Alluvial Soil
This is a soil that has been deposited by water. If, at some time in the past, an area of Eath was covered with water due to a prior river of flood, that water would deposit sediment that remains once the water is gone. This sediment is known as alluvium, and is known to be very agriculturally productive.
Why I Looked It Up
From On Paper: The Everything of Its Two-Thousand-Year History:
In the absence of building stones and forests, the rich alluvial soil became vital to their quality of life.