Or is it just soil moved around? And how deep is the soil? I can see from pics it is deep enough to grow grass. Were crops on it?
Edit: side note: there was a comment here, but it disappeared. Was it anecdotal? It might have been, from the line it showed in my notifs. I would not mind even anecdotal answers. Make of that what you will.
Apparently they took the much harder route presumably they were in a hurry. Considering what it was being built for, they had no time to gradually improve the soil (as is done when people are trying to open up new land for agriculture).
“The builders used the best techniques known to them to make terraces that would last for eternity. Then the workers added layers of rock and clay as subsoil, and from the river below them they hauled up rich dirt over steep embankments half a mile deep. This task would be the equivalent of hauling dirt from the Colorado River to plant fields on top of the Grand Canyon”
Extract from Indian Givers, Chapter 5, by Jack Weatherford