Mycobacterium vaccae is a soil bacteria and has been shown to boost "happy hormones" serotonin and norepinephrinen levels in humans.

Geosmins originate from several soil microbes like actinomycetes and contribute that fresh, earthy smell to healthy soils. Inhaling geosmin or exposure to geosmin-producing soil microbes helps stabilize brain waves, reduce stress, and alleviate depression-like behaviors by lowering neuroinflammation.

Both are found in healthy forest soils in abundance. The forest is delivering anti-depressants in non-trivial amounts to mammals (not sure about birds or insects etc.)

Playing in dirt is good for you.