The healthiest soils that promote the highest crop yields are generally those that are left alone
"The healthiest soils are those with a diversity and abundance of life. Farmers with the healthiest soils nurture that life by creating a diversity of plant life above the soil surface, with year-round ground cover, no tillage, and judicious pesticide use. Understanding that the soil is full of life is a game-changer for farmers who are full of life...These farmers understand that tillage, the turning of the soil that has been the standard for growing crops for years and years, is disruptive to soil microbes and destructive to the soil system. Instead, they disturb the soil as little as possible. And, they grow a diversity of living plants in the soil as much of the time as practical, covering the soil and offering food to soil microbes through living roots. Those soil organisms, in turn, cycle nutrients back to the plant, allowing it to grow and flourish."
Source: U.S. Department of Agriculture, Natural Resources Conservation Service, “Healthy Soils Are: Full of Life,” 1, accessed July 30, 2014, http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1193147.pdf.