The Basics of Composting: How Decomposition Actually Works

Composting looks like magic from the outside. You pile up scraps and leaves, do almost nothing for a few months, and somehow it transforms into dark crumbly soil that smells like a forest floor. Most gardeners learn the practical recipe — what to add, how often to turn — without ever understanding what’s actually happening inside the pile.

Understanding the underlying biology and chemistry doesn’t just satisfy curiosity. It also makes you a much better composter, because when something goes wrong (sour smell, pile won’t heat up, takes forever to finish), you can diagnose what’s happening biologically and fix the root cause rather than guess.

This guide walks through what happens inside a compost pile, from the first hour after you add fresh material through the final stages months later. The picture that emerges is more interesting than the simplified “decomposes into compost” story.

What’s actually doing the decomposing

The work of composting is done almost entirely by microorganisms — bacteria, fungi, archaea, protozoa, and at slightly larger scales, mites, springtails, and worms. The microbes are the primary engine; the larger organisms play supporting roles in physical breakdown and pile mixing.

A typical compost pile in active decomposition contains:

Bacteria — billions to trillions per gram of compost. They’re the workhorses of decomposition, especially in the early hot stage. Several distinct bacterial communities take turns dominating the pile at different temperatures.

Fungi — primarily filamentous fungi (the kind that grow as visible threadlike networks called hyphae). Less abundant by cell count than bacteria but very important for breaking down tough materials like wood, leaves, and other lignin-rich inputs. Fungi dominate later, cooler stages of composting.

Actinomycetes — a specialized class of bacteria that grows in filaments similar to fungi. They’re responsible for that earthy “forest floor” smell of finished compost (they produce geosmin). Most abundant in the cooling/curing phase.

Protozoa, nematodes, mites, springtails, worms — secondary decomposers that consume bacteria and fungi, transport microbial communities around the pile, and physically break down materials. Important for the later stages of decomposition.

The cumulative activity of these organisms breaks down complex organic compounds (proteins, carbohydrates, fats, cellulose, lignin) into simpler compounds (amino acids, sugars, fatty acids), then into even simpler products (CO2, water, ammonia, nitrate, humus).

The carbon-to-nitrogen ratio explained

The single most important variable in composting is the carbon-to-nitrogen (C:N) ratio of the input materials. Microbes consume carbon for energy and nitrogen for building proteins. The ideal ratio of carbon to nitrogen they need is about 30:1 — 30 atoms of carbon for every 1 atom of nitrogen.

Different materials have very different C:N ratios:

High-nitrogen (“green”) materials, low C:N ratios:
– Fresh grass clippings: 15:1
– Vegetable scraps: 15-20:1
– Coffee grounds: 20:1
– Fresh manure (cow, horse, chicken): 12-25:1
– Plant trimmings (fresh): 20-30:1

High-carbon (“brown”) materials, high C:N ratios:
– Dry leaves: 50:1
– Straw: 80:1
– Shredded paper/cardboard: 100-300:1
– Sawdust: 400:1
– Wood chips: 400-700:1

The trick is mixing greens and browns to land at roughly 30:1 overall.

What happens if the ratio is wrong:

Too much carbon (over 50:1): Microbes don’t have enough nitrogen to build new cells. Decomposition slows dramatically. The pile sits cold and inactive. Some texts call this “carbon-starved” but the microbes are actually nitrogen-starved while sitting on too much carbon they can’t fully use.

Too much nitrogen (under 20:1): Microbes have more nitrogen than they can incorporate. Excess nitrogen is released as ammonia (NH3) gas, which is why poorly-balanced piles smell sharply of ammonia. The nitrogen is also lost from the pile (going into the air), reducing the finished compost’s nitrogen value.

Approximately 30:1: Microbial activity peaks. Decomposition proceeds quickly. No ammonia loss. Pile heats up properly. Finished compost has good nutrient content.

The exact ratio varies somewhat — anywhere from 25:1 to 40:1 produces good composting — but 30:1 is the target. Most home composters approximate this by adding 3 parts brown to 1 part green by volume.

Why the pile heats up

When you mix the right C:N ratio with adequate moisture and oxygen, microbial activity ramps up rapidly. Bacteria multiply exponentially when conditions are favorable. They consume sugars, simple carbohydrates, and proteins — the easiest energy sources.

Microbial metabolism releases heat. A single bacterium produces a tiny amount of heat, but trillions of bacteria simultaneously metabolizing produce a measurable temperature rise. In a well-built compost pile, the internal temperature rises from ambient (whatever the outdoor temperature is) to 140-160°F over 24-72 hours.

The temperature rise has three important consequences:

Different microbe communities thrive at different temperatures. The pile shifts through three or four distinct microbial communities as it heats and cools:

• Psychrophilic phase (below 70°F): Cold-loving microbes do initial breakdown of easily-accessible nutrients. This phase is brief in a properly-built pile.

• Mesophilic phase (70-105°F): Mid-temperature microbes dominate. Most familiar soil bacteria fall in this group. This phase lasts a few days as the pile warms up.

• Thermophilic phase (105-160°F): Heat-loving bacteria and a few thermophilic fungi dominate. The most rapid decomposition happens here. Pathogens are killed. Weed seeds are killed. This phase lasts 2-4 weeks in a well-managed pile.

• Cooling and curing phase (drop back to ambient): As easily-decomposable materials are exhausted, microbial activity slows, the pile cools, and the slower-growing organisms (fungi, actinomycetes, secondary decomposers) take over. This phase lasts 4-12 weeks.

Heat kills pathogens and weed seeds. Sustained temperatures above 131°F for three days kill most plant pathogens, most weed seeds, and human pathogens like E. coli or Salmonella that may have been in the input materials. This is why proper hot composting produces “clean” compost safe for vegetable gardens. Cold composting (which never reaches these temperatures) doesn’t kill pathogens reliably.

The pile shrinks as it heats. Water in the pile evaporates faster at high temperatures. The pile may lose 30-50% of its initial volume during the hot phase, partly from water loss and partly from carbon being converted to CO2 and released to the atmosphere. A pile that started at 4 cubic feet may end up at 2.5-3 cubic feet.

What microbes need

For microbes to do their decomposition work effectively, they need four things:

Food. The compostable materials themselves. Diverse inputs — kitchen scraps, leaves, grass clippings, paper, garden trimmings — provide varied carbon and nitrogen sources, supporting a diverse microbial community.

Water. Microbes are mostly water; they need a moist environment to function. Compost piles should be about 40-60% moisture (damp sponge feel). Too dry: microbes desiccate and stop working. Too wet: the pile goes anaerobic.

Oxygen. Most efficient decomposition is aerobic — microbes use oxygen to fully break down organic matter to CO2 and water. Anaerobic decomposition (without oxygen) produces methane, alcohols, and organic acids — smelly, slower, and lower-quality compost. Turning the pile every 1-2 weeks introduces fresh oxygen.

Temperature. Microbes have temperature preferences. Cold piles run cold; hot piles run hot. The transition from one microbial community to another is largely driven by temperature changes within the pile.

When all four conditions are met, microbes thrive and composting proceeds quickly. When any one is missing or out of range, composting slows or stops.

Why turning matters

Turning a compost pile does three useful things:

It re-oxygenates the pile. As microbes consume oxygen, the interior of the pile becomes oxygen-depleted. Anaerobic pockets develop. Turning physically moves material around, mixing oxygen-poor interior with oxygen-rich exterior.

It redistributes materials. Some areas of a pile may be wetter or drier; some may be more thoroughly mixed. Turning evens out the conditions so all materials decompose at similar rates.

It exposes new surfaces to microbes. Decomposition happens at the interface between intact material and the microbial community on its surface. Turning breaks up partially-decomposed clumps and exposes new surfaces.

A well-turned pile may finish in 6-8 weeks. An unturned pile may take 4-6 months for the same material to break down, because oxygen depletion limits the rate.

The role of physical particle size

Microbes work at surfaces. Smaller particles have more surface area relative to volume, so they break down faster than larger particles.

A whole apple sitting in a compost pile breaks down slowly because microbial attack is limited to the apple’s exterior surface. The same apple chopped into 1-inch pieces has roughly 6-8x more surface area; chopped into half-inch pieces, roughly 16x more. Decomposition speed scales accordingly.

Practical implication: chop or shred large inputs before adding to the pile. Whole leaves, branches, melon rinds, corn cobs — all benefit from being reduced to smaller pieces before composting.

For compost liner bags and compostable food containers, the BPI-certified materials are designed to decompose at rates similar to other organic inputs when chopped or torn to expose surface area.

The chemistry: what materials break down to what

Different input materials break down through different pathways:

Sugars and simple carbohydrates (fruit, vegetables) break down first, fastest. Microbes hydrolyze them into individual sugar molecules, then ferment those into CO2 and water, releasing heat in the process. Days 1-7 of composting are dominated by this process.

Proteins (egg shells with residue, meat scraps, lawn clippings high in nitrogen) break down through proteolysis — enzymatic cutting of protein chains into amino acids, then further breakdown to ammonia, then conversion to nitrate by nitrifying bacteria. The nitrogen ends up incorporated into microbial biomass or as nitrate in the finished compost.

Cellulose (paper, leaves, plant stems) breaks down via cellulase enzymes produced by bacteria and fungi. Cellulose is harder to break down than sugars — takes weeks to months in a hot pile, longer in a cold pile. Cellulose is the main component of “browns.”

Hemicellulose (similar to cellulose, with shorter sugar chains) breaks down faster than cellulose. It accompanies cellulose in most plant materials.

Lignin (the structural compound in wood and tough plant tissues) is the hardest to break down. Lignin requires specialized fungal enzymes (laccases, peroxidases) that mostly come from white-rot fungi. Lignin breakdown happens mostly in the cooler, curing phase of composting. The fact that lignin breaks down slowly is what makes finished compost dark — the partially-degraded lignin contributes to the brown-black humus content.

Fats and oils break down slowly via lipase enzymes. They can also cause problems by coating other materials and reducing oxygen availability. This is why fatty or oily food scraps are usually not recommended for home composting.

Eggshells and other minerals are largely inert. The shell calcium carbonate doesn’t biodegrade per se; it dissolves slowly via microbially-produced acid attack. Eggshells break down on a timescale of months to years.

Why finished compost is darker than the inputs

The dark color of finished compost comes from a class of compounds called humic substances — humin, humic acid, fulvic acid. These are complex polymers that form during decomposition when microbial breakdown products (especially partially-degraded lignin and microbial cell wall components) chemically combine.

Humic substances are remarkably stable. Once formed, they persist in soil for hundreds to thousands of years. They give compost (and topsoil generally) its characteristic dark color and play important roles in soil structure, nutrient retention, and water-holding capacity.

The humic substances aren’t just byproducts of decomposition — they’re a key part of what makes compost valuable as a soil amendment. The dark color signals that the compost contains stable humic content that will improve soil for years to come.

Putting it all together: a typical compost pile timeline

Day 1: Materials added in correct C:N ratio, watered to damp-sponge moisture. Microbes from soil and air populate the pile. Mesophilic bacteria dominate.

Day 2-5: Microbial population explodes. Temperature rises to 130-150°F. Sugars and simple proteins consumed rapidly. Thermophilic bacteria take over from mesophiles.

Day 5-21: Hot phase continues. Cellulose and hemicellulose break down. Pile loses about 30% of volume from water loss and CO2 release. Pathogens and weed seeds killed.

Day 21-42: Pile cools as easily-decomposable materials are exhausted. Mesophiles return. Fungi begin actively decomposing lignin. Turning the pile every 1-2 weeks re-oxygenates.

Day 42-90: Curing phase. Cooler temperatures (60-90°F). Slow breakdown of remaining lignin and tough materials. Humus forms from partially-degraded lignin and microbial cell wall material. Actinomycetes produce earthy geosmin smell.

Day 90+: Finished compost. Dark, crumbly, earthy-smelling, no recognizable original materials.

This timeline assumes a well-managed hot pile. Cold piles run roughly 4-8x slower. Tumbler systems run slightly faster than hot piles due to better mixing. Worm bins use a different pathway dominated by worm gut microbes rather than thermophilic bacteria.

What this means in practice

Understanding what’s happening inside the pile lets you diagnose problems:

Pile won’t heat up: Either too dry, too wet, wrong C:N ratio (usually too carbon-heavy), or pile is too small to retain heat. Check moisture, add greens, build the pile larger.

Pile smells sour or like ammonia: Either too wet (anaerobic) or too nitrogen-heavy. Turn the pile, add browns, ensure drainage.

Pile smells like rotting food: Surface scraps not buried deep enough. Bury new additions under existing material.

Pile takes forever: Particle size too large, or moisture/oxygen suboptimal. Chop materials smaller; turn more often.

Finished compost still has chunks: Some materials decompose slower than others. Pick out the slow stragglers (eggshells, fruit pits, large wood) and return to the next pile.

Finished compost is shallow brown rather than dark: Probably finished but with limited lignin content. Useful but lower in humic substances. Compost from more diverse inputs (including some tough woody material) tends to be darker.

The bigger picture

Composting at its core is just a managed acceleration of natural decomposition. Out in the forest, leaves fall to the ground and become humus on a timescale of years. In a compost pile, the same materials become humus in months because we’ve optimized the conditions — concentrated the inputs, balanced the C:N ratio, maintained moisture and oxygen, periodically turned the pile to keep it active.

The microbes do the work. We just set up favorable conditions. Once you see the pile that way — as a microbial ecosystem you’re cultivating rather than a chore you’re managing — composting becomes more satisfying. You’re not making compost; you’re tending a community of organisms that makes compost as a byproduct of their living.

The dark earthy stuff at the end is the visible result. The invisible part — the billions of organisms doing their work — is where the actual transformation happens. Every gardener who composts is also, whether they realize it or not, running a small microbial ecology project in their backyard. That’s worth knowing about.

For B2B sourcing, see our compostable supplies catalog or compostable bags catalog.

Background on the underlying standards: ASTM D6400 defines the U.S. industrial-compost performance bar, EN 13432 harmonises the EU equivalent, and the FTC Green Guides govern how “compostable” can be marketed on packaging in the United States.