# Serotonin in the Soil

Soul Intro

There is a moment, often in childhood, when you press your hand into wet dirt and something settles in your chest. You don't know why. You couldn't explain it if asked. You simply feel, for a second, like you are exactly where you belong.

That moment is not sentimental. It is biochemical. It is the consequence of a four-billion-year conversation between your nervous system and the life in the ground — a conversation most of us no longer have, and whose absence is one of the invisible costs of modern life.

This article traces what the peer-reviewed evidence actually shows. It then gets very practical. Twenty-seven specific actions, tiered by time commitment, so whoever you are and whatever your life looks like, there is something on this page you can do today.

The Core Claim

Roughly 90% of the serotonin in your body is made in your gut, by cells that take their cues from the trillions of bacteria living there. Those bacteria are seeded and maintained by what you eat, what you touch, what you inhale, and — critically — what kind of soil you are exposed to. Specific microbes, most famously *Mycobacterium vaccae*, activate the brain's serotonin system. Modern life has largely severed this circuit. The good news: it reconnects fast.

Arc 1 — The Gut Makes Your Mood

Ninety percent

The most important number on this page: ~90% of peripheral serotonin is synthesized in the enterochromaffin cells of the gut, not in the brain (Yano et al., 2015, *Cell*, doi:10.1016/j.cell.2015.02.047). The gut doesn't just digest — it manufactures, in quantities that dwarf the brain's own production, the neurotransmitter most associated with mood.

And that manufacturing is regulated by the gut microbiome. Germ-free mice — raised in sterile conditions with no gut bacteria — have 60% less circulating serotonin than conventionally-raised mice. Reintroducing the bacteria restores serotonin levels (Yano et al., 2015).

The vagus wire

Gut and brain talk constantly via the vagus nerve — the longest cranial nerve in the body, carrying signals in both directions but flowing predominantly upward (gut-to-brain) at roughly 9 to 1. Gut microbes produce short-chain fatty acids (butyrate, propionate, acetate), neurotransmitter precursors, and immune-signaling molecules that reach the brain via this direct wire (Cryan & Dinan, 2019, *Physiological Reviews*, doi:10.1152/physrev.00018.2018; Bravo et al., 2011, *PNAS*, doi:10.1073/pnas.1102999108).

In humans, not just mice

A 2019 analysis in *Nature Microbiology* examined fecal samples from over 1,000 people and found that bacterial genera known to produce dopamine and GABA precursors were consistently depleted in individuals with diagnosed depression, even after controlling for antidepressant use, diet, and socioeconomic variables (Valles-Colomer et al., 2019, doi:10.1038/s41564-018-0337-x). Clinical trials of specific probiotic strains — *Lactobacillus rhamnosus*, *Bifidobacterium longum*, *B. breve*, *L. helveticus* — show measurable reductions in cortisol, depression scores, and anxiety scores across multiple small-to-medium RCTs (Sarkar et al., 2016, *Trends in Neurosciences*, doi:10.1016/j.tins.2016.09.002; Liu et al., 2019, *Frontiers in Neuroscience*, doi:10.3389/fnins.2019.00776; Messaoudi et al., 2011, *British Journal of Nutrition*, doi:10.1017/S0007114510004319).

This has a name now: psychobiotics. It has its own journals, conferences, and medical-school curricula. Fifteen years ago none of that existed.

Arc 2 — Mycobacterium vaccae: The Dirt Bacterium That Acts Like an Antidepressant

The 2004 discovery

Cancer researchers injecting mice with heat-killed *Mycobacterium vaccae* — a common, harmless soil bacterium — noticed the treated mice were *calmer*. They performed better on cognitive tasks. Their behavior looked less anxious.

Christopher Lowry, a neuroscientist at the University of Colorado, investigated. His 2007 paper in *Neuroscience* showed that *M. vaccae* exposure activated specific mesolimbic serotonergic neurons — producing effects structurally similar to antidepressants, measurable on behavior and on brain chemistry (Lowry et al., 2007, doi:10.1016/j.neuroscience.2007.01.067).

This was the paper that launched a field. A soil bacterium, not a drug — just dirt — activated the brain's serotonin system.

What followed

Matthews and Jenks (2013): mice fed live *M. vaccae* solved maze tasks twice as fast and showed reduced anxiety behaviors (doi:10.1016/j.beproc.2013.01.011).
Reber et al. (2016): *M. vaccae* administration conferred stress resilience — treated mice didn't develop depressive behavior even under experimentally-induced chronic stress (*PNAS*, doi:10.1073/pnas.1600324113).
Frank et al. (2018): *M. vaccae* reduced neuroinflammation and microglial reactivity in stressed mice — evidence of an anti-inflammatory mechanism in the brain itself (*Brain, Behavior, and Immunity*, doi:10.1016/j.bbi.2018.02.004).

We don't yet have large human RCTs of *M. vaccae* as therapy. What we have is the broader "old friends" hypothesis (Rook, 2013, *PNAS*, doi:10.1073/pnas.1313731110): human immune and nervous systems evolved to expect constant exposure to soil-derived microbes, and modern absence of that exposure contributes to rising depression, anxiety, and inflammatory disease.

Natural experiments in human populations

Yanomami of the Amazon — one of the few remaining populations with near-continuous soil contact — carry a gut microbiome roughly twice as diverse as typical urban Americans, with correspondingly low rates of allergic disease and inflammatory conditions (Clemente et al., 2015, *Science Advances*, doi:10.1126/sciadv.1500183).
Amish vs. Hutterite children (genetically related, both farming, but Amish work soil directly while Hutterites use industrial agriculture): Amish children have four times lower asthma prevalence and dramatically different immune profiles (Stein et al., 2016, *NEJM*, doi:10.1056/NEJMoa1508749).
Farm-raised vs. suburban children in Bavaria: raw-milk consumption and barn exposure in early life correlates with a 50–80% reduction in asthma and hay fever across multi-thousand-participant European birth cohorts (Ege et al., 2011, *NEJM*, doi:10.1056/NEJMoa1007302).

Arc 3 — What Modern Life Broke

Three large-scale disruptions have broken the soil-gut-brain loop for most people in industrialized countries:

1. Sanitization. Antibacterial soaps, pasteurized food, sealed everything. Microbial content of the average person's food has dropped by orders of magnitude over three generations (Rook, 2013; Mosca et al., 2016, *Frontiers in Microbiology*, doi:10.3389/fmicb.2016.00455).

2. Antibiotics. Each course of broad-spectrum antibiotics eliminates a significant fraction of gut microbial diversity — some of which never fully recovers. Repeated childhood exposure correlates with increased later-life risk of asthma, IBD, and depression (Dethlefsen & Relman, 2011, *PNAS*, doi:10.1073/pnas.1000087107; Mikkelsen et al., 2016, *BMJ Open*, doi:10.1136/bmjopen-2015-010527).

3. Physical separation from soil. Urbanization, sealed floors, cleaned food, washed hands. Direct daily soil contact has dropped from roughly 12 hours per day (pre-industrial) to near zero (typical urban life).

The "hygiene hypothesis" of the 1990s has been refined into the "old friends" hypothesis: it's not cleanliness per se that's the problem, but the specific loss of co-evolved microbial partners (Rook, 2013, doi:10.1073/pnas.1313731110; Haahtela et al., 2013, *World Allergy Organization Journal*, doi:10.1186/1939-4551-6-3).

Arc 4 — Reconnecting Is Surprisingly Easy (The Evidence)

What the accumulated literature supports:

Regular direct skin contact with living soil (gardening, barefoot on grass, handling houseplants) — even small exposures matter; frequency beats duration (Hanski et al., 2012, *PNAS*, doi:10.1073/pnas.1205624109).
Diverse plant foods grown in healthy soil: the American Gut Project documented that people eating 30+ different plant types per week have measurably more diverse gut microbiomes than those eating fewer than 10 (McDonald et al., 2018, *mSystems*, doi:10.1128/mSystems.00031-18).
Fermented foods: a 2021 Stanford study found a 10-week fermented-food-rich diet (yogurt, kefir, kimchi, sauerkraut, kombucha) increased microbiome diversity and decreased 19 inflammatory proteins in healthy adults (Wastyk et al., 2021, *Cell*, doi:10.1016/j.cell.2021.06.019).
Green-space exposure: just being in and around biodiverse green space increases skin and airway microbial diversity within weeks (Hanski et al., 2012, doi:10.1073/pnas.1205624109).
Avoidance of unnecessary antibiotics and antimicrobials.

Arc 5 — What People Are Actually Doing About It

*Ron Finley's community garden work in South Los Angeles is one visible instance of this whole cascade operationalized. Growing food in your own neighborhood restores soil, restores access to diverse fresh plants, and restores the daily direct soil contact that the "old friends" hypothesis says matters.*

*The Detroit community-garden movement — buying vacant lots for $100 and converting them to productive soil — is another. The science above is not theoretical at scale.*

The evidence does not ask you to become a homesteader. It asks you to close the loop in whatever form fits your life. Twenty-seven specific ways, below.

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Key Takeaway

Your mood, your immune system, and your gut are three views of one system — and that system was built to be in constant conversation with soil-dwelling microbes. Ninety percent of your body's serotonin is made in the gut, regulated by bacteria your ancestors met through food, skin contact, and breath. Modern life has largely broken that conversation. Restoring it doesn't require medication or dramatic life change. It requires, on most days, some deliberate dirt.

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Love In Action: 27 Things You Can Actually Do

Tiered by time. Pick one from each tier this week.

60-second acts (daily)

1. Touch living soil with a bare hand for sixty seconds. A potted plant, a tree well, a park corner. Every time you pass one.

2. Smell a handful of fresh dirt. The earthy smell is *geosmin*, produced by *Streptomyces* bacteria — the same family that gave us most of our antibiotics. Evolved to attract animals that spread spores. Your nose is a co-evolved sensor.

3. Add a spoonful of sauerkraut, kimchi, or yogurt-with-live-cultures to whatever you're already eating. Fermented food on your plate daily is the single easiest continuous input.

4. Skip antibacterial soap. Use plain soap. Equivalent for infection prevention; better for microbiome.

5. Open a window for an hour. Outdoor microbial aerosol is more diverse than indoor (Hanski et al., 2012, doi:10.1073/pnas.1205624109).

5-minute acts (several times a week)

6. Repot a plant. Hands in soil, no gloves, sit with it afterward before washing.

7. Eat one fruit or vegetable you haven't eaten in the last month. Microbiome-diversity proxy is plant-diversity on the plate.

8. Start a jar of fermented garlic honey (one head peeled garlic, cover with raw honey, stir daily for two weeks — edible at any point, live bacteria by day 10).

9. Walk barefoot on grass for 5 minutes before coffee or after work.

10. Sit on the ground instead of a chair for one conversation this week.

15-minute acts (weekly)

11. Weed or plant something — your garden, a planter, someone else's yard. Weekly minimum.

12. Visit a farmers' market once a week. Fresh produce grown in diverse soils still carries meaningful microbial signatures compared to supermarket produce.

13. Make a simple sauerkraut (cabbage + salt in a jar, two weeks on the counter). One jar produces four weeks of daily fermented-food servings.

14. Take off your shoes indoors. Shoe contamination tracks urban pollutants into your home; bare or clean-sock floors also let you feel the ground of the house you live in.

15. Eat one meal a week entirely plants you can identify — no ingredient lists, no processed anything.

Hour-scale acts (monthly at minimum)

16. Start a container garden. A single 12-inch pot on a sunny windowsill grows lettuce, herbs, chard, radishes, or cherry tomatoes.

17. Visit a compost center or community garden. Most cities have public-facing ones. Touch the compost.

18. Plant one perennial (a strawberry plant, a rosemary, a lavender). Perennials maintain soil biology year-round in a way annuals cannot.

19. Swim in natural water — a lake, a river, the ocean. Natural water is microbially rich in ways pool water is not.

20. Walk a hiking trail with exposed dirt rather than a paved path.

Day-scale and community acts (weekly-to-monthly)

21. Volunteer at a community garden. See the Detroit and South LA videos above — these networks exist in most cities and always need hands.

22. Support one regenerative farmer directly (CSA box, farmers' market relationship, farm visit). The farms that rebuild soil carbon are operationally the same farms that produce the most microbiome-diverse food.

23. Teach a child to garden. Childhood soil exposure is the most influential window in the "old friends" literature (Stein et al., 2016, doi:10.1056/NEJMoa1508749). Passing it down matters.

24. Host a fermented-food swap with friends. One person's sauerkraut, another's yogurt, another's kimchi, another's kombucha. Increases the diversity every participant is exposed to.

Long-arc commitments (months to years)

25. Reduce meat from industrial sources. Industrial meat is the single biggest driver of soil degradation and antibiotic overuse worldwide; choosing pasture-raised or skipping is both a soil vote and a microbiome vote.

26. Lobby your city for more unpaved park area, community gardens, schoolyard gardens. Green-space proximity and access is one of the strongest predictors of childhood microbiome diversity (Hanski et al., 2012).

27. If you have any control over a piece of land — yard, balcony, community plot — stop using broad-spectrum pesticides. One household's decision matters locally. A neighborhood's decisions matter regionally.

The honest truth

You cannot do all 27. You don't need to. Three items from the first two tiers, consistently, over four to six weeks, is enough to produce measurable changes in gut microbiome diversity in the published studies (Wastyk et al., 2021, doi:10.1016/j.cell.2021.06.019; McDonald et al., 2018, doi:10.1128/mSystems.00031-18). Start small. Repeat.

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Frequently Asked Questions

Do I need to eat dirt? No. The evidence for direct consumption is thin and the risks (parasites, heavy metals) are real. Skin, airway, and plant-based contact is sufficient for the documented effects.

Will probiotic pills give me the same benefit? Partially. Specific strains (*L. rhamnosus*, *B. longum*, *B. breve*, *L. helveticus*) have clinical evidence for mood effects (Messaoudi et al., 2011, doi:10.1017/S0007114510004319). But the diversity of natural soil-plus-food exposure is orders of magnitude higher than any commercial probiotic. Pills supplement; they don't replace.

How long before I feel anything? Animal studies show behavioral effects within days. Human probiotic trials typically show mood-scale changes within 3–6 weeks of consistent use. The Stanford fermented-food study saw microbiome changes in 10 weeks.

Should I let my kids play in more dirt? Yes. Childhood exposure windows are the most influential. Kids who grow up with regular soil, animal, and traditional-food exposure have dramatically lower rates of later allergic and autoimmune disease (Stein et al., 2016; Ege et al., 2011).

I'm on antidepressants. Should I stop? No. None of this is a replacement for clinically-prescribed medication. What these interventions are is a complement — evidence-backed, side-effect-free, and free. Consult your prescriber about integrating both.

What about fecal microbiota transplant (FMT)? Real, increasingly studied therapy, clinically established for recurrent *C. difficile* infection and being trialed for depression, IBS, and other conditions (Kelly et al., 2015, *American Journal of Gastroenterology*, doi:10.1038/ajg.2014.444). Not a DIY protocol.

I live in an apartment in a dense city. What's realistic? Four things: a pot of live soil on your windowsill, a fermented food daily, a weekly farmers-market trip, and a monthly trip to a park or garden where you touch actual ground. That alone captures most of the evidence base.

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Connected Reading

`/articles/the-soil-foundation-of-love` — the four-arc cascade this article lives inside
`/articles/earth-as-antioxidant` — the electrical-contact dimension of the same bare-skin-to-earth practice
`/articles/gut-microbiome-mental-health-dysbiosis` — the clinical-depression side of the axis
`/articles/psychobiotics-and-mental-clarity-how-to-feed-your-microbiome` — food-specific dietary protocols
`/articles/mycorrhizal-fungi-soil-health` — the fungal networks that make the soil diverse in the first place

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References

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2. Clemente, J. C., Pehrsson, E. C., Blaser, M. J., et al. (2015). The microbiome of uncontacted Amerindians. *Science Advances*, 1(3), e1500183. https://doi.org/10.1126/sciadv.1500183

3. Cryan, J. F., & Dinan, T. G. (2019). The microbiota-gut-brain axis. *Physiological Reviews*, 99(4), 1877–2013. https://doi.org/10.1152/physrev.00018.2018

4. Dethlefsen, L., & Relman, D. A. (2011). Incomplete recovery and individualized responses of the human distal gut microbiota to repeated antibiotic perturbation. *PNAS*, 108(Suppl 1), 4554–4561. https://doi.org/10.1073/pnas.1000087107

5. Ege, M. J., Mayer, M., Normand, A. C., et al. (2011). Exposure to environmental microorganisms and childhood asthma. *New England Journal of Medicine*, 364(8), 701–709. https://doi.org/10.1056/NEJMoa1007302

6. Frank, M. G., Fonken, L. K., Dolzani, S. D., et al. (2018). Immunization with *Mycobacterium vaccae* induces an anti-inflammatory milieu in the CNS: attenuation of stress-induced microglial priming, alarmins and anxiety-like behavior. *Brain, Behavior, and Immunity*, 73, 352–363. https://doi.org/10.1016/j.bbi.2018.02.004

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21. Stein, M. M., Hrusch, C. L., Gozdz, J., et al. (2016). Innate immunity and asthma risk in Amish and Hutterite farm children. *New England Journal of Medicine*, 375(5), 411–421. https://doi.org/10.1056/NEJMoa1508749

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23. Wastyk, H. C., Fragiadakis, G. K., Perelman, D., et al. (2021). Gut-microbiota-targeted diets modulate human immune status. *Cell*, 184(16), 4137–4153. https://doi.org/10.1016/j.cell.2021.06.019

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*If a claim here is not backed by peer-reviewed evidence, it is not here.*