Fertilizer is much more expensive to produce than iron which is cheap and plentiful. Using runoff would be much more expensive (extremely so) as they would have to build millions of miles of piping and collection systems, evaporate it out, and transport it into the middle of the ocean.

And in the end, the effects would be minimal, since the nutrient the phytoplankton are limited by is usually iron.

You are directionally thinking of a geoengineering method called ocean fertilization. Using iron in the middle of the ocean where plankton normally couldn’t grow to stimulate blooms.