Reference point
Gently stir the sludge at the bottom of a stagnant pond and smell what rises.
Do this before you read on. An entry only works when you have just smelled the thing.
Fishy, musty, with a faint rotten-egg note and a dense layer of earth. Heavy, unpleasant and clinging.
The rotten-egg note comes from sulphur compounds produced when bacteria decompose matter without oxygen. It is an important signal: this smell tells you the pond bottom is short of oxygen, information fish farmers need.
Pond sludge and river mud differ clearly. Flowing water carries oxygen, so river mud has much less of the sulphur note.
How to smell it
Gently stir the sludge at the edge of a stagnant pond with a long stick and smell from a metre away. Don’t lean over the water. Compare it with mud at the edge of a flowing river.
Why this one is hard
Because of the urge to avoid it, and because the smell is associated with pollution. Keeping your distance is essential.
Don’t confuse with
This entry
Pond sludge
Earthy family
Both have sulphur compounds from anaerobic bacteria. Pond sludge also has geosmin, giving an earthy note; fermented shrimp paste has trimethylamine, giving a fishy, marine note.
Scientific note
Oxygen-poor pond sludge lets sulphate-reducing bacteria thrive, producing hydrogen sulphide and mercaptans. Their concentration is inversely proportional to the dissolved oxygen at the bottom, so this smell is a fairly reliable qualitative measure of a pond’s health.
In Vietnamese life
Traditional fish and shrimp farmers judge the state of a pond bottom by nose before any measuring instrument. It is a body of sensory knowledge of high practical value that has almost never been written down.