Sun, Soil, and Serotonin

06.24.25

It has been about two weeks since I finished school, and I’ve fully settled into summer mode—which, for me, means spending time outside. Now that I’m keeping this blog, I’ve been more intentional about getting outdoors every day, knowing how much it benefits both body and mind. Lately, I’ve been spending my free time in my very small, very beginner garden at home.

It’s well known that gardening can support mental and physical health. Just being in the sun and soaking up vitamin D reduces stress and improves mood. But a more interesting question is whether something in the soil itself directly impacts cognitive health.

As it turns out, soil is teeming with microorganisms that play crucial roles in nutrient cycling, plant health, and soil structure. Microbes break down organic matter to restore nutrients, improve soil structure, and even protect plants from disease. Different microbes perform different functions, and some also appear to affect human mental health.

One example is the bacterium Mycobacterium vaccae, which has antidepressant-like effects. According to writer Bonnie L. Grant, this microbe influences neurons in a way similar to drugs like Prozac, stimulating serotonin production.

Higher serotonin levels not only improve mood but also support learning. In a study led by Dr. Chris Lowry at the University of Bristol, mice injected with M. vaccae showed significantly reduced stress. Dorothy Matthews and Susan Jenks at Sage Colleges in Troy went further by feeding mice peanut butter sandwiches laced with M. vaccae. Those mice navigated a maze twice as fast and showed half the anxiety behaviors of the control group. Even after the bacterium was removed from their diet, the benefits lasted about three weeks.

As the NWF Blog explains, Matthews and Jenks suggested that humans benefit through everyday contact with soil. Gardeners inhale M. vaccae while digging, encounter it on homegrown vegetables, and may even absorb it through small cuts in the skin. Matthews speculated that creating school environments where students spend more time outdoors could reduce anxiety and improve learning.

First couple minutes of backpacking in Hawaii, and my shoes already got this dirty. Very soon after we were dirty everywhere!

I’ve gone backpacking and camping many times, and I always feel emotionally refreshed afterward. Now I see that those boosts might be partly due to exposure to microbes like M. vaccae, which can elevate serotonin and extend their effects long after the trip ends.

Today I focused on just one of the millions of microbes in soil that may support cognitive health. In future posts, I look forward to exploring others and what they might teach us about the hidden connections between nature and the mind.