There are many studies showing that the amount of nutrients is soil/plants are decreasing - and soil depletion isn't something new. Here's a link to an article written in 1936 discussing the same problem.

 

A little excerpt:

 

Quote: The alarming fact is that foods--fruits and vegetables and grains--now being raised on millions of acres of land that no longer contains enough of certain needed minerals, are starving us--no matter how much of them we eat! This talk about minerals is novel and quite startling. In fact, a realization of the importance of minerals in food is so new that the textbooks on nutritional dietetics contain very little about it. Nevertheless, it is something that concerns all of us, and the further we delve into it the more startling it becomes. You would think, wouldn’t you; that a carrot is a carrot--that one is about as good as another as far as nourishment is concerned? But it isn’t; one carrot may look and taste like another and yet be lacking in the particular mineral element which our system requires and which carrots are supposed to contain. Laboratory tests prove that the fruits, the vegetables, the grains, the eggs and even the milk and the meats of today are not what they were a few generations ago.