Soil
Tropical rainforests have the most biodiversity, out of any biome. Biodiversity is what every ecosystem thrives for, but when it comes to soil it's not the best thing. Since rainforests have such high biodiversity when seeds, vegetation, or animals die instead of decaying into the soil, there are organisms that will eat all decaying matter. Since nothing is being decayed into the soil the soil lacks nutrition, which is the main ingredient for good soil.