Geosmin, which literally translates to “earth smell”, is an organic compound with a distinct earthy flavour and aroma, and is responsible for the earthy taste of beets and a contributor to the strong scent that occurs in the air when rain falls after a dry spell of weather (petrichor) or when soil is disturbed.[1] The human nose is extremely sensitive to geosmin and is able to detect it at concentrations as low as 5 parts per trillion.
[ http://en.wikipedia.org/wiki/Geosmin ]
Petrichor (pronounced /ˈpɛtrɨkər/; from Greek petros “stone” + ichor the fluid that flows in the veins of the gods in Greek mythology) is the name of the scent of rain on dry earth.
The term was coined in 1964 by two Australian researchers, Bear and Thomas, for an article in the journal Nature.[1] In the article, the authors describe how the smell derives from an oil exuded by certain plants during dry periods, whereupon it is absorbed by clay-based soils and rocks. During rain, the oil is released into the air along with another compound, geosmin, producing the distinctive scent. In a follow-up paper, Bear and Thomas (1965) showed that the oil retards seed germination and early plant growth