Agricultural soil requires coverage to prevent erosion and damage to microbes
"When you have a vegetative cover on the soil, especially a living cover, you offer those microbes both food and shelter. Some scientists say when you till the soil and remove crop residues, the effects are as devastating to soil microbes as a combination of an earthquake, hurricane, tornado, and forest fire would be to humans...When a falling raindrop explodes as it hits bare soil, it dislodges unprotected soil particles, and begins the process of soil erosion. Cover crops and plant residue prevent that violent splash on soil, protecting soil aggregates from being pounded by falling raindrops."
Source: U.S. Department of Agriculture, Natural Resources Conservation Service, “Healthy Soils Are: Covered All the Time,” 1, accessed July 30, 2014, http://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/soils/health/?c....