Question

why are northern plains considered as agriculturally productive part of india

Answer 1

because in plains very less members will do farming and even in northern plains very less members will be to do farming so the soil remains fertile and more productivity grows.

Answer 2

There are number of points responsible for that.
1. Northern plain are in the ganga basin. Good water retention properties of local soil and good water available are responsible for high fertility
2. Geological location, less wind erosion