How is compost made? The 4 phases of composting explained simply

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A carrot peel that, a few months later, turns into brown, crumbly soil that smells like the forest floor: there's something magical about compost. Yet there's no secret to it, just a crowd of living creatures that eat, breathe, and take turns inside your composter.

Understanding what goes on inside is the best way to make good compost without asking yourself a hundred questions. Here are the main stages of the transformation, explained simply, along with the few rules that follow from them.

Composting: digestion in the open air

Composting means letting living organisms break down organic matter in the presence of oxygen. Food scraps and yard waste are their food: they draw carbon from them for energy and nitrogen to build their own cells, and they need water and air to live. What they leave behind, after many successive meals, is compost: a stable material, rich in humus, that is then returned to the soil.

The key words are "in the open air." When oxygen runs short, because the pile is too compacted or waterlogged, the organisms that need air suffocate. Other microorganisms then take over. They work without oxygen, as in a landfill or at the bottom of a pond, and produce methane, a powerful greenhouse gas, as well as foul-smelling gases. That's why compost that smells bad is usually short on air. The diagram summarizes the four phases described in detail below: warm-up (1), the hot phase (2), cooling (3), and maturation (4).

Who does the work? Bacteria, fungi, and small critters

Bacteria, the main workforce

A single gram of compost is home to billions of microorganisms. Bacteria make up 80 to 90% of them. They do most of the decomposition and produce the heat you can sometimes feel when you plunge your hand into an active pile.

Fungi and actinomycetes, specialists in tough materials

Fungi (molds and yeasts) tackle the tough debris that bacteria have trouble digesting: dry stems, thick leaves, cardboard. Actinomycetes, bacteria that form fine filaments, break down cellulose and some of the wood. You can often see their whitish, web-like network in the outer layer of the pile toward the end of composting: it's not a disease, it's a sign that the work is moving along.

Small critters that shred and mix

Once the material has cooled, larger animals move in: earthworms, pill bugs, millipedes, mites, and small beetles. They break the scraps into pieces, mix them, and open the way for microorganisms. Earthworms play a leading role here, which we describe in detail in our guide The role of earthworms.

Did you know?

These microorganisms don't come from a packet: they're already present on your peels, on fallen leaves, and in the soil in your yard. A well-balanced compost populates itself. A helping hand is still useful in some cases: sprinkled over food scraps, our bokashi activator, made on the farm, helps limit bad smells and kick-start compost that's slow to get going. We explain when to use it in our guide Compost activators: useful or not?

Phase 1: warm-up

Right after a large addition of fresh scraps, so-called mesophilic microorganisms, which like moderate temperatures, get to work. They first consume whatever breaks down most easily: sugars, starches, and soft materials. This intense activity gives off heat. In a well-stocked pile, this first phase lasts only a few days.

Phase 2: the hot (thermophilic) phase

When the temperature rises above about 104 °F (40 °C), heat-loving species, known as thermophiles, take over. They keep breaking down the more resistant materials. This phase can last anywhere from a few days to several months, depending on the volume and the mix.

Two thresholds are worth knowing. From 131 °F (55 °C), a good share of the germs that are harmful to people and of plant diseases are destroyed. Above about 149 °F (65 °C), however, the heat also kills many of the beneficial microorganisms and decomposition slows down: hotter compost isn't necessarily better compost.

Phases 3 and 4: cooling and maturation

Once the easily digested materials have been used up, activity drops and the temperature gradually falls. Mesophilic organisms return, fungi and actinomycetes tackle the toughest leftovers, and then small critters colonize the whole thing. This is the cooling phase, followed by maturation, during which humus forms.

Some books talk about three phases, grouping cooling and maturation together; others count four. How it's divided up doesn't matter: just remember that this final stage is the longest. It lasts several months, and it's what makes the difference between "fresh" compost, still active, and finished compost, dark and crumbly, ready for the vegetable garden. We explain how to tell them apart in our guide How to tell if your compost is mature.

Why backyard compost barely heats up, and why that's fine

Temperatures of 131–149 °F (55–65 °C) are mainly reached in the large piles at industrial composting facilities, which commonly exceed 140 °F (60 °C). In a home composter, the volume is small and heat easily escapes through the sides and top. Let's be clear: the temperature there generally stays low, and it isn't enough to guarantee that germs and weed seeds are destroyed.

That's not a failure. "Cold" compost works very well: it's simply slower, and small critters play a bigger role in it. There's just one practical takeaway: since your composter doesn't sanitize, avoid putting diseased plants and weeds that have gone to seed in it. If you'd like to try your hand at very hot composting, our guide on hot composting and the Berkeley method lays out its fairly demanding conditions.

Key takeaways

Compost is made in four stages: a quick start, a hot phase, cooling, and then a long maturation. In a backyard composter, the hot phase is low-key: that's normal, but it explains why some scraps still need to be left out.

The 4 rules that follow: air, water, mix, time

Air

Without oxygen, the good workers stop and the smells move in. A light stir with each addition, then a more thorough mix about once a month, with a pitchfork or an aerator such as the Brass'Compost, is usually enough. The right rhythm for your equipment is explained in our guide on compost moisture and aeration.

Water

Microorganisms live in thin films of water. Too dry, and everything stops; too wet, and air can't get through. A good rule of thumb: compost should be as damp as a wrung-out sponge.

Mix

Nitrogen-rich scraps (peels, grass clippings) break down quickly but rot on their own; carbon-rich materials (dry leaves, twigs, cardboard) are slow but provide structure. It's mixing them that properly feeds the microorganisms, as we explain in our guide Green and brown materials.

Time

Backyard compost takes several months: allow four to nine months in a bin, depending on the season and upkeep, and six to twelve months in a pile. A bin such as our Compostys wooden composter, set directly on the soil, lets small critters come up from the ground and finish the job at their own pace.

In summary

Compost is the work of a chain of living organisms: bacteria that get things started and heat up the pile, fungi and actinomycetes that tackle the tough materials, and then worms and small animals that complete the transformation during a long maturation. In a home composter, the heat stays moderate: compost still forms, but it doesn't sanitize everything. For these workers to do their job well, they need air, a reasonable amount of water, a mix of green and brown materials, and a few months of patience.

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