The nose knows: what smell can tell you about soil health and microbial activity (ASK Advice)

Does your favorite smell have a name? Mine does: petrichor. Petrichor is that fresh, earthy smell after rain falls on dry ground. It’s a pleasant, almost sweet smell, that blossoms up through the air while plants below soak up the new water. Some people describe petrichor as forest-like or dewy, since it’s so closely associated with greenery and water—but the smell is actually created by soil microbes, not plants!

 Petrichor: the fresh smell of earth after rain. Image via Wikimedia Commons

Scientists discovered that the smell of fresh rain is mostly thanks to a molecule named geosmin. A filamentous (thread-like) class of microbes called Actinobacteria produce geosmin in the soil, especially Streptomyces species of bacteria. So, when soil smells rich and earthy, know that it’s already teeming with life at the microbial level.

On the other hand, if soil smells sour, foul, or rotten, it may be a sign that oxygen levels are too low due to waterlogging and/or compaction. Without enough air, aerobic microbes slow down and anaerobic microbes take over. Some of these organisms produce hydrogen sulfide gas that gives off the strong, unpleasant odor characteristic of a swamp. In a garden, container, or field, that smell likely points to poor drainage or overwatering.

Audobon Swamp Gardens in South Carolina. Image by Brian Stansberry via Wikimedia Commons.

Another soil smell you might encounter in your garden is a sharp ammonia scent, especially if you have a backyard compost. Similar to the rotten sulfur smell, ammonia might taint the air if bacteria are breaking down organic matter without access to enough oxygen. In the case of compost, it can also indicate an imbalance in the starting mix (too much “green” material compared to “browns” such as dry leaves or straw). That pungent ammonia smell is a sign of rich nutrients and active microbes that just need to breathe a bit better. Time to aerate the soil or turn the compost!

Mature compost. Image by Bernard Dejean via Wikimedia Commons.

The next time you pick up a handful of soil, take a moment to smell it. Healthy soil is neither odorless nor overpowering. It has a mild, earthy aroma that signals a steady level of microbial activity with the right levels of moisture, organic matter, and aeration. That springtime sweet smell of fresh earth reminds us of the connections between water, soil and life, from April showers to May flowers and beyond. Soil bacteria might be too small for us to see with the naked eye, but sometimes we can detect their activity with our nose!