Here’s why soil smells so good after it rains

Klas Flärdh, Lund University and Paul Becher, Swedish University of Agricultural Sciences

Did you ever wonder what causes that earthy smell that rises after a light summer rain? That mysterious scent has been called “petrichor”, and a main component of it is an organic compound called geosmin, which lingers around moist soil.

Geosmin comes from the ancient Greek “geo”, meaning earth, and “osme”, meaning smell. We use this scent as an ingredient in perfumes and it is what gives beetroot its earthy flavour. Geosmin can also be perceived as an “off” flavour in water and wine.

Animals can detect geosmin. Fruit flies, for example, dislike geosmin and they avoid anything that smells of it, possibly to avoid contaminated and potentially toxic food. But why is geosmin made in the soil? As part of a team of scientists from Sweden, the UK and Hungary, we discovered the fascinating biology behind this enigmatic compound.

Scientists have known since the 1960s that geosmin is made by microorganisms in the soil, primarily by bacteria with the scientific name Streptomyces. These bacteria are abundant in soil and are among nature´s best chemists, as they make a wide range of molecules (called specialised metabolites) from which many antibiotics derive. Streptomycetes and their close relatives make thousands of different specialised metabolites – a true treasure trove for the potential discovery of new antibiotics.

It turns out that all streptomycetes have the gene for making geosmin, suggesting that it has an important function. But what do these bacteria gain from producing geosmin? This has been a longstanding mystery.

In our recent study, we found that geosmin is part of the chemical language in a mutually beneficial relationship between