What is composting?
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Published: 19 May 2024
Contributors: Julie Rogers, Alexandra Jonker

What is composting?

Composting is the process of recycling organic materials into decomposed matter called compost. Compost is a natural, nutrient-rich fertilizer that can be used to fortify soil and plants.

Composting is done through a controlled, aerobic (oxygen required) process that encourages natural decomposition of organic solid wastes. This process can be sped up by building an ideal environment for bacteria, fungi, worms and other decomposition organisms.

Composting is a viable way to divert organic material out of the waste stream. There are many ways to compost, such as vermicomposting and hot composting. There area also many locations where composting takes place, including commercial composting facilities, municipal composting facilities and at-home composting, which, as the name suggests, takes place at private homes.

The matter resulting from composting is crumbly like fresh soil and can be used in gardening, landscaping, horticulture and agriculture to encourage plant growth.

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How does the composting process work?

Composting is an ongoing process; the full breakdown of organic materials into compost takes months. Microorganisms feed on materials added to the compost pile over time. These microorganisms use water to digest the material, oxygen to breathe and carbon and nitrogen as food. There must be a balance of these four ingredients for the process to be effective.

Ideally, a compost bin would maintain the proper ratio of materials: two to three parts carbon-rich material to one part nitrogen-rich material. Carbon-rich materials, known as “greens,” are things like fruit and vegetable scraps, grass clippings and coffee grounds. Nitrogen-rich materials, known as “browns,” are smaller pieces of plant matter, like yard waste, dry leaves, wood chips, twigs and shredded paper.

Meanwhile, water adds moisture and feeds the microorganisms. A well-fed composting pile maintains a damp moisture level and feels like a wrung-out sponge. In addition, composters use aeration to make sure the microorganisms get enough oxygen. They turn the pile over, or aerate it, once a week in the summer, while in the winter, once every three to four weeks is adequate.

Why is composting important?

The benefits of composting include:

Waste reduction

Composting reduces organic waste. Processing food waste, including kitchen scraps and garden waste, is a burden on the environment. Composting can be a cost-effective waste management solution, lowering a business’ carbon footprint by reducing the amount of waste that goes into landfills. According to the US Environmental Protection Agency (EPA), food and garden material make up about 24% of municipal solid waste in landfills.1

Better soil health

Composting can be essential for improving agriculture systems and maintaining healthy soil. When added to mulch or potting soil, compost improves soil fertility and reduces the dependency on commercial and chemical fertilizers. It also introduces beneficial microbes into the soil that help suppress pathogens, reducing some soil-borne diseases. The soil amendment adds three primary nutrients needed by plant material—nitrogen, phosphorus and potassium—and trace amounts of other essential elements, such as iron and zinc.

Reduction of greenhouse gas emissions

Reducing the volume of food waste and other green material that gets disposed of in landfills helps prevent powerful greenhouse gases from being emitted into the atmosphere. This is because organic matter decomposes through aerobic decomposition, which means it gets broken down by microorganisms that consume oxygen.

If compostable waste, such as food scraps, goes to a landfill, it gets buried under other garbage, cutting off the oxygen supply, and creates biogas as a byproduct. This biogas is approximately 50% methane and 50% carbon dioxide. While many large-scale landfills use methane capture systems, they do not capture all the gas, making landfills the third-largest source of human-generated methane emissions, according to the EPA.

Mitigation of climate change impacts

Because compost helps soil absorb and retain water, it helps communities adapt to adverse effects of climate change such as extreme weather events. Soil fertilized with compost holds more water, mitigating the effects of drought, and prevents runoff of pollutants during floods. Compost also sequesters carbon in the soil, which helps reduce greenhouse gas emissions.

What can be composted?

High temperatures resulting from the decomposition process can help reduce the presence of pathogens, pesticides and weed seeds in the compost material. This is especially true when compost is kept at temperatures around 55°C (130°F) or higher, which is known as hot composting.

However, animal products can still carry pathogens that might survive the composting process, and it's best to avoid composting animal products, such as meat, fish, eggs, bones, grease and dairy products. Brown material that might compromise the finished compost, such as plant material with plant diseases, should also be avoided. 

Inorganic material like plastic can’t be composted, but neither can materials like treated wood, glossy or taped paper and cardboard products. Paper tea bags are okay, without the staples, as are eggshells. Pet waste should not be used in backyard composting, but drop-off composting programs might accept it, depending on local guidelines.

What is vermicomposting?

Another form of a composting system is vermicomposting, or worm composting. Vermicomposting uses a worm bin, made from untreated wood or plastic storage containers, which is kept either indoors or outdoors. The bedding is typically made of sawdust, shredded cardboard, straw or yard trimmings like dry leaves. It generally takes three to six months for the vermicompost—or worm castings—to be ready to harvest from the bottom of the worm bin.

While there are many species of earthworms, only certain kinds can be used for vermicomposting. The most common used in vermicompost are red wigglers, as they quickly ingest waste.2

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1 Quantifying Methane Emissions from Landfilled Food Waste (link resides outside ibm.com), EPA, 22 January 2024.

How to Create and Maintain an Indoor Worm Composting Bin (link resides outside ibm.com), EPA, 3 January 2024.