Courtesy : en.wikipedia.org

Composting

Compost is a mixture of ingredients used as plant fertilizer and to improve soil’s physical, chemical and biological properties. It is commonly prepared by decomposing plant, food waste, recycling organic materials and manure. The resulting mixture is rich in plant nutrients and beneficial organisms, such as bacteria, protozoa, nematodes and fungi. Compost improves soil fertility in gardens, landscaping, horticulture, urban agriculture, and organic farming, reducing dependency on commercial chemical fertilizers.The benefits of compost include providing nutrients to crops as fertilizer, acting as a soil conditioner, increasing the humus or humic acid contents of the soil, and introducing beneficial microbes that help to suppress pathogens in the soil and reduce soil-borne diseases.

At the simplest level, composting requires gathering a mix of ‘greens’ (green waste) and ‘browns’ (brown waste). Greens are materials rich in nitrogen such as leaves, grass, and food scraps. Browns are woody materials rich in carbon, such as stalks, paper, and wood chips. The materials break down into humus in a process taking months. Composting can be a multi-step, closely monitored process with measured inputs of water, air, and carbon- and nitrogen-rich materials. The decomposition process is aided by shredding the plant matter, adding water, and ensuring proper aeration by regularly turning the mixture in a process using open piles or “windrows.”Fungi, earthworms, and other detritivores further break up the organic material. Aerobic bacteria and fungi manage the chemical process by converting the inputs into heat, carbon dioxide, and ammonium.

Composting is an important part of waste management, since food and other compostable materials make up about 20% of waste in landfills, and these materials take longer to biodegrade in the landfill.Composting offers an environmentally superior alternative to using organic material for landfill because composting reduces anaerobic methane emissions, and provides economic and environmental co-benefits.For example, compost can also be used for land and stream reclamation, wetland construction, and landfill cover.

Fundamentals

Home compost barrel

Compost bins at the Evergreen State College Organic Farm in Washington State

Materials in a compost pile

Food scraps compost heap

Composting is an aerobic method of decomposing organic solid wastes.It can therefore be used to recycle organic material. The process involves decomposing organic material into a humus-like material, known as compost, which is a good fertilizer for plants.

Composting organisms require four equally important ingredients to work effectively:

• Carbon is needed for energy; the microbial oxidation of carbon produces the heat required for other parts of the composting process. High carbon materials tend to be brown and dry.
• Nitrogen is needed to grow and reproduce more organisms to oxidize the carbon.High nitrogen materials tend to be green and wet. They can also include colourful fruits and vegetables.
• Oxygen is required for oxidizing the carbon, the decomposition process. Aerobic bacteria need oxygen levels above 5% to perform the processes needed for composting.^[3]
• Water is necessary in the right amounts to maintain activity without causing anaerobic conditions.

Certain ratios of these materials will allow microorganisms to work at a rate that will heat up the compost pile. Active management of the pile (e.g., turning over the compost heap with a pitchfork) is needed to maintain sufficient oxygen and the right moisture level. The air/water balance is critical to maintaining high temperatures 130–160 °F (54–71 °C) until the materials are broken down.

Composting is most efficient with a carbon-to-nitrogen ratio of about 25:1.Hot composting focuses on retaining heat to increase the decomposition rate thus producing compost more quickly. Rapid composting is favored by having a carbon-to-nitrogen ratio of ~30 carbon units or less. Above 30, the substrate is nitrogen starved. Below 15, it is likely to outgas a portion of nitrogen as ammonia.

Nearly all dead plant and animal materials have both carbon and nitrogen in different amounts.Fresh grass clippings have an average ratio of about 15:1 and dry autumn leaves about 50:1 depending upon species. Composting is an ongoing and dynamic process, adding new sources of carbon and nitrogen consistently as well as active management is important.

Organisms

Organisms can break down organic matter in compost if provided with the correct mixture of water, oxygen, carbon, and nitrogen.^[3] They fall into two broad categories: chemical decomposers which perform chemical processes on the organic waste, and physical decomposers which process the waste into smaller pieces through methods such as grinding, tearing, chewing, and digesting.^[3]

Chemical decomposers

• Bacteria – the most abundant and important of all the microorganisms found in compost.Bacteria process carbon and nitrogen and excrete plant-available nutrients such as nitrogen, phosphorus, and magnesium.Depending on the phase of composting, mesophilic or thermophilic bacteria may be the most prominent.
  • Mesophilic bacteria get compost to the thermophilic stage through oxidation of organic material. Afterwards, they cure it, which makes the fresh compost more bio-available for plants.
  • Thermophilic bacteria do not reproduce and are not active between −5 to 25 °C (23 to 77 °F), yet are found throughout soil. They activate once the mesophilic bacteria have begun to breakdown organic matter and increase the temperature to their optimal range.^[13] They have been shown to enter soils via rainwater. They are present so broadly because of many factors including their spores being resilient.^[Thermophilic bacteria thrive at higher temperatures, reaching 40–60 °C (104–140 °F) in typical mixes. Large-scale composting operations, such as windrow composting, may exceed this temperature, potentially killing beneficial soil microorganisms but also pasteurizing the waste.
  • Actinomycetota are needed to break down paper prducts such as newspaper, bark, etc and other large molecules such as lignin and cellulose that are more difficult to decompose. The “pleasant earthy smell of compost” is attributed to Actinomycetota.They make carbon, ammonia, and nitrogen nutrients available to plants.
• Fungi such as mold and yeast help break down materials that bacteria cannot, especially cellulose and lignin in woody material
• Protozoa – contribute to biodegradation of organic matter as well as consuming non-active bacteria, fungi, and micro-organic particulates.

Physical decomposers

• Ants – create nests, making the soil more porous and transporting nutrients to different areas of the compost.
• Beetles – grubs feed on decaying vegetables.
• Earthworms – ingest partly composted material and excrete worm castings, making nitrogen, calcium, phosphorus, and magnesium available to plants.The tunnels they create as they move through the compost also increase aeration and drainage.
• Flies – feed on almost all organic material and input bacteria into the compost. Their population is kept in check by mites and the thermophilic temperatures that are unsuitable for fly larvae.
• Millipedes – break down plant material.
• Rotifers – feed on plant particles.
• Snails and slugs – feed on living or fresh plant material.They should be removed from compost before use as they can damage plants and crops.
• Sow bugs – feed on rotting wood, and decaying vegetation.
• Springtails – feed on fungi, mold, and decomposing plants.

Phases of composting

Three year old household compost

Under ideal conditions, composting proceeds through three major phases:

1. Mesophilic phase: an initial, mesophilic phase, in which the decomposition is carried out under moderate temperatures by mesophilic microorganisms.
2. Thermophilic phase: as the temperature rises, a second, thermophilic phase starts, in which various thermophilic bacteria carry out the decomposition under higher temperatures (50 to 60 °C (122 to 140 °F).)
3. Maturation phase: as the supply of high-energy compounds dwindles, the temperature starts to decrease, and the mesophilic bacteria once again predominate in the maturation phase.