How do I tell if manure is well rotted enough to use?

The look, smell, heat and structure of manure that is ready — and why 'well rotted' answers the scorching question but says nothing at all about herbicide residue.

Difficulty: beginner
Time: 15 min
Read: 4 min
Safety: warning

Short answer

Ready manure is dark brown to black, crumbly, smells earthy rather than of ammonia or of animal, is cool right through the heap, and has no recognisable straw or bedding left. Fresh manure is pale, strong-smelling, warm in the middle and clearly still bedding. Six months is a minimum and a year is better. None of this tells you whether it carries herbicide residue.

There are two entirely separate questions people merge into one when they ask whether manure is ready. Is it going to scorch my plants and carry weed seed and pathogens — which rotting answers. And is it going to distort my tomatoes because of what was sprayed on the hay three years ago — which rotting does not answer at all, and which needs a test.

Safety

Fresh manure carries E. coli, salmonella and other pathogens, and it must be fully rotted before it goes anywhere near food crops. Wear gloves, keep it off cuts, wash your hands thoroughly, and wash all produce grown in manured ground. Keep tetanus cover current. Do not use manure from animals that have been recently wormed, and never use dog, cat or pig waste on a food garden.

Step by step

Look at the colour and texture.Well-rotted manure is dark brown to almost black, crumbly and uniform, and falls apart in the hand. You should not be able to identify individual straws or shavings. Pale, stringy, obviously fibrous material is not ready.
Smell it, from the middle of the heap.Ready manure smells earthy and slightly sweet — the same smell as good compost. Ammonia, or a strong farmyard smell, means active decomposition and free nitrogen that will scorch roots. A sour or rotten-egg smell means it has gone anaerobic and needs turning and air rather than time.
Put your hand into the heap.A heap still working is noticeably warm at 300mm in and can be hot. Ready manure is at ambient temperature all the way through. Heat is the single quickest test and almost nobody does it.
Judge the age against the bedding.Manure on straw rots faster than manure on wood shavings, because shavings are high in carbon and slow to break down. Straw-based: six months is often enough, a year is comfortable. Shavings-based: a full year minimum, and eighteen months is better.
Check for worms.A heap full of brandling worms is finished or nearly so. Worms will not colonise material that is still hot or still strongly ammoniacal, so their presence is a good passive indicator.
Test for viability of weed seed if it matters.A cold-rotted heap will be full of viable weed seed and it will germinate on your beds. Spread a tray of it, keep it damp and warm for two weeks and see what comes up. Hot-composted manure is much cleaner.
Test for herbicide residue separately.This is not the same question and no amount of looking answers it. Run a bean bioassay on any manure going onto a vegetable plot, regardless of how beautifully rotted it looks. Well-rotted contaminated manure is exactly as damaging as fresh contaminated manure.
Use it in the right place at the right stage.Fully rotted: spread as a surface mulch anywhere, including around food crops. Partly rotted: stack it for longer, or spread it on empty ground in autumn where it has the winter to finish. Fresh: stack it, or use it in a hot compost heap as the nitrogen component. Never dig fresh manure in around growing plants.

Tips

Stack fresh manure in a heap at least a cubic metre in size, cover it to keep rain from washing the nitrogen out, and leave it. Turning it once at three months speeds it considerably.
Autumn spreading of not-quite-finished manure onto empty vegetable ground works well: the winter finishes the job and the nitrogen that would have scorched is taken up or held by the time you plant in spring.
Manure from animals wormed within the last few weeks can suppress the worms and dung beetles in your own heap. Ask, and let it stand a few extra months if the answer is recent.
Poultry manure is far stronger than horse or cow and needs longer and more careful handling. Fresh poultry manure will kill plants outright.
Free manure from a livery yard is usually straw or shavings with a modest amount of dung in it. Bagged, sold 'well-rotted farmyard manure' is usually much richer. They are not interchangeable by volume.

Common mistakes

Digging fresh manure into a bed in spring and planting immediately — Free ammonia scorches roots and germinating seed, and the decomposition draws nitrogen out of the soil to process the bedding. The bed does worse than it would have with nothing added.
Treating 'well rotted' as 'safe' — It answers scorching, pathogens and weed seed. It says nothing about aminopyralid, which is the failure mode that costs people a whole season.
Using fresh manure where root vegetables will grow — Carrots and parsnips fork and fang in freshly manured ground. Manure the bed the autumn before, and grow roots in the second year of the rotation.

Questions people ask

Is a year always long enough?

For scorching and pathogens, usually yes on straw and probably yes on shavings. For weed seed, only if the heap got hot. For herbicide residue, no — that is a separate question with a separate test.

Can I speed it up?

Yes. Build the heap in one go rather than adding to it, get it to at least a cubic metre, keep it damp but covered from rain, and turn it once or twice. A properly built heap finishes in three to six months rather than a year.

Is bagged 'well-rotted manure' from a garden centre reliable?

Generally it is genuinely rotted — that part is reliable. Herbicide residue has been found in bagged products too, so the bean test is still worth running before it goes onto a vegetable plot.