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How does composting work?

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Quick answer

Composting is the aerobic decomposition of organic matter by microorganisms. Bacteria and fungi consume plant scraps, using oxygen and releasing heat, carbon dioxide, and water. This process breaks down materials into humus, a dark, nutrient-rich substance that improves soil health and supports plant growth.

Composting works when bacteria, fungi, and other decomposers break down organic matter into simpler substances, using oxygen and releasing heat, carbon dioxide, and water. The result is humus, a dark, nutrient-rich material that improves soil structure and feeds plants.

Step by step

  1. Collect a mix of 'greens' (nitrogen-rich materials like food scraps and grass) and 'browns' (carbon-rich materials like leaves and cardboard).
  2. Chop or shred materials into smaller pieces to increase surface area for microbes to attack.
  3. Layer greens and browns in a pile or bin, keeping it as moist as a wrung-out sponge and turning it regularly to supply oxygen.
  4. Microbes multiply and heat the pile to 55–65°C (131–149°F), speeding decomposition and killing pathogens.
  5. As easy food runs out, fungi break down tougher compounds, and the pile cools and matures.
  6. After weeks to months, the compost becomes dark, crumbly humus, ready to use in gardens.

Key things to understand

  1. Composting starts when microorganisms like bacteria and fungi begin consuming easily degradable compounds such as sugars and starches.
  2. As they feed, microbes multiply rapidly, and their metabolism releases heat, raising the pile's temperature to 55–65°C (131–149°F).
  3. High temperatures speed up decomposition and kill many pathogens and weed seeds, while oxygen keeps the process aerobic and odor-free.
  4. Once the easy food is gone, fungi and actinomycetes break down tougher materials like cellulose and lignin, and the pile cools.
  5. The finished compost, called humus, is a stable, dark, crumbly material rich in nutrients and beneficial microbes.

Labelled diagram

  1. Organic waste

    Greens and browns are mixed; greens provide nitrogen, browns provide carbon.

  2. Microbial activity

    Bacteria and fungi consume easy compounds, multiplying and releasing heat.

  3. Hot phase

    Temperatures reach 55–65°C, speeding decomposition and killing pathogens.

  4. Cooling and maturing

    Fungi break down tough materials like cellulose and lignin as the pile cools.

  5. Finished compost

    Dark, crumbly humus is ready to enrich soil and feed plants.

The composting process from start to finish

Real-world examples

  • A backyard compost bin in a temperate climate turns kitchen scraps and yard waste into compost in about 2–3 months during summer.
  • San Francisco's mandatory composting program collects food scraps from residents and businesses, turning them into compost for local farms.
  • Vermicomposting with red wigglers in a worm bin is popular for apartment dwellers to compost food scraps indoors without odors.
  • Large-scale windrow composting at farms uses mechanical turners to aerate long piles of manure and crop residues, producing compost for fields.

Common mistake

✗ Myth: Composting is just letting food rot.

✓ Fact: Composting is a controlled aerobic process that requires a balance of materials, oxygen, and moisture to work efficiently. Unmanaged rotting heaps often turn anaerobic and smell foul, while proper composting stays aerobic and smells earthy.

People also ask

What causes compost to heat up?
Compost heats up because microorganisms, mainly bacteria, rapidly consume organic matter and release energy as heat. As they break down sugars and starches, their metabolism generates temperatures that can reach 55–65°C (131–149°F). This heat is a sign of active, aerobic decomposition and helps kill pathogens and weed seeds.
Why is my compost not breaking down?
If your compost isn't breaking down, it likely lacks nitrogen, oxygen, or moisture. A pile with too many browns (carbon-rich) and not enough greens (nitrogen-rich) will decompose slowly. Turn the pile to add oxygen, and mix in grass clippings or food scraps to boost nitrogen. Keep it as moist as a wrung-out sponge.
Can you compost in winter?
Yes, you can compost in winter, but the process slows down significantly because cold temperatures reduce microbial activity. The pile may freeze and stop decomposing, then resume in spring. To keep it active, insulate the pile with a tarp or build it large enough to retain heat. You can also continue adding materials and let them decompose later.
What can I compost?
You can compost most plant-based materials, including fruit and vegetable scraps, coffee grounds, grass clippings, leaves, and shredded paper. Avoid meat, dairy, and oily foods because they attract pests and cause odors. Also skip pet waste and diseased plants, which can carry pathogens.
Why does my compost pile smell bad?
A bad smell usually means the pile has gone anaerobic, meaning it lacks oxygen. This happens when the pile is too wet or too compacted. Turn the pile to add oxygen, and mix in dry, brown materials like leaves or cardboard to restore balance.
How long does composting take?
Composting can take anywhere from a few weeks to a year, depending on conditions. A hot, well-managed pile with the right mix of greens and browns can produce compost in 4–8 weeks. Cooler, less managed piles may take 6–12 months.
Do I need to add worms to compost?
No, worms are not required for composting. Bacteria and fungi do most of the work. However, adding red wigglers (vermicomposting) can speed up the process for kitchen scraps and produce nutrient-rich castings.

Part of the guide: Everyday Science: Light, Heat & Matter →