Noun. 1. alluvial soil-a fine-grained fertile soil deposited by water flowing over flood plains or in river beds. alluvial deposit, alluvial sediment, alluvium, alluvion-clay or silt or gravel carried by rushing streams and deposited where the stream slows down.. alluvial soil-a fine-grained fertile soil deposited by water flowing over flood plains or in river beds. alluvial deposit, alluvial sediment, alluvium, alluvion-clay or silt or gravel carried by rushing streams and deposited where the stream slows down.