Your comment intrigued me, so I looked it up--I found it pretty interesting, so thanks for the inspiration.

Looks like one of the main factors is due to the soil acidity being similar to the pH of the plant roots. Also, while there is a ton of dead organic material in the soil, the high temperatures and precipitation causes it to decompose so rapidly that most of the nutrients are lost.

Finally, the clay particles in the soil are poor for trapping the minimal nutrients remaining from the decomposing material, and what is trapped is frequently washed away from the precipitation.

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.