Where do you get the idea that they are over fertilizing? The vast majority of them are on slim margins and watch everything like a hawk so I find it unlikely, given the costs, and their equipment that they are rampantly over fertilizing.
Not to mention the fact that over fertilizing will often reduce crop yields. The idea that the runoff of fertilizer products into groundwater, streams, lakes and oceans must be the result of over fertilization is false. It's the result of gravity.