But the musty scent becomes far stronger after the rain when the dry ground becomes wet and clouds of the bacterial spores and geosmin are released into the air.

Bacterial spores lie dormant in soil for many years and typically gain entry through penetrating wounds (the clichéd nail through the foot).

As for the bacteria themselves, the scent of geosmins in the soil could play a role in attracting earthworms and springtails to pick up and spread the bacterial spores.

Many mould and bacterial spores, even live bacteria, are small enough to be carried long distances and to great heights on the wind.

Flour naturally contains a variety of yeasts and bacterial spores.