It hasn't been measured very consistently, and that's the problem. More monitoring should be done to assess long-term productivity losses associated with soil erosion. There haven't been enough studies on the subject. Some were done in the 1980s, but they need to be updated.
Another problem is that tractors are getting bigger all the time. Because there are fewer and fewer farm workers, we tend to use bigger vehicles with over three tonnes load per wheel, which leads to greater and greater soil compaction. That's put us in a downward spiral where heavier tractors are causing more compaction and fertilizers don't work as well.