This is a wonderful example of the way an ecosystem works as a community, not just of plants but of all types, and sizes, of organisms. A true permaculturepolyculture!
Some trees growing in nutrient-poor forest soil may get what they need by cultivating specific root microbes to create compounds they require. These microbes are exceptionally efficient at turning inorganic minerals into nutrients that the trees can use.
Full article, from the July 2010 issue of the journal Applied and Environmental Microbiology here.