Healthy soil has microbes that decompose organic material
"Healthy soil holds, filters and regulates water, mitigates drought and flooding, reduces runoff and erosion, cycles nutrients, sequesters carbon and suppresses weeds and pests naturally—all while supporting our homes and buildings...Healthy soil should have a sweet earthy smell, indicating the presence of geosmin, a byproduct of soil microbes called actinomycetes. These microbes decompose the tough plant and animal residues in and on the soil and bring nitrogen from the air into the soil to feed plants. An unhealthy, out-of-balance soil smells sour or metallic, or like kitchen cleanser."
Source: U.S. Department of Agriculture, Natural Resources Conservation Service California, “Before Spring Planting Expert Says, ‘Dig a little. Learn a lot,’” News Release, April 3, 2013, accessed July 30, 2014, http://www.nrcs.usda.gov/wps/portal/nrcs/detail/ca/people/employees/?cid....