Which is why you have a bootstrapping problem bringing back the forest. The soil is only poor because there is such a vibrant ecosystem that nutrients are constantly getting absorbed into living material as fast as they can be retrieved from dead material. The vibrancy consumes everything rapidly. But if you destroy the forest, you destroy the stored nutrients.

But as to raw material, surprisingly little of the physical bulk of trees actually comes from soil. It comes from the air! This is where the Carbon in the CO2 -> O2 process comes into play.