The Smell of Healthy Soil

Actinobacteria are a hyphae-producing soil bacteria that, in appearance and behavior, appear to have more in common with soil fungi. Like fungi, they decompose some of the more resistant forms of organic plant residues. Like fungi, they form branching filaments, which resemble the mycelia of fungi. Actinobacteria were originally classified as fungi under the older name Actinomycetes.

As bacteria, Actinobacteria have cell walls. They grow best in soil when conditions are damp and warm, playing an important role in decomposition of organic materials, such as cellulose and chitin. When the soil dries they produce spores. The wetness and force of rainfall kick these tiny spores up into the air. The moist air easily carries the spores to us so we breathe them in. These spores have a distinctive, earthy smell we often associate with rainfall. The smell comes from a compound, geosmin, which translates to "earth smell". The human nose is exquisitely sensitive to geosmin, able to detect it at concentrations down to 10 parts per trillion. Since the bacteria thrives in moist soil but releases the spores once the soil dries out, the smell is most acute after a rain that follows a dry spell, although you'll notice it to some degree after most rainstorms. Actinobacteria are important to healthy soil function, and are ubiquitous. Thus the smell of healthy soil is similar the world over.

Though they play an important role in soil quality, Actinobacteria are more commonly known for what it produces in the laboratory. Actinobacteria are unsurpassed in their ability to produce many compounds that have pharmaceutically useful properties. In 1940 Selman Waksman discovered that they made actinomycin, a discovery for which he was awarded a Nobel Prize. Hundreds of naturally occurring antibiotics have been discovered in these terrestrial microorganisms, especially from the genus Streptomyces.

Actinobacteria are also involved in nitrogen fixation; they convert atmospheric nitrogen into a form that can be used by plants.

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