Question

why is alluvial soil fertile?

Answer 1

Alluvial soil is fertile because of enormous amount of minerals present in it.
These minerals are formed by the sedimentation process of rivers which contains minerals in it.

Answer 2

Alluvial fertile is fertile because of the presence of minerals in it.It is a fertile loamy soil.When the river falls from miuntains,they bring silt and clay particles with them.
These are deposited in the river and valleys and flood plains.This is how alluvial soil  is formed.It is found in Punjab,Uttar Pradesh,Assam etc.(In India.)