Stop and Smell the Geosmin

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Spring is a time of transformation. The sleepy silence of the cold white winter recedes into a decadent symphony of sound and color, slowly at first, but with continually increasing vigor right on through until summer. There is so much to see and hear during this time of the year that it is easy to overlook one of my favorite spring sensations: the smell.

Usually described as “fresh earth” or “muddy” the smell is one we all know well. I associate it with those hardy little flowers known as spring ephemerals that come up as soon as the ground thaws, producing both flower and fruit before the trees above open their leaves and soak up all of the available sunlight. Trout lilies and spring beauties and trilliums—just writing their names brings the smell of spring to my mind; but it is not these flowers that give us that springy smell. So just what is it that we have to thank for spring’s classic scent?

It turns out the scent of spring is the very same substance responsible for the earthy taste of beets: a terpene known as geosmin. The name geosmin comes from the Greek words for “earth” and “smell.” Geosmin is produced by blue green algae and certain kinds of soil bacteria called actinomycetes. These bacteria break down plant material, and in the process convert another soil chemical (farnesyl diphosphate) to geosmin in a two-step process.

Geosmin exists in the soil year-round, but its smell is only noticeable under certain conditions—you might notice it when digging into soil, during spring thaw, and after a rain. This is because generally the gas pools in spaces beneath the soil surface and is only detectable when these pools are disturbed, like when digging turns over the soil or water soaks into it, displacing the gasses within. Sometimes the smell of geosmin is apparent before a storm hits, the result of decreasing pressure associated with the approaching weather causing the soils to off-gas geosmin and carbon dioxide that has built up beneath the surface.

With spring in full swing I hope you take the time to get out into the wild, where forests are coming to life and there is so much to see and explore. And while you are out there don’t forget to stop and smell the geosmin.