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Scent of the soil

It’s been raining and the colours in the garden are more vivid than ever. The water cranks up the green yet blackens the soil. It’s a garden of contrasts.

When the rain stops and the temperature lifts a little, the smell of damp soil beckons me outside. I can thank the molecule geosmin for that.

Scientists have been interested in the chemical signal of damp soil for more than 100 years. But it wasn’t until the 1960s that they’d isolated the molecule responsible for it – geosmin – named after the Greek words for ‘earth odour’.

Soil bacteria make geosmin then release it when they die, a volatile legacy that our noses have evolved to pick up.

They’re pretty good at it and can sniff out fewer than 10 parts per trillion. Imagine a drop of water diluted into several Olympic-sized swimming pools and you get the scale I mean.

Doesn’t it make you wonder why we evolved to be super-sensitive to the smell of damp soil?

The clue’s in the ‘damp’. Geosmin is also in water. Too much of it gives a musty taste. No wonder water authorities routinely screen for it to check that our drinking water tastes fresh rather than off.

So if geosmin is a tell-tale sign of water, that might help explain why humans are so good at smelling it. Evolutionary speaking, it makes sense for humans to be able to find a far-off drink, especially if they live in a dry climate.

Thirsty camels

Crazier still is the idea that camels evolved to smell geosmin too. Think of a camel, short on water, being able to find a distant oasis by following its nose.

As far-removed as a train of camels is from my garden, it’s not that bit of the story that intrigues me the most.

Part of me finds it hard to believe that a single molecule is responsible for that familiar garden smell. Not a Who’s Who of volatile compounds that intermingle to produce a multilayered scent. Just one.

I feel cheated. Wasn’t nature supposed to be more complex than that?

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