Many have suggested that digging in the dirt and gardening stir up the microbes that live there, which can influence how we feel. Therefore, part of the therapeutic benefits of gardening may indeed come from contact with some of the metabolic products or actual members of the soil microbiome (the community of microorganisms in the soil environment).
In a well-designed study using psychophysiological and metabolomic approaches, Kim and colleagues1 investigated the responses of a population of 30 adults when they mixed soil without and with the soil bacterium Streptomyces rimosus. The genus Streptomyces might seem familiar because a different member of genus Streptomyces griseus is the source of the broad-spectrum antibiotic streptomycin first isolated in the 1940s, and S. rimosus is the source of the widely used antibiotic oxytetracycline. Two other metabolites of S. rimosus, geosmin and 2-methylisoborneol (2-MIB), contribute to the earthy aroma of freshly dug or cultivated soil.
The study design was a single-blinded crossover experiment meaning all participants would experience both the control and the treatment activities but not know the identity of each activity. A preliminary survey of four Streptomyces species growing in a culture medium revealed S. rimosus produced the highest amount of geosmin and 2-MIB and was chosen for the experiment. The participants were randomly assigned so that some would experience the control activity first and some would experience the treatment activity first. The control activity used sterilized peat moss and perlite soil mix inoculated with a small amount of sterile bacterial culture medium. The treatment soil was exactly the same, except the added culture medium likely contained tens of millions of live S. rimosus cells. The two inoculated soils without and with S. rimosus were incubated for three days before use in the human subject experiments.
A small, controlled environment treatment space was prepared for the approved human subject experiments that maintained constant light, temperature, relative humidity, and visual environment. Randomly assigned participants were fitted with a wireless EEG device, seated in front of a basin containing either the control or the S. rimosus-containing soil, monitored, and followed a precisely scripted soil mixing activity trial protocol. After mixing the soil for 5 minutes, blood was drawn for metabolite analysis, and the participant’s emotional and stress state was assessed using the Semantic Differential Method and a Numerical Rating Scale. A second soil mixing activity trial with EEG monitoring and post-mixing blood draw and emotional state analyses was conducted for each participant to have experienced mixing the control and S. rimosus-containing soils.
What did the researchers find? Chemical analysis showed the soil containing S. rimosus emitted greater amounts of volatile geosmin and 2-MIB than the control soil without the bacteria added and modified the levels of many other volatile organic compounds. Using an ASEF50 index of the EEG data that corresponds to brain comfort and 50% of alpha power of the occipital lobes, the ASEF50 was found to be significantly higher for the right occipital lobe when participants were mixing soil containing S. rimosus. Serum levels of the neurotransmitter serotonin were higher following mixing soil with S. rimosus, while brain-derived neurotrophic factor levels were unchanged from the control. Serum C-reactive protein levels were lower after mixing soil with S. rimosus than in the control soil without S. rimosus. Non-targeted serum low-molecular-weight metabolite analyses revealed a complex pattern of metabolite level changes following mixing soil without and with S. rimosus that will require further studies. Ultimately participant emotional comfort was found to be greater and subjective stress lower when mixing soil containing S. rimosus.
This well-designed pilot study provides compelling empirical evidence that even a short intimate exposure to soil, its microbiome, and microbial metabolites can positively influence a person’s emotional state and modify serum metabolite levels. This study should serve as crucial foundational evidence justifying further research studies and more large-scale randomized controlled trials on the therapeutic role of soil and the soil microbiome on human health and wellbeing.
1Kim, S.O., Kim, M.J., Choi, N.Y., Kim, J.H., Oh, M.S., Lee, C.H., Park, S.A. (2022). Psychophysiological and Metabolomics Responses of Adults during Horticultural Activities Using Soil Inoculated with Streptomyces rimosus: A Pilot Study. International Journal of Environmental Research and Public Health, 19(19):12901. https://doi.org/10.3390/ijerph191912901
Charles Guy, Emeritus and Courtesy Professor
Department of Environmental Horticulture, University of Florida