Iowa has some of the best topsoil in the world. In the past century, half of it’s been lost, from an average of 18 to 10 inches deep (Pate 2004, Klee 1991).
Productivity drops off sharply when topsoil reaches 6 inches or less, the average crop root zone depth (Sundquist 2005).
Crop productivity continually declines as topsoil is lost and residues are removed. (Al-Kaisi May 2001, Ball 2005, Blanco-Canqui 2006, BOA 1986, Calviño 2003, Franzleubbers 2006, Grandy 2006, Johnson 2004, Johnson 2005, Miranowski 1984, Power 1998, Sadras 2001, Troeh 2005, Wilhelm 2004).
On over half of America’s best crop land, the erosion rate is 27 times the natural rate, 11,000 pounds per acre (NCRS 2006). The natural, geological erosion rate is about 400 pounds of soil per acre per year (Troeh 2005). Some is due to farmers not being paid enough to conserve their land, but most is due to investors who farm for profit. Erosion control cuts into profits.
Erosion is happening ten to twenty times faster than the rate topsoil can be formed by natural processes (Pimentel 2006). That might make the average person concerned. But not the USDA -- they’ve defined erosion as the average soil loss that could occur without causing a decline in long term productivity.