http://www.scientificamerican.com/article.cfm?id=soil-depletion-and-nutrition-loss

The link talks about the decline in the nutrients in veg/fruits.
Mostly due being breed to grow faster and bigger, which doesn't allow them to get the minerals in as great a quantity.
If you want to feed 300 million people something has to give.
It would be great if we all could grow our own food.