Why are alluvial soils very fertile?

Alluvial soils are very fertile because of following reasons:

1. It is rich in minerals like iron, magnesia, alumina, potash and lime. It is also rich in organic matter and humus.
2. It is loamy.
3. It has sufficient depth.
4. It is renewed annually / replenished by floods.
5. It is transported soil which brings along lots of minerals, alluvium and silt.
6. It is non-porous and water retentive if all the minerals are present.