[MichaelMcG]

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.