Compost is a mixture of materials used to help plants grow and improve soil quality. It is made by breaking down plant and food waste, recycling organic materials, and using manure. The final product contains nutrients that plants need and helpful living things like bacteria, protozoa, nematodes, and fungi. Compost helps make soil healthier in gardens, landscaping, horticulture, urban farming, and organic farming. It reduces the need for chemical fertilizers. Benefits of compost include giving plants nutrients, improving soil structure, increasing the amount of humus or humic acid in soil, and adding microbes that help protect plants from harmful germs and diseases.

To make compost, you need to mix green waste, such as leaves, grass, and food scraps (which are rich in nitrogen), with brown waste, like stalks, paper, and wood chips (which are rich in carbon). These materials take months to turn into humus. Composting can be a detailed process that carefully controls water, air, and the balance of carbon and nitrogen. Breaking plant material into smaller pieces, adding water, and turning the pile regularly helps the process. Fungi, earthworms, and other organisms that eat dead material help break things down further. Bacteria and fungi use oxygen to change the materials into heat, carbon dioxide, and ammonium.

Composting is important for managing waste because food and other compostable materials make up about 20% of landfill waste. In landfills, these materials break down slowly because there is no oxygen. Composting is a better choice than sending organic waste to landfills because it reduces harmful gas emissions and offers environmental and economic benefits. For example, compost can be used to restore land and water areas, build wetlands, and cover landfills.

Fundamentals

Composting is a way to break down organic waste using oxygen. This process turns organic materials into a rich soil-like material called compost, which helps plants grow.

Microorganisms need four key ingredients to work well:

• Carbon: Provides energy. Materials high in carbon are usually brown and dry.
• Nitrogen: Helps microorganisms grow and multiply. Materials high in nitrogen are often green and wet, such as fruits and vegetables.
• Oxygen: Needed for breaking down materials. Aerobic bacteria need oxygen levels above 5% to work properly.
• Water: Must be in the right amount to keep microorganisms active without causing areas with no oxygen.

The right mix of these materials helps microorganisms heat up the compost pile. Turning the pile regularly keeps oxygen and moisture levels balanced. This balance is important for keeping temperatures between 130–160 °F (54–71 °C) until the materials break down completely.

Composting works best when the carbon-to-nitrogen ratio is about 25:1. Hot composting keeps the pile warm to speed up decomposition, making compost faster. If the ratio is too low (below 15), nitrogen may escape as ammonia. If the ratio is too high (above 30), the process slows.

Most plant and animal materials have 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.

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

• Chemical decomposers: Use chemical processes to break down waste.
• Physical decomposers: Break waste into smaller pieces by grinding, tearing, chewing, or digesting.

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 start the composting process.
• Thermophilic bacteria: Work best at high temperatures (40–60 °C or 104–140 °F) and help kill harmful germs.

Actinomycetota help break down tough materials like paper, bark, and wood. They also give compost its "earthy" smell.

Fungi like molds and yeasts break down materials bacteria cannot, such as wood.
Protozoa help break down organic matter and eat bacteria, fungi, and tiny particles.

Other organisms in compost include:
– Ants: Make soil more porous and move nutrients.
– Beetles: Eat decaying vegetables.
– Earthworms: Eat compost and excrete nutrients for plants. Their tunnels improve aeration.
– Flies: Add bacteria to compost, but their numbers are controlled by heat and mites.
– Millipedes: Break down plant material.
– Rotifers: Eat plant particles.
– Snails and slugs: Eat fresh plant material and should be removed before using compost.
– Sow bugs: Eat rotting wood and plants.
– Springtails: Eat fungi and decaying plants.

Composting goes through four main stages:

• Mesophilic phase: Begins with moderate temperatures (2–8 days).
• Thermophilic phase: Temperatures rise to 50–60 °C (122–140 °F).
• Cooling phase: Temperatures start to drop.
• Maturation phase: Compost cools as nutrients are used up.

Semicomposting is a slower process that does not reach high temperatures. Only mesophilic microorganisms work on the waste.

The time needed to compost depends on the size of the pile, the size of the materials, and how much it is mixed. Large piles reach higher temperatures and stay hot longer, which is called hot composting. This is common in large-scale operations.

The Berkeley method makes finished compost in 18 days. It requires at least 1 cubic meter (35 cubic feet) of material and turning every two days after four days. This method uses smaller, uniform materials and carefully controls the carbon-to-nitrogen ratio (30:1 or less).

Cold composting is slower and can take up to a year. It happens in small piles, like those in homes, and does not reach high temperatures. Turning is not needed, but some parts of the pile may become too wet or lack oxygen.

Composting can kill germs and seeds if temperatures reach 50–70 °C (122–158 °F). Stabilized compost, which has reached these temperatures, is safe to use because it kills harmful germs. The time and temperature needed to kill germs depend on the type of germ and the compost’s pH level.

Compost products like compost tea and extracts have been found to…

Environmental benefits

Compost improves soil by adding organic material and increasing the nutrients and variety of tiny living things in the soil. Composting at home helps reduce the amount of plant waste sent to landfills or composting centers. When less waste is collected, trucks make fewer trips, which reduces the total pollution from the vehicles used in waste management.

Materials that can be composted

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

Organic solid waste is divided into two main groups: green and brown. Green waste is a source of nitrogen and includes food waste, grass clippings, garden trimmings, and fresh leaves. Animal remains, such as roadkill or butcher waste, can also be composted and are nitrogen sources.

Brown waste provides carbon. Examples include dried leaves, straw, wood chips, pine needles, and sawdust, but not charcoal ash. Paper and plain cardboard are also carbon sources.

On farms, composting often uses animal manure (a nitrogen source) and bedding materials (a carbon source). Common bedding includes straw and sawdust. Other materials like 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 mixed with bedding work well for composting. Swine manure, which is very wet, needs to be mixed with straw or similar materials. Poultry manure must be combined with high-carbon, low-nitrogen materials.

Human waste, sometimes called "humanure," can be added to compost because it is rich in nutrients. Solid human waste contains nitrogen, which helps plants make amino acids. Liquid human waste contains phosphorus, which helps plants convert sunlight into energy.

Solid human waste can be collected directly in composting toilets or indirectly as sewage sludge after treatment in wastewater plants. Both methods require careful design to manage health risks. At home, feces may contain harmful microorganisms like bacteria or parasites. Improper composting can cause health issues. In large sewage treatment plants, composted sludge, called biosolids, may contain metals or drugs. Poor processing of biosolids can cause problems when used on land.

Urine can be added to compost piles or used directly as fertilizer. Urine increases compost pile temperatures, which helps destroy harmful germs and seeds. Unlike feces, urine does not attract disease-carrying flies or contain the most stubborn pathogens, like parasitic worm eggs.

Animal carcasses can be composted as a disposal method. These materials are high in nitrogen.

Human composting, also called "natural organic reduction" in legal terms, is a process that turns human remains into compost using microbes and organic material. Developed in the early 2000s as an eco-friendly alternative to burial and cremation, it is legal in some U.S. states and Sweden. The practice has sparked discussions about its environmental effects, legal status, and religious views.

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. Fresh air is often pushed into the compost through buried tubes, and exhaust air is removed through a biofilter. Probes are used to monitor temperature and moisture, helping maintain the best conditions for breaking down waste. Some systems use augers or rotating parts to mix the material, speeding up the composting process. Examples of these systems include rotary drum composters from companies like XACT and auger-based systems such as Earth Flow from Green Mountain Technologies and Biomax G from Sorain Cecchinni.

In-vessel composting is commonly used to process organic waste, such as food scraps, sewage sludge, or the organic part of household trash. It helps kill harmful germs, turning waste into safe soil material while reducing smells, runoff, and pollution. This method can also refer to aerated static pile composting inside buildings or under covers, like systems used by farmers in Thailand with support from the National Science and Technology Development Agency. Recently, smaller in-vessel systems have been developed, using containers like roll-off dumpsters or modified shipping containers.

Aerated static pile (ASP) composting uses forced aeration to keep compost materials oxygen-rich without physically mixing them. This is done by placing waste on pipes with holes, where air is pushed, pulled, or moved back and forth through the pile. If air is pulled through the pile, it is often passed through a biofilter to remove odors. ASP systems are usually built outdoors in large beds or bunkers, sometimes inside open buildings, enclosed tunnels, or windrows. Many are covered to protect them from rain, reducing water runoff and allowing excess water to evaporate.

In agriculture, windrow composting involves piling organic materials, like manure or crop waste, into long rows called windrows. This method is also known as open windrow composting (OWC) or open air windrow composting (OAWC) because it happens in the open air.

Hügelkultur is a gardening technique where raised beds or mounds are filled with decaying wood, covered with soil. This method helps retain water and warm the soil because the buried wood acts like a sponge, holding water for plants to use later.

A composting toilet is a type of dry toilet that uses composting to break down human waste. Microorganisms like bacteria and fungi decompose the waste under controlled conditions, turning it into compost-like material. These toilets often use carbon-rich materials like sawdust or coconut coir to help with decomposition and reduce odors. Most rely on mesophilic composting, which is slower but effective at killing germs. After composting, the final product may go through another step to further reduce germs. If allowed by local rules, this compost can be used to enrich soil. Some systems separate urine to control moisture, and others use earthworms in a process called vermifiltration.

Vermicompost is the result of earthworms breaking down organic matter. It is richer in nutrients and has fewer contaminants than the original material. Black soldier fly larvae can quickly consume organic waste, producing compost that can be used for biogas or further composting. Bokashi is a fermentation process that preserves energy and nutrients in food waste, including noncompostable items. This process uses lactic acid to help soil microbes and improve soil health. Co-composting mixes organic waste with other materials like sewage sludge. Anaerobic digestion, often paired with waste sorting, is used in developed countries to reduce landfill methane emissions. Small farms may use turned compost piles, mixing materials in the right ratios, monitoring temperature, and turning the pile weekly.

Uses

Compost can be spread over open ground where wheat, corn, soybeans, and similar crops are grown. Spreader trucks or spreaders attached to tractors are used for this. The compost layer is usually very thin, about 6 mm (0.24 in), and mixed into the soil before planting. When rebuilding poor soil or controlling erosion, compost layers of 25 mm (0.98 in) or more are sometimes used. In countries like Germany, compost is used more often on open ground because it is partially supported by fees for handling waste, and it helps maintain soil nutrients over time.

In plasticulture, crops such as strawberries, tomatoes, peppers, melons, and other fruits and vegetables 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 during bed construction and plastic installation, or used as a top dressing.

Some crops are not planted directly in the field but are grown in seed trays inside greenhouses. When seedlings are ready, they are moved to the field. Compost may be part of the 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 ratio. The compost must be mature enough to avoid oxygen shortages or harmful substances that could harm plants.

Compost can be added to soil, coir, or peat to improve soil structure, providing humus and nutrients. It acts as an absorbent material that holds moisture and minerals, offering support and nutrients to plants. While compost is rarely used alone, it can be mixed with other materials like sand, grit, bark chips, vermiculite, perlite, or clay granules to create loam. Compost can be tilled directly into soil to increase organic matter and soil fertility. Ready-to-use compost is typically dark brown or black and has an earthy smell.

Directly planting seeds into compost is not recommended because compost can dry quickly, immature compost may contain harmful substances that prevent germination, and decomposing materials might tie up nitrogen needed by plants. It is common to use compost blends of 20–30% when transplanting seedlings.

Compost can help plants resist diseases and pests.

Compost tea is made by extracting fermented water from composted materials. Compost teas can be aerated (with air added) or non-aerated, depending on the fermentation process. They are usually made by mixing compost with water in a ratio of 1:4–1:10, sometimes stirring to release microbes.

There is debate about whether aerating compost tea is beneficial. Non-aerated compost tea is cheaper and easier to make, but studies show it may have risks of harmful substances or regrowth of disease-causing microbes. Aerated compost tea produces more microbes faster but may also allow disease-causing microbes to regrow, especially if extra nutrients are added.

Field studies show that adding compost tea to crops improves soil health by increasing organic matter, nutrients, and microbial activity. It can also reduce the presence of harmful soil-borne diseases. The effectiveness depends on how the tea is prepared, the compost source, brewing conditions, and the environment where crops grow. Adding nutrients to compost tea may help control diseases but could also lead to the regrowth of harmful microbes like E. coli and Salmonella.

Compost extracts are non-fermented liquids made by dissolving compost materials in a solvent.

Compost is sold in bagged potting mixes at garden centers. These mixes may include composted materials like manure or peat, as well as loam, fertilizers, sand, and grit. Different types are available, such as multi-purpose composts, John Innes formulations, grow bags for planting crops like tomatoes, and specialized composts for vegetables, orchids, houseplants, hanging baskets, roses, ericaceous plants, seedlings, and potting on.

Community composting uses organic waste collected from local areas, such as schools or community gardens. These programs often teach the public about composting. The compost is used locally and is typically smaller in scale than commercial composting operations.

Compost is also used for land and stream restoration, wetland construction, and covering landfills.

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

Regulations

In Europe, rules and standards for composting and product quality began in the early 1980s in Germany, the Netherlands, and Switzerland. These rules were later created in the United Kingdom and the United States. In both the UK and the US, private groups in the industry set basic standards, some say to prevent government agencies from creating stricter rules that would better protect consumers. Compost is also regulated in Canada and Australia.

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 US EPA banned dumping sludge into the ocean. About 26 states now require compost to follow these federal rules for controlling harmful germs and pests, even though these rules were not tested for use with non-sludge materials. For example, green waste compost is used at much higher levels than sludge compost was ever expected to be used. Rules about compost quality also exist in the UK, Canada, Australia, and many European countries.

In the US, some compost makers take part in a testing program run by a private group called the U.S. Composting Council. The USCC was started in 1991 by Procter & Gamble to support composting disposable diapers after states banned throwing them in landfills, which caused public concern. The idea of composting diapers was later stopped because it was not proven to work and was seen as a marketing effort. After this, the focus returned to composting organic waste from landfills. The USCC offers a program called "Seal of Testing Assurance" (STA), which allows companies to display its logo on products after paying a fee. This requires the company to share lab results about compost quality, including nutrients, pH, salt levels, and other factors.

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

The United States is the only major Western country that does not separate compost made from sludge (biosolids) from compost made from green waste. About half of US states expect compost to meet some requirements from the EPA 503 rule, which was created in 1984 for sludge products.

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

History

Composting has been practiced for a long time, dating back to the early Roman Empire. It was first written about in a book called De Agri Cultura, written by Cato the Elder in 160 BCE. In the past, composting meant collecting organic materials, such as food scraps and plant waste, and letting them decompose over time until they were ready to be added to soil for planting. This method was used in farming traditions around the world.

Composting began to change in the 1920s in Europe as part of organic farming efforts. The first industrial facility to turn city organic waste into compost was built in Wels, Austria, in 1921. Early supporters of composting in farming included Rudolf Steiner, who created a farming method called biodynamics, and Annie Francé-Harrar, who helped Mexico establish a group focused on improving soil health to prevent erosion and damage from the 1950s to 1958. Sir Albert Howard, who worked in India to develop sustainable farming practices, and Lady Eve Balfour also promoted composting. In the United States, modern scientific composting was introduced by J. I. Rodale, founder of Rodale, Inc. and Organic Gardening, as well as 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 farming methods that protect the environment. For many years, people in India have used traditional methods, such as turning plant waste, animal manure, and organic materials into compost, to keep soil healthy and the environment balanced. In recent years, government programs like the National Mission for Sustainable Agriculture and the Paramparagat Krishi Vikas Yojana have worked to encourage organic farming and the use of natural fertilizers across the country.

Companies such as Coromandel International, National Fertilizers Limited, and Rashtriya Chemicals & Fertilizers also help by researching, making, and selling eco-friendly fertilizers, including compost-based and organic products. India is now a major exporter of organic fertilizers and soil improvement products, sending items like vermicompost and bio-fertilizers to countries in Asia, Europe, and North America. This growing ability to export shows how important India is in helping the world move toward farming practices that are better for the environment.