Learn / The basics
What is compost?
Compost is what is left when microbes finish breaking down organic waste: a dark, crumbly, earthy-smelling material that feeds the soil. Here is what it is, how it forms, and what it can and can’t do for your land.
Rotting, on purpose
Everything organic breaks down eventually. Composting speeds that up and steers it. You pile plant and animal waste in the right mix, keep it damp and aerated, and let billions of bacteria and fungi do the work. What you get is stable, safe to handle and good for soil.
Kenya’s draft organic fertilizer standard (KS 2290) puts it in similar terms: well-decomposed organic waste, such as plant residues and animal slurry from livestock, used as a fertilizer.
The numbers of a heap that is working well:
SOURCES: KEBS DRAFT KS 2290:2023 (WTO NOTIFICATION); UNIVERSITY OF NEBRASKA–LINCOLN EXTENSION, GUIDE G1315. TEMPERATURES CONVERTED FROM °F.
Four stages, from cold pile to finished compost
You don’t need to manage these stages by hand. Knowing they exist tells you what the heap is doing and when something is wrong.
- Warm-upMicrobes that feed on the easy materials (sugars, soft green waste) multiply fast and the heap begins to heat. This can take a day or two.
- The hot phaseThe heap climbs into the range of roughly 40 to 65°C, and the best results come at about 55 to 60°C. Holding about 55°C or more for at least three days kills most disease organisms. Weed seeds need it hotter, around 63°C. This is the busiest stage.
- CoolingAs the easy food runs out, the heat fades. If the heap stops heating up even after you turn it, the main work is done.
- CuringSlower microbes and fungi finish the job. Extension guidance suggests letting the heap rest for at least a month after it stops heating, so the material fully stabilises before use.
Four things: carbon, nitrogen, water and air
Carbon
Dry, woody, fibrous “browns”. They give microbes energy and keep the heap airy.
Nitrogen
Wet, soft “greens” and manure. Microbes use nitrogen to build their own bodies and multiply.
Water
Damp like a wrung-out sponge. If you squeeze a handful, a drop or two should come out.
Air
Most of the helpful microbes need oxygen. Turning the heap, or building in air channels, supplies it.
| Browns (carbon) | Greens (nitrogen) |
|---|---|
| Dry maize stalks and cobs, wheat or rice straw, dry grass, dry leaves, sawdust (very slow), shredded cardboard | Kitchen and market vegetable waste, fresh weeds (before they seed), fresh grass and Napier cuttings, cow, goat or chicken manure, coffee pulp |
A practical rule of thumb is about two to three parts brown to one part green by volume, then adjust by smell and feel. Chicken manure is much richer in nitrogen than cow manure, so use less of it.
It builds the soil your crops grow in
It rebuilds organic matter. About 75% of Kenya’s soils are low in organic carbon. Adding compost is one of the most effective ways to correct that.
It improves structure. Compost loosens heavy clay and helps sandy soil hold water. Roots go deeper and water soaks in rather than running off.
It feeds soil life. Microbes and fungi in compost help release nutrients to plants and keep the soil working.
It releases nutrients slowly. A little at a time through the season, rather than all at once.
What compost can’t do
Compost is bulky and relatively low in nutrients per kilogram, so it doesn’t replace fertilizer where a crop needs a lot of nitrogen, phosphorus or potassium quickly. It does not correct strongly acid soil on its own either: about 63% of Kenya’s arable land is acidic, and soils below roughly pH 5.2 usually need lime. The best results come from combining compost with balanced nutrition and a soil test.
SOURCES: HEINRICH BÖLL STIFTUNG KENYA, “KENYA’S DIVERSE SOILS”; KALRO SOIL FERTILITY OVERVIEW (IFDC). SEE SOILS OF KENYA.
Compost, manure, mulch and fertilizer
| Term | What it is | Main job |
|---|---|---|
| Compost | Fully decomposed, stable organic material. | Builds soil. Safe to use around plants. |
| Fresh manure | Raw animal dung. It is not compost. | Needs composting or ageing first. It can burn plants and carry pathogens. |
| Mulch | Any material spread on the soil surface, such as straw or dry grass. | Keeps soil moist and cool and suppresses weeds. Breaks down slowly. |
| Chemical fertilizer | Manufactured, concentrated nutrients. | Feeds the crop quickly. Does not build soil structure. |
| Organic fertilizer | Fertilizer from natural materials, including composts, manures and bone meal. | Feeds crops and, depending on the type, also improves soil. |
How to tell compost is finished
Using compost too early is the most common mistake. Unfinished material can still be active, can tie up nitrogen the crop needs, and can harm seedlings.
- Smell. Earthy and pleasant. No sour, rotten or ammonia smell.
- Look. Dark and crumbly. You can no longer recognise what went in, apart from a few woody pieces.
- Heat. The heap stays near air temperature after you turn it. Extension guidance puts the cut-off at about 40°C.
- Germination. Seeds such as beans or garden cress sprout and grow as well in your compost mix as in plain soil. Researchers use a germination index of 0.8 or higher as the sign of no growth inhibition.
How long does it take?
It depends on your materials and management. In one Ethiopian study of farmers’ organic fertilizers, a managed, turned compost matured in about 50 days, while traditional pit composting (kosi) took 70 to 90 days. The authors suggested applying compost about a month before sowing.
SOURCES: UNIVERSITY OF NEBRASKA–LINCOLN EXTENSION (G1315); EVALUATION OF MATURITY AND STABILITY OF ORGANIC FERTILISERS IN THE SEMI-ARID ETHIOPIAN RIFT VALLEY (SCIENTIFIC REPORTS, 2021). ONE SMALL STUDY, SO TREAT THE TIMINGS AS A GUIDE ONLY.
Common problems and fixes
| Problem | Likely cause | Fix |
|---|---|---|
| Smells of ammonia | Too much nitrogen (greens or manure). | Mix in dry browns and turn the heap. |
| Smells sour or rotten | Too wet or compacted, so no air. | Turn it, add dry browns, and cover it from heavy rain. |
| Won’t heat up | Too dry, too much carbon, or the heap is too small. | Add water and some greens, and build a bigger heap. About a metre across and high is a common minimum. |
| Too hot (above about 63°C) | Heap is large and very active. | Turn it to release heat and add air. |
| Flies, rats or dogs | Exposed food scraps, meat or cooked food. | Bury scraps in the middle of the heap, and keep meat and bones out. |
| Weeds sprout from it | Heap never got hot enough to kill seeds. | Keep seeding weeds out, or make sure the whole heap reaches the hot phase by turning outer material into the middle. |
What to keep out
Meat, bones and dairy (pests and smell), cooked oily food, pet and human waste (disease risk), plastic, metal and glass, treated or painted wood, and plants sprayed with persistent weedkillers.
Questions people ask
Do I need an activator or microbial product?
Most heaps work without one, because the microbes are already in your materials and the soil. Microbial products are sold and some farmers use them, but the evidence on how much they help is mixed. Get the mix, moisture and air right first.
Can I compost in the rainy season?
Yes, but cover the heap so it doesn’t get waterlogged, and add more dry browns. In the dry season, check moisture more often and add water when the heap dries out.
How much compost should I apply?
It depends on your crop, your soil and how rich your compost is. A soil test gives the best starting point. We will be publishing crop-by-crop guidance, and we can advise on your farm directly.
Is compost the same as vermicompost or bokashi?
No. Vermicompost is made by earthworms, and bokashi is fermented in a sealed bucket and is not finished compost until it has been buried or mixed into soil. See the methods section on the Learn page.
Can I make compost from kitchen waste only?
You can, but kitchen waste is wet and high in nitrogen, so mix it with dry browns such as dry leaves, shredded paper or cardboard to avoid a sour, smelly heap.
Ready to make your own?
Pick a method that suits your space and waste, or let us show you in person.