When the Europeans first arrived in North America, the average depth of the topsoil was 53.34 cm (1¾ ft) and it was rich in the types of symbiotic organisms necessary for plant roots to absorb minerals from the soil. Today North America averages around 15.24 cm (6 in) of topsoil and most if it is exhausted of nutrients and much is devoid of life.~ Thom Hartmann (1951-05-07 age:66) Threshold: The Progressive Plan to Pull America Back from the Brink page 22
As any gardener can tell you, you can’t get soil much thinner than 45.72 cm (1½ ft) and still grow a decent crop of vegetables. Yet, even knowing the rate of erosion, farmers persist in the agricultural practices that strip the soil. They refuse to look ahead any more than one year.