Question

why do we consider khadar as a most fertile part of northern plains ​

Answer 1

The Northern Plain was formed by the deposits brought in by the three major rivers and their tributaries. Alluvium was deposited at the foot–hills of the Himalayas for millions of years. These deposits are now the fertile Northern Plains. Alluvium is soil or sediments deposited by a river or other running water.

Khadar soil consists of new alluvial deposits and is often very fertile. Khadar plains are low-lying next to a river. Khadar areas are prone to flooding and sometimes include portions of former river-beds that became available for agriculture when a river changed course.