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    The One Where the Nitrogen is Fixed

                    You probably read our post about cool season legumes and how they can fix you nitrogen, but how do legumes actually give you a nitrogen credit? Is it their leaves? Roots? Magic? Kind of! It is actually a symbiotic relationship that legumes can form with rhizobia in the soil. Let see how that works…

    It starts with the air -

          The air we breathe is around 78% nitrogen. Wouldn’t you save a lot on fertilizer costs if plants could use that atmospheric nitrogen? The reason they can’t is atmospheric nitrogen is tightly connected by a triple bond that can only be disconnected by high pressure and heat or... Bacteria! 

    u.2.InkedAtmospheric N_LI.jpg 

          Remember those rhizobia that I mentioned? Those are the happy little bacteria that legumes allow to attach to their roots and make a trade (see below). The deal is, the rhizobia take nitrogen from the air in the soil and break it apart to make it useable for the plant to uptake into leaves and stems. In exchange, the plant provides the other means of life that the rhizobia need. Pretty good deal, right?

    u.2.Rhizobia Infection.JPG 

    When the deal is done -

          So, your rhizobia have attached and your legume is full of nitrogen. It is a common misconception that at this point your legumes are directly feeding your grasses and broadleaves that may be growing nearby. Unfortunately, the amount of this that happens is very small. I repeat, legumes do not feed your grasses a large amount in season. That being said, they can feed a grass that is planted after the legume is terminated, but Beyond Agronomy will get into that in the next post; Come back on Thursday for The One where the Nitrogen is Released to see how this atmospheric nitrogen truly gets back into you soil!

    Visuals created by Associate Professor Julie Grossman and her team of grad students at the University of Minnesota Department of Horticulture Science.