Question

what makes the plains so fertile?

Answer 1

The deposition of silt by the rivers on its banks make the soil more fertile . Most of the plains are formed by the rivers which carry the silt and deposits on the banks. Such plains are appropriate for agriculture. As a result these plains are quickly inhabit by the people

Answer 2

when the dirt and impure substance comes from the mountain it store that impure substance causes the formation of fertile land mainly in the bank's of river.