Question

what are harmful effects of using excessive fertilizers?

Answer 1

Excessive fertilizers causes the process of biological magnification.

Answer 2

Excessive use of fertilizers decrease the fertility of the soil. It kills all the insects that create the soil fertile. These helps to grow the crops but do not sustain the ferlility of the soil. They remain in the soil for decades of years.
They are drained into waterbodies and destoy the equatic life.