I'm fixin' to fix some nitrogen.

We know that plants in the legume family (i.e. black locust & soybeans) covert atmospheric nitrogen into a form that is available for root absorption in the soil. However it's not the plant fixing nitrogen, but bacteria known as rhizobia that form a symbiotic host relationship with the plant doing the work.

The nitrogen cycle borrowed from: http://kamuspengetahuan.blogspot.com/2011/08/daur-siklus-nitrogen.html

Similar in away to mycorrizal fungi, the plant provides sugars to the bacteria, and in return gains benefits from increased uptake of nitrogen. This relationship is only formed on sites that are deficient in available nitrogen. Root hairs are prompted to grow around the bacteria forming characteristic nodules.

But wait, there's more. Less thought about Frankia, a species of actinobacteria, form the same symbiotic relationship with 24 genera of actinorhizal plants.  Trees in this group are varied, and include Alnus (sample species: red alder), Myrica (sample species: wax myrtle), and the often loathed Eleagnus (sample species: autumn olive).  This plant bacteria relationship is a major source of nitrogen fixation world wide, and many if these species are commonly found in our landscapes.

Root Nodules

To see a list of actinorhizal plants check out this site: http://web.uconn.edu/mcbstaff/benson/Frankia/PlantBiology.htm .  If you find these guys, do some digging around, and see if you can find the unmistakable nodules.  Because the list of actinorhizal plants is quite varied, and includes several species of ornamentals, incorporating these plants in to our landscape plans can be a natural and sustainable why to get available nitrogen into our soils.