Agriculturally, rain forests are a huge negative because of their extremely thin soils. In addition to being much more acidic than normal soils, the nutrients that normally form the soil base are recycled very quickly. As biotic organisms decay in a rain forest, they do so quickly that a process that in relative terms takes a long time is shortened drastically. Part of the reason for this is the abundance of decomposers, such as termites and fungi. As soon as an animal or vegetation dies, the decomposers start their work immediately. All of the nutrients are broken down and processed, and returned to living plants, thus stripping the soil of these nutrients. The thin soil is especially bad for tall trees, as they are often shallow rooted. To counteract this predicament, they spread their roots out in order to give themselves a much wider and more stabilized base.
http://www.public-domain-image.com/cache/flora-plants-public-domain-images-pictures/trees-public-domain-images-pictures/roots-of-big-old-tree_w725_h544.jpg