Compost is a mixture of materials used as plant fertilizer and to improve soil's physical, chemical, and biological properties. It is made by breaking down plant and food waste, recycling organic materials, and using manure. The final product is rich in nutrients and helpful microorganisms, such as bacteria, protozoa, nematodes, and fungi. Compost improves soil fertility in gardens, landscaping, horticulture, urban agriculture, and organic farming, reducing the need for chemical fertilizers. Benefits of compost include providing nutrients to crops, acting as a soil conditioner, increasing the amount of humus or humic acid in soil, and adding microbes that help reduce harmful soil diseases.

At the simplest level, composting requires collecting green waste (materials high in nitrogen, such as leaves, grass, and food scraps) and brown waste (materials high in carbon, such as stalks, paper, and wood chips). These materials break down into humus over several months. Composting can involve careful steps, including adding water, air, and balanced amounts of carbon- and nitrogen-rich materials. Shredding plant material, adding water, and turning the pile regularly helps the process. Fungi, earthworms, and other organisms that eat dead material further break down the organic matter. Aerobic bacteria and fungi convert the materials into heat, carbon dioxide, and ammonium ions.

Composting is important for waste management because food and other compostable materials make up about 20% of landfill waste. In landfills, these materials take longer to break down due to the lack of oxygen. Composting is a better environmental choice than landfilling because it reduces methane emissions, which are harmful gases produced without oxygen. Compost also has other benefits, such as helping to restore land and streams, build wetlands, and cover landfills.

Fundamentals

Composting is a process that uses oxygen to break down organic waste, such as food scraps and plant materials, into a rich soil-like material called compost. This compost is useful as fertilizer for plants.

To work well, composting needs four key ingredients:

• Carbon provides energy. Materials high in carbon are usually brown and dry, like dried leaves or wood chips.
• Nitrogen helps organisms grow and reproduce. Materials high in nitrogen are often green and wet, such as fresh grass clippings or fruit and vegetable scraps.
• Oxygen is needed for the decomposition process. Aerobic bacteria require oxygen levels above 5% to function properly.
• Water must be present in the right amount. Too much water can create anaerobic conditions, while too little can slow down the process.

The right balance of these materials helps microorganisms work efficiently, creating heat in the compost pile. Turning the pile regularly helps keep oxygen levels high and moisture balanced. Maintaining temperatures between 130–160 °F (54–71 °C) is important for breaking down materials quickly.

Composting works best when the carbon-to-nitrogen ratio is about 25:1. Hot composting keeps the pile warm to speed up decomposition, while rapid composting uses a ratio of about 30:1 or less. Too much nitrogen (below 15:1) can cause nitrogen to escape as ammonia gas.

Most plant and animal materials contain both carbon and nitrogen. Fresh grass clippings have a ratio of about 15:1, while dry autumn leaves have a ratio of about 50:1. Composting is an ongoing process that requires adding new materials and managing the pile regularly.

Microorganisms break down organic matter if given the right mix of water, oxygen, carbon, and nitrogen. These organisms are divided into two groups:

• Chemical decomposers use chemical processes to break down waste.
• Physical decomposers break waste into smaller pieces through actions like grinding or chewing.

Bacteria are the most important microorganisms in compost. They process carbon and nitrogen, releasing nutrients like nitrogen, phosphorus, and magnesium for plants. Two types of bacteria are involved:

• Mesophilic bacteria work at moderate temperatures and help start the composting process.
• Thermophilic bacteria thrive at higher temperatures (40–60 °C or 104–140 °F) and are active once the pile is warm. They can be found in soil and spread through rainwater.

Other microorganisms, like Actinomycetota, help break down tough materials such as paper and wood. They also create the "earthy" smell of compost. Fungi (like molds and yeasts) break down materials such as wood and plant fibers. Protozoa help break down organic matter and consume other microorganisms.

Insects and other animals also play a role:

• Ants make the soil more porous and move nutrients around.
• Beetles eat decaying vegetables.
• Earthworms consume compost and excrete nutrient-rich waste, improving soil aeration.
• Flies introduce bacteria but are controlled by high temperatures and mites.
• Millipedes and rotifers break down plant material.
• Snails and slugs eat fresh plants and should be removed from compost.
• Sow bugs and springtails help decompose wood and fungi.

Composting happens in four main phases:

• Mesophilic phase (2–8 days): Moderate-temperature bacteria begin the process.
• Thermophilic phase: Temperatures rise to 50–60 °C (122–140 °F), and heat-loving bacteria break down materials.
• Cooling phase: Temperatures start to drop.
• Maturation phase: The compost stabilizes as nutrients become available for plants.

Semicomposting is a slower process that doesn’t reach high temperatures, relying only on moderate-temperature bacteria.

The time needed for composting depends on the size of the pile, the size of the materials (smaller pieces break down faster), and how often the pile is turned. Large piles can reach higher temperatures, making them ideal for hot composting used in large-scale operations.

The Berkeley method creates finished compost in 18 days. It requires a pile of at least 1 cubic meter (35 cubic feet) and turning the pile every two days after an initial four-day period. This method uses smaller, uniform materials and carefully controls the carbon-to-nitrogen ratio.

Cold composting is slower and can take up to a year. It works best with small piles, like those in home gardens, and doesn’t require turning. However, it may not reach high enough temperatures to kill all pathogens or seeds.

Composting can kill some harmful bacteria and seeds if the pile reaches temperatures between 50–70 °C (122–158 °F). Stabilized compost, which has finished breaking down, is safe to use because it has already killed most pathogens.

Compost products like compost tea and extracts are made from compost and can be used to improve soil health.

Environmental benefits

Compost improves soil by adding organic material and making it richer in nutrients. It also helps more types of tiny living things grow in the soil. When people compost at home, less yard waste is sent to landfills or composting centers. This means trucks carry less waste, so they make fewer trips. Fewer trips lead to less pollution from the trucks that collect waste.

Materials that can be composted

Compostable materials, also called feedstocks, come from homes, farms, and businesses. Food and yard waste from homes can be composted at home or sent to large composting centers in cities. In some areas, this waste may also be used in local composting projects.

Organic waste is divided into two main groups: green and brown. Green waste provides nitrogen, a nutrient important for plant growth. Examples include food scraps, grass clippings, garden trimmings, and fresh leaves. Animal remains, such as roadkill or butcher waste, can also be composted and are a source of nitrogen.

Brown waste provides carbon, another essential nutrient for plants. Examples include dried leaves, straw, wood chips, pine needles, and sawdust. Paper and plain cardboard are also carbon sources. Charcoal ash is not used in composting.

On farms, compost is often made using animal manure as a nitrogen source and bedding materials, like straw or sawdust, as a carbon source. Other materials, such as newspaper or cardboard, may also be used. The amount of manure composted depends on factors like cleaning schedules, land space, and weather. Different manures have unique properties. For example, cattle and horse manure mix well with bedding, while swine manure, which is very wet, needs to be mixed with straw. Poultry manure requires high-carbon, low-nitrogen materials to balance it.

Human waste, sometimes called "humanure," can be used in composting because it contains nutrients like nitrogen and phosphorus. Nitrogen helps plants make proteins, and phosphorus helps plants use sunlight to create energy. Solid human waste can be collected in composting toilets or treated in sewage plants and turned into compost. Both methods require careful handling to avoid health risks. Composting human waste at home can expose people to harmful germs if not done properly. Composting waste from large sewage plants may also involve managing materials that contain metals or medicines.

Urine can be added to compost piles or used directly as fertilizer. Urine can raise the temperature of compost, helping to kill harmful germs and seeds. Unlike feces, urine does not attract disease-carrying insects or contain the strongest germs, like those from parasitic worms.

Animal carcasses can be composted as a way to dispose of them. These materials are rich in nitrogen.

Human composting, also called natural organic reduction, is a process that turns human remains into compost using microbes and organic materials. It was developed in the early 2000s as an eco-friendly alternative to burial and cremation. It is legal in some U.S. states and Sweden. This practice has raised questions about its environmental effects, legal status, and religious beliefs.

Composting technologies

In-vessel composting is a method that keeps composting materials inside a building, container, or vessel. These systems can use metal, plastic, or concrete structures where air flow and temperature are controlled, similar to how a "bioreactor" works. Air is often pushed into the system through buried tubes, and exhaust air is removed through a biofilter. Probes in the compost monitor temperature and moisture to help maintain the best conditions for decomposition. Many systems use augers or rotating parts to mix the material, which helps speed up the breakdown process. Examples of this include rotary drum composters made by companies like XACT and auger-based systems like Earth Flow from Green Mountain Technologies and Biomax G from Sorain Cecchinni.

In-vessel composting is often used to process organic waste from cities, such as sewage sludge, food scraps, or the organic part of household trash. It helps kill harmful germs so the compost can be used as fertilizer, while reducing smells, water runoff, and pollution. It can also refer to composting in large piles covered by buildings or removable covers, like systems used by farmers in Thailand with support from their science agency. Recently, smaller versions of in-vessel composting have been developed, sometimes using modified waste containers or shipping containers.

Aerated static pile (ASP) composting uses forced air to keep compost materials oxygen-rich without physically turning the pile. This is done by placing materials on pipes with holes, and air is either pushed up, pulled down, or moved back and forth through the pile. If air is pulled through the pile, it is often passed through a filter to remove smells. ASP systems are usually built outdoors in large beds or enclosed spaces like tunnels or windrows. Many are covered to protect from rain, which helps reduce water runoff and allows moisture to evaporate naturally.

In agriculture, windrow composting involves piling organic materials like manure or plant waste into long rows. This method is also called Open Windrow Composting (OWC) or Open Air Windrow Composting (OAWC) because it happens outdoors.

Hügelkultur is a gardening technique where decomposing wood is buried under soil to create raised garden beds. The wood acts like a sponge, holding water and warming the soil, which helps plants grow better.

A composting toilet is a type of toilet that uses composting to break down human waste. Microorganisms like bacteria and fungi decompose the waste under controlled conditions. These toilets usually do not use water for flushing. After each use, materials like sawdust or coconut coir are added to help the composting process by creating air pockets and balancing nutrients. Most systems use moderate-temperature composting, which helps reduce harmful germs over time. The final product can be used as fertilizer if allowed by local rules. Some toilets separate urine to control moisture, and others use worms to aid decomposition.

Vermicompost is the result of earthworms breaking down organic matter. It contains more nutrients and fewer contaminants than the original material. Black soldier fly larvae can quickly consume organic waste and produce compost that can be used for biogas or further composting. Bokashi is a fermentation process that preserves food waste’s nutrients and energy, often used for noncompostable items. Co-composting mixes organic waste with materials like sewage sludge. Anaerobic digestion, combined with sorting waste, is used in some countries to reduce landfill methane emissions. Small farms may use turned compost piles, mixing materials with the right ratios, checking temperature, and turning the pile weekly.

Uses

Compost can be spread over the soil on open fields where crops like wheat, corn, and soybeans are grown. This is done using trucks or machines pulled by tractors. The compost layer is usually very thin, about 6 mm (0.24 in), and mixed into the soil before planting. When soil is poor or erosion is a problem, thicker layers of compost, up to 25 mm (0.98 in) or more, may be used. In countries like Germany, compost is used more often on open fields because governments sometimes help pay for composting through waste fees, aiming to keep nutrients in the soil sustainably.

In plasticulture, crops such as strawberries, tomatoes, and peppers are grown under plastic to control temperature, keep moisture, and prevent weeds. Compost can be applied in strips along rows and mixed into the soil before planting, added when creating raised beds and laying plastic, or used as a top layer on the soil.

Some crops are not planted directly in the field. Instead, seeds are started in greenhouses using seed trays. When the seedlings are ready, they are moved to the field. Compost may be part of the soil mix used to grow seedlings, but it is not usually the only material. The type of crop and the seeds’ sensitivity to nutrients and salts determine the mix. The compost must be mature enough to avoid problems like lack of oxygen or harmful chemicals that could harm plants.

Compost can be added to soil, coir, or peat to improve soil structure. It provides nutrients and helps hold moisture. Compost is often mixed with other materials like sand, bark chips, or perlite to create a balanced growing medium. It is rarely used alone but can be tilled directly into the soil to increase organic matter and fertility. Ready-to-use compost is dark brown or black and has an earthy smell.

Planting seeds directly into compost is not usually recommended. Compost may dry too quickly, and immature compost can contain harmful chemicals that stop seeds from growing. It can also tie up nitrogen, making it unavailable to plants. Instead, compost is often mixed with other materials in amounts between 20% and 30% for transplanting seedlings.

Compost can help plants resist diseases and pests.

Compost tea is made by mixing compost with water. The compost can be aerated (mixed with air) or not, depending on the brewing method. Tea is typically made with a 1:4 to 1:10 ratio of compost to water, and the mixture is stirred to release microbes.

There is debate about whether aerating compost tea is better. Non-aerated tea is cheaper and easier to make, but some studies suggest it may contain harmful chemicals or regrow disease-causing microbes. Aerated tea produces more microbes quickly but may also regrow harmful microbes if extra nutrients are added.

Studies show that compost tea can improve soil health by adding organic matter, increasing nutrients, and boosting microbial activity. It may also help prevent plant diseases. However, the effectiveness depends on how the tea is made, the compost source, and the conditions where it is used. Adding nutrients to compost tea can help fight diseases but may also cause harmful microbes like E. coli or Salmonella to grow.

Compost extracts are made by dissolving compost materials in water or other solvents without fermenting them.

Compost is sold in bags at garden stores. These mixes often include materials like manure, peat, or loam, and may also have sand, fertilizers, or other additives. Different types of compost are available, such as multi-purpose mixes, special blends for specific plants like roses or orchids, and grow bags designed for planting crops like tomatoes directly.

Community composting uses organic waste from local areas, such as schools or community gardens. The compost is used locally and is usually part of educational programs. These operations are smaller than commercial composting facilities.

Compost is also used to restore land, repair wetlands, cover landfills, and improve streams.

The heat produced during composting can be used to warm greenhouses by placing compost around the edges.

Regulations

In Europe, rules for making and using products have been in place since the early 1980s in countries like Germany, the Netherlands, and Switzerland. In the United Kingdom and the United States, such rules were created more recently. In these two countries, private groups in the industry have set less strict standards. Some people believe these standards were created temporarily to prevent government agencies from making stricter rules that would better protect consumers. Canada and Australia also regulate compost.

In the United States, the EPA Class A and B guidelines were created to manage the processing and reuse of sludge, now called biosolids, after the EPA banned dumping sludge into the ocean. About 26 U.S. states now require compost to follow these federal rules for controlling harmful germs and pests, even though these rules have not been tested for use with non-sludge materials. For example, green waste compost is used at much higher rates than sludge compost was ever expected to be used. Guidelines for compost quality also exist in the United Kingdom, Canada, Australia, and many European countries.

In the United States, some compost makers take part in a testing program run by a private group called the U.S. Composting Council. This group was started in 1991 by Procter & Gamble to support composting disposable diapers after states banned throwing them in landfills, which caused public concern. However, composting diapers was later stopped because it was not proven to work scientifically and was seen as a marketing idea. After this, the focus returned to composting organic waste that was previously sent to landfills. There are no real quality standards for compost in the United States, but the U.S. Composting Council sells a logo called "Seal of Testing Assurance" (STA). Companies pay a fee to use this logo, which means they agree to provide customers with recent lab results showing details like nutrients, respiration rate, salt levels, pH, and a few other measurements.

In countries like Wales and cities such as Seattle and San Francisco, laws require people to separate food and yard waste for composting (as required by San Francisco’s Mandatory Recycling and Composting Ordinance).

The United States is the only Western country that does not treat compost made from sludge differently from compost made from green waste. By law, 50% of U.S. states expect compost to meet some requirements from the EPA’s 1984 rule for sludge products.

There are health concerns about high levels of PFASs, also called "forever chemicals," in compost made from biosolids in sewage sludge. The EPA has not set safety standards for these chemicals in compost. The Sierra Club advises home gardeners to avoid using compost made from sewage sludge because of the risk of high PFAS levels. The EPA’s PFAS Strategic Roadmap, which runs from 2021 to 2024, will study the full life cycle of PFAS, including health risks from PFAS in wastewater sludge.

History

Composting began in the early Roman Empire and was written about in a book called De Agri Cultura by Cato the Elder in 160 BCE. Traditionally, composting meant collecting organic materials, such as food scraps and plant waste, and letting them break down over time until they were ready to be added to soil before planting. This practice was part of farming traditions in many countries around the world.

Composting became more advanced in the 1920s in Europe as part of organic farming efforts. The first large facility to turn city waste into compost was built in Wels, Austria, in 1921. Important early supporters of composting included Rudolf Steiner, who created a farming method called biodynamics, and Annie Francé-Harrar, who helped Mexico establish a program to improve soil health between 1950 and 1958. Sir Albert Howard, who worked in India on farming practices, and Lady Eve Balfour also promoted composting. In the United States, modern composting methods were introduced by J. I. Rodale, who started Rodale, Inc. and Organic Gardening, and others involved in the organic farming movement.

India plays the major role in the development of compost and natural fertilizers.

India has an important role in creating and using compost and natural fertilizers. This is because of its long history of farming and growing focus on ways to farm that protect the environment. For many years, people in India have used methods like composting leftover crops, animal waste, and other organic materials to keep soil healthy and balance the ecosystem. In recent years, national programs such as the National Mission for Sustainable Agriculture and the Paramparagat Krishi Vikas Yojana have worked to encourage organic farming and the use of bio-fertilizers throughout the country.

In addition to government efforts, companies like Coromandel International, National Fertilizers Limited, and Rashtriya Chemicals & Fertilizers have helped develop, make, and share eco-friendly fertilizers, including compost-based and bio-organic products. India has also become a major exporter of organic fertilizers and soil enhancers, sending items like vermicompost and bio-fertilizers to countries in Asia, Europe, and North America. This increase in exports shows how important India is in helping the world move toward farming practices that are better for the environment.