Stone-Ideas.com 
Researchers have gained new insights into Geosmin and Petrichor, which make the ground smell after rain
In nature, there are many different types of communication: people speak directly to each other, birds sing as inhabitants of confusing terrain, and plants everywhere attract insects with scents for pollination, to name just a few examples. This type of communication also exists under our feet: The substance Geosmin is released by bacteria in the soil to warn nematodes as predators: “we are poisonous,“ the bacteria tell the Nematodes, blinds worms which are only a few millimeters long (and make up an estimated four-fifths of the animal world on our planet). Researchers at Concordia University in Montreal have reported on this.
For us humans, Geosmin has an earthy to musty smell, and researchers from the Leibniz Institute for Food Systems Biology in Germany, recently discovered which of the many receptors in our noses it is that smells the substance. The smell is pleasant to us when we are working in the garden and it is attached to beetroot and some cacti, but it is unpleasant to us when it is associated with fish, water or wine, because these are then usually spoiled.
Even the smallest concentrations of 4 to 10 ng/L are enough for a person to perceive the smell in water. That corresponds to about a teaspoon of Geosmin in the amount of water in 200 Olympic swimming pools.
But we find it most pleasant when it comes from summer rain, when the earth smells for a short moment after a period of heat.
Geosmin is often not solely responsible for this smell. Petrichor, a mysterious plant sap that scientists discovered as early as 1860, may also be involved. In 1964, Australian researchers Isabel Bear and Richard G. Thomas were able to solve the mystery of the yellow and oily substance: in dry periods, some plants release it and use this chemical weapon to hinder growth around them – in order to reserve the scarce water for themselves.
The Petrichor settles on the ground or on stones until the next rain falls and it – together with Geosmin – is thrown up by the drops. Then it can reach our noses.
This also explains why the earth smells strong when there is a light rain, but not when there is a heavy downpour: fine droplets stir up more tiny particles and let them drift further than a wall of rain.
The name Petrichor is poetic, by the way: it goes back to the Greek terms “pétra“ (rock, stone) and “ichor.“ Ichor was the name of the liquid that flowed through the veins of the gods.
Leibniz-Institut für Lebensmittel-Systembiologie
(04.10.2024, USA: 10.04.2024)