Ideally you'd get a soil test done on it - call you local county ag extension office for one. That will tell you exactly what all needs to be added in the way of soil improvers, organic matter, and nutrients.

There are all sorts of soil improvers available including compost, peat, composted manures, perlite, lime, etc. but adding many of them without knowing what you are starting with can really skew things.

But whatever you decide to add needs to be well mixed in with the existing. Don't just layer new stuff on top as that can create all sorts of additional issues.

Dave

Also, 6" of space to fill is quite a lot. How large an area are we talking?