Bacon,
Despite me tending to agree with you that farmers are cost conscious, the simple fact is that fertilizer is well over applied even in the rural areas since no farmer wants to think his crop has not reached its full potential because it was nutrient limited. Hence the tendency for it to find its way into the rivers and lakes not near a town. Application of fertilizer in the fall (in particular) is a great way for it to lie and wait for the spring runoff where it gets mobilized (also not saying you do this, but a heck of a lot of farmers do). About the only way to accurately put the correct amount of fertilizer on a growing crop is to do it through fertigation or some other form of incremental application. Putting it on when the crops haven't even been planted is a surefire recipe for enhancing the phosphorous and nitrogen in the surface waters.
There is no easy answer to all the problems of eutrophication, city dwelling grass farmers don't think they're doing harm with the small amounts they individually put on, nor do the ag producers who can't afford to not maximize yield. But overall, we suck at taking care of this part of the environment!
