A bacteria commonly found in dirt has been shown to reduce depression, according to an article in Discover Magazine. Mycobacterium vaccae is found in soil, and may be inhaled when you take a walk out of doors, work in the garden, or when children play in the dirt. Scientists have long known that farm kids have fewer allergies and have healthier immune systems than kids raised in more sterile suburban or urban environments. Working in the garden has health benefits beyond the food that it produces.