines to ambient temperature. By the time composting is
completed, the pile becomes more uniform and less active biologically although
mesophilic organisms recolonize the compost. The material becomes dark brown to
black in colour. The particles reduce in size and become consistent and
soil-like in texture. In the process, the amount of humus increases, the ratio
of carbon to nitrogen (C:N) decreases, pH neutralizes, and the exchange
capacity of the material increases.
FIGURE 1
Temperature changes and
fungi populations in wheat straw compost
Note:
Solid line = temperature; broken line = mesophilic fungi population; dotted
line = thermophilic fungi population; left y-axis = fungal populations
(logarithm of colony forming units (cfu) per gram of compost plated onto agar);
right y-axis = temperature in centre of compost. a, b, c and d = heating
phases.
Factors affecting aerobic composting
Aeration
Aerobic composting
requires large amounts of O, particularly at the initial stage. Aeration is the
source of O, and, thus, indispensable for aerobic composting. Where the supply
of O is not sufficient, the growth of aerobic micro-organisms is limited, resulting
in slower decomposition. Moreover, aeration removes excessive heat, water
vapour and other gases trapped in the pile. Heat removal is particularly
important in warm climates as the risk of overheating and fire is higher.
Therefore, good aeration is indispensable for efficient composting. It may be
achieved by controlling the physical quality of the materials (particle size
and moisture content), pile size and ventilation and by ensuring adequate
frequency of turning.
Moisture is necessary to
support the metabolic activity of the micro-organisms. Composting materials
should maintain a moisture content of 40-65 percent. Where the pile is too dry,
composting occurs more slowly, while a moisture content in excess of 65 percent
develops anaerobic conditions. In practice, it is advisable to start the pile
with a moisture content of 50-60 percent, finishing at about 30 percent.
Micro-organisms require
C, N, phosphorus (P) and potassium (K) as the primary nutrients. Of particular
importance is the C:N ratio of raw materials. The optimal C:N ratio of raw
materials is between 25:1 and 30:1 although ratios between 20:1 and 40:1 are
also acceptable. Where the ratio is higher than 40:1, the growth of
micro-organisms is limited, resulting in a longer composting time. A C:N ratio
of less than 20:1 leads to underutilization of N and the excess may be lost to
the atmosphere as ammonia or nitrous oxide, and odour can be a problem. The C:N
ratio of the final product should be between about 10:1 and 15:1.
The process of
composting involves two temperature ranges: mesophilic and thermophilic. While
the ideal temperature for the initial composting stage is 20-45 °C, at
subsequent stages with the thermophilic organisms taking over, a temperature range
of 50-70 °C may be ideal. High temperatures characterize the aerobic composting
process and serve as signs of vigorous microbial activities. Pathogens are
normally destroyed at 55 °C and above, while the critical point for elimination
of weed seeds is 62 °C. Turnings and aeration can be used to regulate
temperature.
Lignin is one of the
main constituents of plant cell walls, and its complex chemical structure makes
it highly resistant to microbial degradation (Richard, 1996). This nature of
lignin has two implications. One is that lignin reduces the bioavailability of
the other cell-wall constituents, making the actual C:N ratio (viz. ratio of
biodegradable C to N) lower than the one normally cited. The other is that
lignin serves as a porosity enhancer, which creates favourable conditions for
aerobic composting. Therefore, while the addition of lignin-decomposing fungi
may in some cases increase available C, accelerate composting and reduce N
loss, in other cases it may result in a higher actual C:N ratio and poor
porosity, both of which prolong composting time.
Polyphenols include
hydrolysable and condensed tannins (Schorth, 2003). Insoluble condensed tannins
bind the cell walls and proteins and make them physically or chemically less
accessible to decomposers. Soluble condensed and hydrolysable tannins react
with proteins and reduce their microbial degradation and thus N release.
Polyphenols and lignin are attracting more attention as inhibiting factors.
Palm et al. (2001) suggest that the contents of these two
substances be used to classify organic materials for more efficient on-farm
natural resource utilization, including composting.
Although the natural
buffering effect of the composting process lends itself to accepting material
with a wide range of pH, the pH level should not exceed eight. At higher pH
levels, more ammonia gas is generated and may be lost to the atmosphere.
Techniques for effective aerobic composting
Simple replication of
composting practices does not always give the right answer to potential
composters. This is because composting takes place at various locations and
under diverse climates, using different materials with dissimilar physical,
chemical and biological properties. An understanding of the principles and
technical options and their appropriate application may be helpful in providing
the optimal environment to the compost pile.
In order to obtain the
end product of uniform quality, the whole of the pile should receive a
sufficient amount of O so that aerobic micro-organisms flourish uniformly. The
methodologies deliberated in this publication made use of the techniques as
presented below.
Pile size and porosity
of the material
The size of the pile is
of great significance and finds mention in the sections on passive composting
of manure piles (Chapter 2) and turned wind-rows (Chapter 3). Where the pile or
wind-row is too large, anaerobic zones occur near its centre, which slows the
process in these zones. On the other hand, piles or wind-rows that are too
small lose heat quickly and may not achieve a temperature high enough to
evaporate moisture and kill pathogens and weed seeds. The optimal size of the
piles and wind-rows should also consider such parameters as the physical
property (porosity) of the materials and the way of forming the pile. While
more porous materials allow bigger piles, heavy weights should not be put on
top and materials should be kept as loose as possible. Climate is also a
factor. With a view to minimizing heat loss, larger piles are suitable for cold
weather. However, in a warmer climate, the same piles may overheat and in some
extreme cases (75 °C and above) catch fire.
Ventilation
Provision of ventilation
complements efforts to optimize pile size. Ventilation methods are varied. The
simplest method is to punch holes in the pile at several points. The high
temperature compost method of Chinese rural composting (Chapter 2) involves
inserting a number of bamboo poles deep into the pile and withdrawing them a
day later, leaving the pile with ventilation holes. Aeration is improved by
supplying more air to the base of the pile where O deficiency occurs most
often. In addition to the above-mentioned vertical poles, Ecuador on-farm
composting (Chapter 2) uses a lattice of old branches at the base to allow more
pile surface to come into contact with the air, and the composting period is
reduced to two to three months in warm seasons. This technique is also
practised in the rapid composting method developed by the Institute of
Biological Sciences (IBS) in the Philippines (Chapter 2), where the platform
should be 30 cm above the ground. The passively aerated wind-rows method
(Chapter 3) uses a more sophisticated technique. It entails embedding
perforated pipes throughout the pile. As the pipe ends are open, air flow is
induced and O is supplied to the pile continuously. The aerated static pile
method (Chapter 3) takes this aeration system a step further; a blower
generates air flow to create negative pressure (suction) in the pile and fresh
air is supplied from outside.
Turning
Once the pile is formed
and decomposition starts, the only technique for improving aeration is turning.
As Table 1 shows, frequency of turning is crucial for composting time. While
the Indian Bangalore method (Chapter 2) requires six to eight months to mature,
the Indian Coimbatore method (Chapter 2) (turning once) reduces the time to four
months, and the Chinese rural composting pit method (turning three times)
reduces the time to three months. An extreme example is the Berkley rapid
composting method (Chapter 2), which employs daily turning to complete the
process in two weeks. In some cases, turning not only distributes air
throughout the pile, it also prevents overheating as it kills all the microbes
in the pile and terminates decomposition. However, turning too frequently might
result in a lower temperature.
Inoculation
While some composters
find improved aeration enough for enhanced microbial activities, others may
need inoculation of micro-organisms. Inoculum organisms utilized for composting
are mainly fungi such as Trichoderma sp. (IBS
rapid composting and composting weeds (Chapter 2)) and Pleurotus sp.
(composting Coir Pith (Chapter 2) and composting weeds). This publication also
features 'effective micro-organisms' (EMs) (EM-based quick compost production
process (Chapter 2)). The inoculums are an affordable choice for those with access
to the market and also for resource-poor farmers. The production cost could be
reduced by using inoculums taken from compost pits (pit method of the Indian
Indore method (Chapter 2)), by purchasing the commercial product and
multiplying it on the farm (EM-based quick compost production process), and by
utilizing native inoculums derived from soils or plant leaves.
The techniques mentioned
above often need to be complemented by the provision of nutrients. One of the
most common practices is to add inorganic fertilizers, particularly N, in order
to modify a high C:N ratio. Similarly, P is sometimes applied as the C:P ratio
of the material mix is also considered important (the ratio should be between
75:1 and 150:1). When micro-organisms are inoculated, they require sugar and
amino acids in order to boost their initial activities; molasses is often added
for this purpose.
Table 1 Salient features
of selected small-scale aerobic composting techniques
Method
|
Salient
features
|
Duration
|
Substrate
size reduction
|
Turnings
at intervals of (days)
|
Added
aeration provision
|
Microbial
inoculation
|
Supporting
microbial nutrition
|
Indore pit
|
|
+15, +30, +60
|
|
Inoculum from old pit
|
|
4 months
|
Indore heap
|
Shredded
|
+42, +84
|
|
|
|
4 months
|
Chinese pit
|
|
+30, +60, +75
|
|
|
Superphosphate
|
3 months
|
Chinese high temperature compost
|
Shredded
|
+15
|
Aeration holes in heap through
bamboo poles/maize stalks
|
|
Superphosphate
|
2 months
|
Ecuador on-farm composting
|
|
+21
|
Lattice of old branches/poles at
heap base
|
|
|
2-3 months in summer;
5-6 months in winter
|
Berkley rapid composting
|
Shredded to small size
|
Daily or alternate day turning
|
|
|
|
2 weeks with daily turning & 3
weeks with alternate day turning
|
North Dakota State University hot
composting
|
Shredded
|
+3 or +4
|
4-5 holes punched in centre of
pile
|
|
0.12 kg N per 90 cm dry matter
|
4-6 weeks
|
EM-based quick composting
|
|
+14, +21
|
|
EM
|
Molasses
|
4-5 weeks
|
IBS rapid composting
|
Shredded
|
+7, +14, then every 2 weeks
|
Raised platform ground/perforated
bamboo trunks
|
Trichodermasp.
|
|
3-7 weeks
|
Shredding
Downsizing, or chopping
up the materials, is a sound and widely-practised technique. It increases the
surface area available for microbial action and provides better aeration. This
technique is particularly effective and necessary for harder materials such as
wood.
An example of other
measures mentioned in this publication is the practice of adding lime. Lime is
thought to weaken the lignin structure of the plant materials and enhance the
microbial population. However, in some cases, liming is not recommended as the
pile may become too alkaline, resulting in significant N loss.
