Lupin, a nodulating legume -- Creative Commons photo by Flickr user aussiegall
Nitrogen seems like a strange element to have problems acquiring. After all, it’s by far the most abundant element in the atmosphere -- about two thousand times as abundant as CO2 -- and, on average, plants only need one nitrogen atom per 30 carbon atoms. Yet, as any gardener will tell you, nitrogen is also one of the most important limiting resources for plants, and is the sole reason we have a lucrative international trade in bird crap. Despite being bathed in nitrogen for their entire lives, plants lack the metabolic machinery to break the strong triple bond between the two nitrogen atoms. They need someone else do the hard work for them.