How-to Guide
Is My Compost Done? How to Tell
The short answer
You can tell compost is finished by look, smell, and temperature. It should be dark brown, loose, and crumbly like rich soil, with the original ingredients no longer recognizable, and it should smell pleasantly earthy, not sour, sharp, or like ammonia. A finished pile has also cooled off and shrunk to roughly half its starting volume, because the microbes have run out of easy food and stopped generating heat.
When you turn a pile and it does not heat back up within a day or so, decomposition is essentially complete. To be sure before using it on seedlings, run a germination test: sow fast-sprouting seeds like cress in a pot of the compost and compare against seeds in ordinary soil. If germination is similar, the compost is mature. Unfinished compost still contains active microbes that pull nitrogen from the soil and can release compounds that inhibit seed germination and stunt young plants.
The look, smell, and temperature signs
- Appearance: dark brown, uniform, crumbly; you can no longer identify the food scraps or leaves that went in.
- Smell: earthy, like a forest floor; a sour, sharp, ammonia, or rotten-egg odor means it is not done.
- Volume: the pile has shrunk to roughly half its original size.
- Temperature: a finished pile stays cool; if turning it makes it heat up again above about 120 F, microbes are still actively working.
The seed germination test
The most reliable home check is a germination bioassay. Fill a small pot with your compost and an identical pot with plain garden soil or seed-starting mix as a control, sow about ten fast-germinating seeds (cress, or something like cucumber) in each, water both equally, and keep them at room temperature for five to seven days. If the seeds in the compost sprout at a rate close to the control, the compost is mature and safe to use. Poor or delayed germination in the compost signals that it is still unfinished and contains substances that suppress seed growth.
Why unfinished compost hurts plants
Immature compost is still biologically hot with microbes that need nitrogen to keep breaking down carbon-rich material. Dug into a bed, they compete with your plants for that nitrogen and can leave crops temporarily starved and yellowed. Unfinished compost can also carry phytotoxic compounds and viable weed seeds, and its ongoing decomposition can damage tender roots and inhibit germination. If you need to use compost that is not quite ready, spread it as a surface mulch where it can finish, rather than mixing it into the root zone. For building the pile in the first place, see our composting guide.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- Cornell Cooperative Extension (Tompkins) - Testing for Finished Compost — Finished compost is dark, crumbly, and earthy-smelling with unrecognizable original materials, stays cool when turned, and can be confirmed with a seed-germination test comparing sprouting in compost against a control.
- University of Illinois Extension - Composting for the Homeowner: Finished Compost — Compost is ready when it is dark, crumbly, and earthy with original materials no longer recognizable; using unfinished compost can tie up nitrogen and harm plants.