Composting: feeding the soil by imitating nature

Le compostage : nourrir le sol en imitant la nature

Compost that smells bad, does not heat up, or takes two years to form: in almost every case, the cause is the same and comes down to three parameters—the carbon-to-nitrogen ratio, moisture, and oxygen. Understanding how these three interact is enough to diagnose any problem and fix it with a single adjustment.

This guide starts with how it works, then moves on to practical matters: what to add, in what proportions, how to tell whether it is working, and how to use the result.

What really happens in a pile

Composting is not passive decomposition: it is a succession of microbial populations that take over from one another, each consuming what the previous one has made accessible. Three phases follow one another.

Phase 1 — the temperature rise (a few days to a few weeks)

Mesophilic bacteria first attack sugars and proteins, the easiest molecules to break down. Their activity generates heat; the pile warms up, and above 40 °C they give way to thermophilic bacteria, which take over up to 55–65 °C.

This heat is not merely aesthetic. Maintained above 55 °C for several days, it destroys most weed seeds and a large proportion of plant pathogens. That is the whole difference between hot compost and cold compost.

To heat up, a pile must reach a critical volume of about 1 m³. Below that, heat dissipates faster than it is produced: the compost will still form, but slowly and without any disinfecting effect. This is the main limitation of small balcony composters.

Phase 2 — active decomposition (several months)

The temperature drops as the thermophiles run out of easily available material. Fungi and actinomycetes take over: they alone can break down cellulose and lignin, the tough molecules in wood and stems. Invertebrates arrive—worms, woodlice, springtails—and mechanically fragment the material.

This is the longest phase, and the one in which turning the pile can usefully restart activity.

Phase 3 — maturation (2 to 4 months)

The humification processes come to an end: the remaining molecules recombine into stable humic components that will persist in the soil for years. The compost develops a crumbly texture, a uniform dark brown color, and a distinct woodland-floor smell.

Compost used before this phase is complete remains biologically active—with the consequences described below.

The C/N ratio: the only number you need to know

Microorganisms need carbon as an energy source and nitrogen to build their proteins. They consume approximately 25 to 30 parts carbon for 1 part nitrogen. This is the famous C/N ratio, and it is the parameter that explains 80% of failed composting.

Material	Approximate C/N ratio	Category
Fresh grass clippings	15–20	Green material
Kitchen scraps	15–20	Green material
Coffee grounds	20	Green material
Poultry manure	7–10	Very nitrogen-rich green material
Fallen leaves	40–80	Brown material
Straw	80–100	Brown material
Brown cardboard	300–400	Very carbon-poor brown material
Sawdust and wood chips	400–500	Very carbon-poor brown material

In practice, you don't calculate it: aim for about two-thirds brown material to one-third green material, bearing in mind that browns are much more voluminous at the same weight. A pile that receives only kitchen scraps will always contain too much nitrogen — hence the value of keeping a bag of fallen leaves beside the composter and adding a handful with each contribution.

Quick diagnosis

Symptom	Cause	Correction
Ammonia smell	Too much nitrogen	Add browns and turn
Smell of rotten eggs or stagnant water	Lack of oxygen (anaerobic)	Turn it thoroughly and add coarse materials
Nothing is happening, pile cold and dry	Too much carbon or too dry	Add greens and water
Cloud of gnats	Fresh waste exposed on the surface	Cover each addition with a layer of brown material
Ant colony established	Pile too dry	Water thoroughly

Moisture and oxygen: the other two levers

The classic guideline — as damp as a wrung-out sponge — corresponds to about 50–60% humidity. The test: a squeezed handful should release one or two drops, not run with water.

These two parameters are linked: above 65% humidity, water drives air out of the gaps and the pile shifts to anaerobic fermentation — odors, acidification, and the process stops. This is why structure matters as much as the proportions: fine branches, hollow stems, or shredded material create gaps that maintain air circulation, even when the pile is damp.

Turning — every 4 to 6 weeks is enough for garden compost. It re-oxygenates and homogenizes the pile, bringing peripheral materials that have never heated up back to the center. Compost that is never turned will still decompose, but it will take twice as long and be very uneven.

Which method suits your situation

Method	Who it's for	What you need to know
Open pile	Large garden	The only one that easily reaches a cubic meter and therefore heats up. Free.
Enclosed composter	Urban garden, close neighbors	Discreet and protected from the rain. Heats up very little: cold compost, weed seeds not destroyed.
Surface composting	Permaculture, living soil	The waste is spread directly beneath the mulch. No handling, but no rise in temperature either.
Integrated into the mounds	Gardens in mounded beds	The material decomposes in situ, at the heart of the raised bed.
Vermicomposting	Apartment, balcony	Produces very rich compost and liquid worm tea. Requires regular monitoring and tolerates neither large quantities of citrus nor overloading.
Bokashi	Kitchen, very small quantities	This is not composting: it is lactic fermentation in a closed environment. The resulting product must still be buried or composted before use.

What can be added — and the real exceptions

Lists of banned items circulate without nuance. Here is what actually justifies them.

Meat, fish, dairy products — the problem is not decomposition (it happens very well) but attracting rodents and causing odors. In a closed, well-managed hot compost system, it is possible; in an open garden pile, avoid it.
Citrus fruits — tolerated in small quantities. The real problem concerns vermicompost, where the acidity bothers the worms.
Diseased plants — acceptable only in verified hot compost (55 °C for several days). When in doubt, leave them out: blight and clubroot survive in lukewarm compost.
Weeds that have gone to seed — the same principle applies. In cold compost, you will reseed your weeds the following year.
Treated wood, glossy paper, cigarette butts, “biodegradable” bags — no, in any case. Industrially compostable bags require temperatures that home compost never reaches.
Eggshells — of no real use unless finely crushed: they reappear intact several years later.

Recognizing mature compost

Criterion	Mature compost	Unfinished compost
Appearance	Dark brown, uniform, crumbly	Heterogeneous, identifiable pieces
Smell	Forest floor, potting soil	Sour, ammoniacal, fermenting
Core temperature	Ambient	Still lukewarm or hot
Earthworms	Present and established	Absent (too hot for them)

The cress test, more reliable than appearance: sow a pinch of garden cress seeds in a dish of moist compost, and the same amount in a control potting mix. If germination and growth are comparable, the compost is mature. If they are significantly lower, it still contains phytotoxic components.

Unfinished compost applied to young plants causes nitrogen hunger: the microorganisms that continue their work use the soil’s nitrogen for themselves, and the crops turn yellow. The phenomenon is temporary but can cost you a season.

Use compost at the right time

Autumn — the main period. Spread 2 to 5 cm on the surface without incorporating it, then cover with mulch. Soil fauna will incorporate it itself over the winter.
Spring—before planting heavy-feeding crops (tomatoes, squash, cabbages), placed at the bottom of the planting hole or lightly scratched into the surface.
During the growing season—as light mulch around plants, using mature compost only.

Do not bury it deeply. Compost is an aerobic product: buried 20 cm deep, it ends up in a low-oxygen zone where decomposition slows and deteriorates. It belongs on the surface or in the top five centimeters, where the microorganisms that will make use of it live.

Mature compost is not actually a fertilizer: it is a soil amendment. It provides few immediately available nutrients, but it permanently improves soil structure and nourishes soil life—see The different types of soil.

Frequently asked questions

How long does it take to make compost?

From 3 to 4 months for hot compost turned regularly, up to 18 months for a cold pile that is never turned. The difference depends almost entirely on the initial volume and how often it is turned.

My compost never heats up—is that a problem?

No, it will still compost. You simply lose the disinfecting effect, so do not add diseased plants or weeds that have gone to seed.

Do you need a compost activator?

Unnecessary in the vast majority of cases. What compost activators provide—nitrogen and microorganisms—is already supplied free of charge by a handful of fresh nettles, grass clippings, or simply a shovelful of mature compost from the previous cycle.

Can you compost in winter?

Yes, but activity slows considerably below 10°C. Keep adding material to the pile; it will start up again in spring. A large pile well insulated with a layer of leaves also maintains a surprisingly stable internal temperature.

Compost or manure?

Fresh manure is too rich and too “hot” for direct use: it must be composted for at least six months. Well-managed homemade compost is more balanced and free from the risk of veterinary drug residues.

Key takeaway

Three parameters, and nothing else: two-thirds brown material to one-third green material, the moisture of a wrung-out sponge, and air. When compost isn’t working, simply ask yourself which of the three is missing—the answer is always there.

For further information

To complete the work of compost: our green manure seeds, which nourish and structure the soil between two crops.