I work in the fertilizer industry directly with Farmers. I can say that over the last 10 years the usage of phosphate based fertilizers has dropped every year. Phosphourous bonds very tightly to the soil particles in the ground. The only way for it to run off is for the dirt to run off. In the spring of the year with heavy rains this is typically a problem...look at the satelite images in april and may. By June, the rains are sporadic enough that there is very little runoff out of the fields. When you look at the tile drains in ditches during june july august rains they predominantly run fairly clear. Exceptions are the abnormally heavy rains we have had this past september. Also in the last 10 years the amount of acres that have had drainage done to them has shot through the roof. This fact is contributing more to the issues with the Lake Erie watershed than any other single factor. Farmers see immediate economic benefits from tiling, and right now farming is a very profitable business and most are doing whatever they need to increase the productivity of their ground.
I am not saying that other areas cause all the problems with the lake, but Farmers are not needlessly overapplying fertilizers. Commercial phosphate fetilizer has become increasingly costly and trust me the farmers watch their pennies and spend their money in areas of their operations that benefit them the most. Drainage is the number one area of a farm that will make money for a farmer.
I didn't mean to stir up a hornet nest here, but wanted to put an opinion in from the ag industry. Trust me I love to fish way more than I like to work....so I am for protecting the lake in practical manners.