The practices of 150 years ago destroyed the soil. Farmers had to stop those in the 1930s. Since then plows have gone out of style and reduced soil loss greatly. Farmers who get 4x the yields are actually building soil, they have to because the soil they were left with by the 1980s wouldn't support those yields without doing things to build up the soil. It is years to build up though, if you can add 1mm of soil in a decade that is good. However you can also make what is left better without adding more.