The smell of earth

Or, Streptomyces: a brief history

It finally feels like fall in Los Angeles.

In the morning, I leave my apartment in a sweater. Thick socks and even a turtleneck. Sometimes, the rain comes and the garden brightens with green and the plants seem especially tall and sturdy. Everywhere is the smell of the soil. That’s my favorite part, of course.

There seems to be no way to accurately describe this odor, except for with other words that all just seem to be its synonyms. Earth smells “earth-y” (go figure), or smelling it makes me feel “grounded.” I guess it could be said to smell rich, warm, and …musty? But what a sad little word for something so sensorily captivating.

It’s such a good smell, we know that much. Well, we know a few other things, too.

The smell of earth comes largely from a chemical compound called geosmin, which is produced by microscopic soil bacteria. We’ve been aware of and responsive to this scent probably as long as we have existed, although we’ve only started exploring it scientifically within the last hundred years or so.

If you are fluent in French.

On April the 23, 1891, the French chemist Pierre Eugène Marcellin Berthelot and Monsieur G. Andre read a short paper at the meeting the French Academie des Sciences, titled “Sur l’odeur propre de la terre” — or, in English, “On the earth’s own smell.”

They theorized the scent of earth was related to a plant’s “essential oils” and released into the air when displaced by rain. They were directionally correct, though the actual chemical compound and its associated processes remained undiscovered for the next eighty years. In 1965 the chemists Nancy Gerber and Hubert Lechevalier, also French, isolated and named “geosmin” (meaning “earth smell”), having determined that it derived from a special genus of bacteria, Streptomyces, that live in the soil.

Bacteria in the Streptomyces genus play an expansive and central role in decomposition, in addition to good smells, breaking down complex organic matter, helping to release nutrients into the soil, making other nutrients available for plant uptake, and producing a range of essential bioactive compounds, including antibiotics. Fascinatingly, scientists also think Streptomyces might represent one of the earliest “trial runs” in nature’s move from being single-celled to being multicellular. In other words, it was probably one of several early experiments in evolution where life was trying out how to organize multiple cells into a single, more complex organism.

Geosmin is a byproduct of the life cycle of these bacteria, released simultaneously as they form spores for reproduction. Its earthy odor acts like a signal, attracting tiny soil animals like springtails, which brush against the spores, carry them on their bodies, and spread them to new places.

Humans, too, can detect geosmin at incredibly low levels—far lower than most other animals, as low as 100 parts per trillion. That’s better than sharks can smell blood in water. How extraordinary. The implication, of course, is that the odor is significant to our survival. Centuries ago, it likely helped us locate safe drinking water and fertile land for crops. In that sense, geosmin doesn’t just signal “earth” to us, then, but specifically good earth. It means healthy, rich soil—teeming with life, invisible but deeply felt, threaded all the way into the oldest parts of our molecular formation.

I see evidence of Streptomyces in my compost all the time, most often as I’m turning the heap. As they grow in volume and mature, they become visible as powdery gray blooms (see above). You’ve probably seen them, too. As ever, it feels remarkable that such a simple and intuitive process as piling organic matter into a heap can produce such a wildly complex and essential property for all of life. Miracles, all around us.

Fall, even in the increasingly dry region of Los Angeles, proffers all the necessary ingredients for this odor to become abundant. The leaves come down and begin their long, slow process of decomposition. The soil enrichens. The rain falls. The earth retreats into a period of generative and active breakdown. Everywhere, everywhere - the smell of earth.

From the Microbiology Society: “We can safely assume that the time-traveller visiting the planet as it was about 440,000,000 years ago would recognise the familiar smell of soil, as the earliest land plants collaborated with the first streptomycetes to generate protocompost.”

That’s all for now!

Love,
Cass

PS.

Scent of earth for sale. Not as good as the REAL stuff.