We're renovating (mostly just filling in low spots) a soccer field and have two batches of dirt to work with - one that's a typical midwestern clay, not bad but not great and another batch thats a lot better - browner and doesn't harden when dry or stay balled up when wet. We've used both by themselves already in our first go round, the new grass planted in the brown stuff seemed to do a lot better. Does it make sense to mix them so there's some of the good stuff twice as deep? I figured just a 50-50 mix and the only method would be to mix it in place. i.e. if we need 4 wheelbarrows to fill in an area, bring over two of each and use our trusty tiller. Or just use the clay as a base and use the good stuff on top?