TommyMacLuckie

The reason you don't dig a hole and replace it with other soils is because when the roots of said plant reach the native/existing soil they stop.

You can blend in organic matter with a tiller as deep as you wish but you're best bet is to plant in the existing soil with a fine pine bark mix tilled in and just fertilize and mulch them well.

Or remove 18 inches down 10 feet across by however long and replace with all new soil and enough of it that it's mounded up quite high rather than simply making it replace exactly what you removed.