What causes pleasant earthy smell after rain?

By | May 4, 2021
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The first rain after a dry spell gives a beautiful earthy smell and many wonder, what is the secret behind this sweet aroma. The reason is not all the dust particles washed away by spell of rain and gives a fresh feel, in fact it’s the smell of Geosmin. This is a chemical compound produced by gram positive bacteria Streptomyces, a genus of Actinobacteria which makes most of normal flora of the soil. During the dry season when the soil dries out and to overcome these tough conditions bacteria forms spores.

It is a stripped-down, dormant form to which the bacterium can reduce itself to survive during unfavorable conditions. Spores revive to normal vegetative state once the conditions turns favorable.

The earthy smelling Geosmin is chemically Dimethyl-9-decalol and forms a major component of spore coat of these soil bacteria. When the raindrops hit the ground, scatters these microscopic spores in air. They remain present as a soil-water aerosol and gives us smell of Geosmin present in the spores upon inhaling the aerosols. One thing to note is that during frequent rain spells we don’t come across same sweet smell of Geosmin as in moist conditions bacteria remains in normal vegetative state and hence no spores, so no geosmin in spores to give sweet smell in frequent rains.