Today I summarized some of the reasons for the nutrient-poor soil of the tropical rainforest. The over-riding issue is the steady rainfall. As we discussed last semester, water is a super efficient solvent, and whatever nutrients reach the soil of the tropical rainforest are dissolved and leached into deeper levels of the soil where plant roots can't reach them.
Another reason for the nutrient-poor soils of the tropical rainforest: Most of the plants are evergreen. Rather than returning nutrients to the soil, they hold onto their leaves, preserving the hard-to-obtain nutrients like nitrogen and phosphorus within their own plant body. Sometimes leaves will stay on a plant for years, enough time for lichen colonies like this one to establish and thrive. You can determine through a quick visual that there are several species of lichen on this leaf, another example of rainforest diversity and biological complexity.
One of the most striking features of the rainforest floor is the fact that any organic matter that falls is quickly utilized by micro-organisms like fungi or the many insects that make use of plant material for their nutrition. Here's a picture I took in Mexico of a termite "highway." Under the protective layer termites are rushing to and from their nests with leaf material.
Perhaps most exciting of all was the chance to see the activity of leaf-cutter ants, which play a complex and vital role in the rainforest ecosystem.