Does the countryside have to smell? What manure odour tells us

No. A sharp manure or slurry smell is not an inevitable feature of farming. It tells us which compounds are escaping and under what conditions decomposition is taking place.

What actually smells?

Wet material left with poor aeration can produce or release:

• ammonia, with a sharp, irritating smell; nitrogen is lost with it,
• hydrogen sulphide, which smells of rotten eggs,
• volatile acids and other decomposition products, creating sour, rotten odours.

Methane also forms without oxygen, but is itself odourless.

Smell does not tell us everything, but a strong stench signals that part of the process is moving in an unwanted direction.

Storage and composting are different

A heap of manure left alone does not automatically become properly managed compost.

Active composting needs oxygen. If material becomes too compacted or conditions turn anaerobic, the processes responsible for rotten odours return.

Simply waiting is therefore different from managing a biological process.

Smell is not a safety test

An absence of bad odour does not mean material is safe or mature.

Raw manure can contain pathogens. A properly managed thermophilic composting stage can greatly reduce some of them, but requires the process to actually take place, not just material sitting in a heap.

We documented this with our own manure: what changed after 3, 7 and 21 days of composting.

When a compost pile starts to smell

If compost develops a strong odour, masking it is not the answer. The cause of the changed conditions needs to be found.

Usually this means considering moisture, air supply, structure and material composition. Odour alone cannot identify which is the problem.

Sources

• Cornell CALS, Science and Engineering of Composting
• Cornell CALS, Pathogens
• BioFarmLab, original Polish article “Czy na wsi musi śmierdzieć?”, 17 May 2021