What’s behind the “fresh earth” smell in spring?

Question: What makes that “fresh earth” smell in spring?

actinomycetes
Actinomycetes as seen under a microscope. Photo courtesy of Tom Loynachan.

Answer: In a word, geosmin.

What is that?

Geosmin is an organic compound released by a group of soil microorganisms called actinomycetes.

Actinomycetes are filamentous “prokaryotes.” In other words, they are microscopic organisms whose cells have no “nucleus” enclosing their DNA. Organisms that do have cell nuclei—like plants, fungi, and humans and other animals—are called “eukaryotes.”

Actinomycetes are also composed of “hyphae”—or small filaments or threads. Under a microscope, these microbes look a bit like a cross between fungi and bacteria, and were once called thread bacteria.

Many people associate the smell of geosmin with digging in a garden or plowing a field, or even an old cellar. For others, it’s what they smell during a rain. That’s because as rainwater enters the soil, the air in the soil is displaced, releasing the air and geosmin to the atmosphere.

People may even smell geosmin before a rain as a low pressure system causes degassing of the soil air.

–Answered by Clay Robinson, a.k.a., Dr. Dirt and Tom Loynachan, Iowa State University

Have a question for Soils Matter? Post it to the comments section below or email us at soils-matter@soils.org

11 thoughts on “What’s behind the “fresh earth” smell in spring?

  1. Geosmin seems to have a couple of methyl groups. Is there some relationship to anaerobic decomposition and methane? Similarly, how does geosmin break down in the atmosphere.

      1. Actinomycetes release aromatic compound vie. geosmin hence smell is come out …

  2. Hello, I run a blog called Green Infrastructure for Your Community (www.deeproot.com/blog) where we discuss issues around trees, soil, and stormwater. I’d love to reprint this question, as well as one or two others from your blog. Would you be willing to grant me permission to do so? Of course I would be happy to credit Soils Matter and link back to the original post.

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