Soils have been degrading for a long time, more rapidly over the last 100 years or so, but well before then.
One thing to keep in the forefront of your minds is that with each subsequent generation of seed, from plants grown in poor soil, seed quality falls dramatically as well.
The seeds planted today ARE NOT as good as they were even a few generations ago. That is of course if you are using non-GMO seed. Genetically-Modified-Organisms plants are designed NOT to seed in order to keep the farmer locked into purchasing seed year after year...
That is why if you are going to plant, you should work hard to find the best seed possible. If you have poor seed, it may take a few generations of plantings in most excellent soil to end up with a nutritious product on a consistant basis.