Compost

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 and 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 green waste and brown waste. The materials break down into humus in a process taking months. Composting can be a multistep, 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 ions.
Composting is an important part of waste management, since food and other compostable materials make up about 20% of waste in landfills, and due to anaerobic conditions, these materials take longer to biodegrade in the landfill. Composting offers an environmentally superior alternative to using organic material for landfill because composting reduces methane emissions due to anaerobic conditions, 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

Composting is an aerobic method of decomposing organic solid wastes, so it can 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.
Water is necessary in the right amounts to maintain activity without causing locally anaerobic conditions.

Certain ratios of these materials allow microorganisms to work at a rate that will heat up the compost pile. Active management of the pile is needed to maintain sufficient oxygen and the right moisture level. The air/water balance is critical to maintaining high temperatures 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 about 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. 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.

Chemical decomposers

Bacteria are 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 bioavailable for plants.
* Thermophilic bacteria do not reproduce and are not active between, yet are found throughout soil. They activate once the mesophilic bacteria have begun to break down organic matter and increase the temperature to their optimal range. 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 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 products 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 molds and yeasts help break down materials that bacteria cannot, especially cellulose and lignin in woody material.
Protozoa contribute to biodegradation of organic matter and consume inactive 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 such as 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 put 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, molds, and decomposing plants.
Phases of composting

Under ideal conditions, composting proceeds through three major phases:

Mesophilic phase: The initial, mesophilic phase is when the decomposition is carried out under moderate temperatures by mesophilic microorganisms. 2 to 8 days
Thermophilic phase: As the temperature rises, a second, thermophilic phase starts, in which various thermophilic bacteria carry out the decomposition under higher temperatures
Cooling phase
Maturation phase: As the supply of high-energy compounds dwindles, the temperature starts to decrease.

Semicomposting is the degradation process that handles volumes of organic waste lower than that recommended for composting and therefore does not present a thermophilic stage, because mesophilic microorganisms are the only responsible ones, for the degradation of organic matter.

Hot and cold composting – impact on timing

The time required to compost material relates to the volume of material, the particle size of the inputs, and the amount of mixing and aeration. Generally, larger piles reach higher temperatures and remain in a thermophilic stage for days or weeks. This is hot composting and is the usual method for large-scale municipal facilities and agricultural operations.
The Berkeley method produces finished compost in 18 days. It requires assembly of at least of material at the outset and needs turning every two days after an initial four-day phase. Such short processes involve some changes to traditional methods, including smaller, more homogenized particle sizes in the input materials, controlling carbon-to-nitrogen ratio at 30:1 or less, and careful monitoring of the moisture level.
Cold composting is a slower process that can take up to a year to complete. It results from smaller piles, including many residential compost piles that receive small amounts of kitchen and garden waste over extended periods. Piles smaller than tend not to reach and maintain high temperatures. Turning is not necessary with cold composting, although a risk exists that parts of the pile may go anaerobic as it becomes compacted or waterlogged.

Pathogen removal

Composting can destroy some pathogens and seeds, by reaching temperatures above.
Dealing with stabilized compost – i.e. composted material in which microorganisms have finished digesting the organic matter and the temperature has reached between – poses very little risk, as these temperatures kill pathogens and even make oocysts unviable. The temperature at which a pathogen dies depends on the pathogen, how long the temperature is maintained, and pH.
Compost products such as compost tea and compost extracts have been found to have an inhibitory effect on Fusarium oxysporum, Rhizoctonia species, and Pythium debaryanum, plant pathogens that can cause crop diseases. Aerated compost teas are more effective than compost extracts. The microbiota and enzymes present in compost extracts also have a suppressive effect on fungal plant pathogens. Compost is a good source of biocontrol agents like B. subtilis, B. licheniformis, and P. chrysogenum that fight plant pathogens. Sterilizing the compost, compost tea, or compost extracts reduces the effect of pathogen suppression.