I don't have any references for this, just the farmers I have spoken to, so take this with a grain of salt. The massive mono-crop farming apparently have the draw-back of not following some of the recycling processes that old school, small farm farmers did to keep the soil rich and diverse in nutrients. i.e. rotating out crops, bringing in animals to eat old crops, flowers, provide diverse waste product as fertilizer. Now it is all just bags of specific minerals added back in by machine, based on the bare minimum that they are required to by people that test the soil. Depending on who I talk to, I hear that there may be less than 60 harvests left before crops basically have little nutritional value. I've also heard that this is basically already happening, in that, crops have less nutritional value than they did say 50 years ago. But again, I don't have any references so this could all be nonsense.