Compost

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]

• Bacteria – the most abundant and important of all the microorganisms found in compost.[3] Bacteria process carbon and nitrogen and excrete plant available nutrients such as nitrogen, phosphorus, and magnesium.[3] 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.[3] Afterwards, they cure it which makes the fresh compost more bio-available for plants.[3][13]
  • Thermophilic bacteria do not reproduce and are not active between −5 to 25 °C (23 to 77 °F),[14] 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.[13] They are present so broadly because of many factors including their spores being resilient.[15] 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.[13]
  • 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.[3] The "pleasant earthy smell of compost" is attributed to Actinomycetota.[3] They make carbon, ammonia, and nitrogen nutrients available to plants.[3]
• Fungi such as mold and yeast help break down materials that bacteria cannot, especially cellulose and lignin in woody material.[3]
• Protozoa – contribute to biodegradation of organic matter as well as consuming non-active bacteria, fungi, and micro-organic particulates.[16]

• Ants – create nests, making the soil more porous and transporting nutrients to different areas of the compost.[3]
• Beetles – grubs feed on decaying vegetables.[3]
• Earthworms – ingest partly composted material and excrete worm castings,[3] making nitrogen, calcium, phosphorus, and magnesium available to plants.[3] The tunnels they create as they move through the compost also increase aeration and drainage.[3]
• Flies – feed on almost all organic material and input bacteria into the compost.[3] Their population is kept in check by mites and the thermophilic temperatures that are unsuitable for fly larvae.[3]
• Millipedes – break down plant material.[3]
• Rotifers – feed on plant particles.[3]
• Snails and slugs – feed on living or fresh plant material.[3] They should be removed from compost before use as they can damage plants and crops.[3]
• Sow bugs – feed on rotting wood, and decaying vegetation.[3]
• Springtails – feed on fungi, mold, and decomposing plants.[3]

Three year old household compost

Under ideal conditions, composting proceeds through three major phases:[16]

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.

The time required to compost material relates to the volume of material, the particle size of the inputs (e.g. wood chips break down faster than branches), and the amount of mixing and aeration.[3] Generally, larger piles will 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 eighteen days. It requires assembly of at least 1 cubic metre (35 cu ft) of material at the outset and needs turning every two days after an initial four-day phase.[17] Such short processes involve some changes to traditional methods, including smaller, more homogenized particle sizes in the input materials, controlling carbon-to-nitrogen ratio (C:N) 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.[18] 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 1 cubic metre (35 cu ft) tend not to reach and maintain high temperatures.[19] Turning is not necessary with cold composting, although there is a risk that parts of the pile may go anaerobic as they become compacted or water-logged.

Composting can destroy some pathogens and seeds, by reaching temperatures above 50 °C (122 °F).[20] Dealing with stabilized compost – i.e. composted material in which microorganisms have finished digesting the organic matter and the temperature has reached between 50–70 °C (122–158 °F) – poses very little risk, as these temperatures kill pathogens and even make oocysts unviable.[21] The temperature at which a pathogen dies depends on the pathogen, how long the temperature is maintained (seconds to weeks), and pH.[22]

Compost products like compost tea and compost extracts have been found to have an inhibitory effect on Fusarium oxysporum, Rhizoctonia sp., and Pythium debaryanum, plant pathogens that can cause crop diseases.[23] Aerated compost teas are more effective than compost extracts.[23] The microbiota and enzymes present in compost extracts also have a suppressive effect on fungal plant pathogens.[24] Compost is a good source of biocontrol agents like B. subtilis, B. licheniformis, and P. chrysogenum that fight plant pathogens.[23] Sterilizing the compost, compost tea, or compost extracts reduces the effect of pathogen suppression.[23]

When turning compost that has not gone through phases where temperatures above 50 °C (122 °F) are reached, a mouth mask and gloves must be worn to protect from diseases that can be contracted from handling compost, including:[25]

• Aspergillosis
• Farmer's lung
• Histoplasmosis – a fungus that grows in guano and bird droppings
• Legionnaires' disease
• Paronychia – via infection around the fingernails and toenails
• Tetanus – a central nervous system disease

Oocytes are rendered unviable by temperatures over 50 °C (122 °F).[21]

A large compost pile that is steaming with the heat generated by thermophilic microorganisms.

There are two broad categories of organic solid waste: green waste and brown waste.

Green waste is generally considered a source of nitrogen and includes pre and post-consumer food waste, grass clippings, garden trimmings, and fresh leaves.[1] Animal carcasses, roadkill, and butcher residue can also be composted and these are considered nitrogen sources.[29]

Brown waste is a carbon source. Typical examples are dried vegetation and woody material such as fallen leaves, straw, woodchips, limbs, logs, pine needles, sawdust, and wood ash but not charcoal ash.[1][30] Products derived from wood such as paper and plain cardboard are also considered carbon sources.[1]

On many farms, the basic composting ingredients are animal manure generated on the farm as a nitrogen source, and bedding as the carbon source. Straw and sawdust are common bedding materials. Non-traditional bedding materials are also used, including newspaper and chopped cardboard.[1] The amount of manure composted on a livestock farm is often determined by cleaning schedules, land availability, and weather conditions. Each type of manure has its own physical, chemical, and biological characteristics. Cattle and horse manures, when mixed with bedding, possess good qualities for composting. Swine manure, which is very wet and usually not mixed with bedding material, must be mixed with straw or similar raw materials. Poultry manure must be blended with high-carbon, low-nitrogen materials.[31]

Human excreta, sometimes called "humanure" in the composting context,[32][33] can be added as an input to the composting process since it is a nitrogen-rich organic material. It can be either composted directly in composting toilets, or indirectly in the form of sewage sludge after it has undergone treatment in a sewage treatment plant. Both processes require capable design as there are potential health risks that need to be managed. In the case of home composting, a wide range of microorganisms including bacteria, viruses and parasitic worms can be present in feces, and improper processing can pose significant health risks.[34] In the case of large sewage treatment facilities that collect wastewater from a range of residential, commercial and industrial sources, there are additional considerations. The composted sewage sludge, referred to as biosolids, can be contaminated with a variety of metals and pharmaceutical compounds.[35][36] Insufficient processing of biosolids can also lead to problems when the material is applied to land.[37]

Urine can be put on compost piles or directly used as fertilizer.[38] Adding urine to compost can increase temperatures and therefore increase its ability to destroy pathogens and unwanted seeds. Unlike feces, urine does not attract disease-spreading flies (such as houseflies or blowflies), and it does not contain the most hardy of pathogens, such as parasitic worm eggs.[39]

Animal carcasses may be composed as a disposal option. Such material is rich in nitrogen.[40]

Composting, or formally "natural organic reduction", is an emerging approach to the environmentally-friendly disposal of human corpses. Mixed with wood chips and aerated, a human corpse turns into compost in a month.[41] The idea is growing in popularity, particularly in the United States where a number of states have either legalized the process or are in the process of doing so.[42]

On September 9th, 2022, California governor Gavin Newsom signed a bill that would allow human composting in California. Burial, cremation and alkaline hydrolysis were the only choices of death care, but with the new bill signed into action, human composting, or natural organic reduction, will be an additional option for “individuals who want a different method to honor their remains after death.” The bill is set to take affect in 2027. [43]

An almost completed hügelkultur bed; the bed does not have soil on it yet.

The practice of making raised garden beds or mounds filled with rotting wood is also called hügelkultur in German.[47][48] It is in effect creating a nurse log that is covered with soil.

Benefits of hügelkultur garden beds include water retention and warming of soil.[47][49] Buried wood acts like a sponge as it decomposes, able to capture water and store it for later use by crops planted on top of the hügelkultur bed.[47][50]

• Vermicompost (also called worm castings, worm humus, worm manure, or worm faeces) is the end-product of the breakdown of organic matter by earthworms.[52] These castings have been shown to contain reduced levels of contaminants and a higher saturation of nutrients than the organic materials before vermicomposting.[53]
• Black soldier fly (Hermetia illucens) larvae are able to rapidly consume large amounts of organic material and can be used to treat human waste. The resulting compost still contains nutrients and can be used for biogas production, or further traditional composting or vermicomposting[54][55]
• Bokashi is a fermentation process rather than a decomposition process, and so retains the feedstock's energy, nutrient and carbon contents. There must be sufficient carbohydrate for fermentation to complete and therefore the process is typically applied to food waste, including non-compostable items. Carbohydrate is transformed into lactic acid, which dissociates naturally to form lactate, a biological energy carrier. The preserved result is therefore readily consumed by soil microbes and from there by the entire soil food web, leading to a significant increase in soil organic carbon and turbation. The process completes in weeks and returns soil acidity to normal.
• Co-composting is a technique that processes organic solid waste together with other input materials such as dewatered fecal sludge or sewage sludge.[10]
• Anaerobic digestion combined with mechanical sorting of mixed waste streams is increasingly being used in developed countries due to regulations controlling the amount of organic matter allowed in landfills. Treating biodegradable waste before it enters a landfill reduces global warming from fugitive methane; untreated waste breaks down anaerobically in a landfill, producing landfill gas that contains methane, a potent greenhouse gas. The methane produced in an anaerobic digester can be converted to biogas.[56]

Compost used as fertilizer

On open ground for growing wheat, corn, soybeans, and similar crops, compost can be broadcast across the top of the soil using spreader trucks or spreaders pulled behind a tractor. It is expected that the spread layer is very thin (approximately 6 mm (0.24 in)) and worked into the soil prior to planting. Application rates of 25 mm (0.98 in) or more are not unusual when trying to rebuild poor soils or control erosion. Due to the extremely high cost of compost per unit of nutrients in the United States, on-farm use is relatively rare since rates over 4 tons/acre may not be affordable. This results from an over-emphasis on "recycling organic matter" than on "sustainable nutrients." In countries such as Germany, where compost distribution and spreading are partially subsidized in the original waste fees, compost is used more frequently on open ground on the premise of nutrient "sustainability".[57]

In plasticulture, strawberries, tomatoes, peppers, melons, and other fruits and vegetables are grown under plastic to control temperature, retain moisture and control weeds. Compost may be banded (applied in strips along rows) and worked into the soil prior to bedding and planting, be applied at the same time the beds are constructed and plastic laid down, or used as a top dressing.

Many crops are not seeded directly in the field but are started in seed trays in a greenhouse. When the seedlings reach a certain stage of growth, they are transplanted in the field. Compost may be part of the mix used to grow the seedlings, but is not normally used as the only planting substrate. The particular crop and the seeds' sensitivity to nutrients, salts, etc. dictates the ratio of the blend, and maturity is important to insure that oxygen deprivation will not occur or that no lingering phyto-toxins remain.[58]

Compost can be added to soil, coir, or peat, as a tilth improver, supplying humus and nutrients.[59] It provides a rich growing medium as absorbent material. This material contains moisture and soluble minerals, which provide support and nutrients. Although it is rarely used alone, plants can flourish from mixed soil, sand, grit, bark chips, vermiculite, perlite, or clay granules to produce loam. Compost can be tilled directly into the soil or growing medium to boost the level of organic matter and the overall fertility of the soil. Compost that is ready to be used as an additive is dark brown or even black with an earthy smell.[1][59]

Generally, direct seeding into a compost is not recommended due to the speed with which it may dry, the possible presence of phytotoxins in immature compost that may inhibit germination,[60][61][62] and the possible tie up of nitrogen by incompletely decomposed lignin.[63] It is very common to see blends of 20–30% compost used for transplanting seedlings.

Compost can be used to increase plant immunity to diseases and pests.[64]

Compost tea is made up of extracts of fermented water leached from composted materials.[59][65] Composts can be either aerated or non-aerated depending on its fermentation process.[66] Compost teas are generally produced from adding compost to water in a ratio of 1:4–1:10, occasionally stirring to release microbes.[66]

There is debate about the benefits of aerating the mixture.[65] Non-aerated compost tea is cheaper and less labor intensive, but there are conflicting studies regarding the risks of phytotoxicity and human pathogen regrowth.[66] Aerated compost tea brews faster and generates more microbes, but has potential for human pathogen regrowth.[66]

Field studies have shown the benefits of adding compost teas to crops due to organic matter input, increased nutrient availability, and increased microbial activity.[59][65] They have also been shown to have a suppressive effect on plant pathogens[67] and soil-borne diseases.[66] The efficacy is influenced by a number of factors, such as the preparation process, the type of source the conditions of the brewing process, and the environment of the crops.[66] Adding nutrients to compost tea can be beneficial for disease suppression, although it can trigger the regrowth of human pathogens like E. coli and Salmonella.[66]

Compost extracts are unfermented or non-brewed extracts of leached compost contents dissolved in any solvent.[66]

Compost is sold as bagged potting mixes in garden centers and other outlets.[68][59] This may include composted materials such as manure and peat but is also likely to contain loam, fertilizers, sand, grit, etc. Varieties include multi-purpose composts designed for most aspects of planting, John Innes formulations,[68] grow bags, designed to have crops such as tomatoes directly planted into them. There are also a range of specialist composts available, e.g. for vegetables, orchids, houseplants, hanging baskets, roses, ericaceous plants, seedlings, potting on, etc.

Compost can also be used for land and stream reclamation, wetland construction, and landfill cover.[69]

The temperatures generated by compost can be used to heat greenhouses, such as by being placed around the outside edges.[70]