I've just finished reading the book Miracle Apples where Akinori Kimura goes through his decade long struggle to learn many of the things we have handed to us on a silver platter by the likes of Mollison and Holzer.  And I should add Hemenway, Jacke and Toensmeier to that list as their books let us skip past a lot of the painful learning process.

Anyway, something Kimura commented on regarding nitrogen fixation was new to me.  I don't recall hearing about it in any of the Permaculture stuff I've read thus far.  Kimura uses soybeans for his N-fixing needs (not surprising in Japan) and observed that when he first started doing this he had many many colonies of bacteria on the roots fixing nitrogen, but over the course of 5 years these diminished to the point of not having any.  He observes the nodules on the roots to know when more nitrogen is needed and he plants more or fewer soybeans as required.

Now this rings true because we know that things done naturally will find the right balance.  But I've never even considered having 'too much' nitrogen.  Always not enough nitrogen: need N-fixers.  I haven't been growing things long enough to observe this trend of bacterial nodules myself (I'm in my 2nd season, so I've been happy to simply recognize the nodules at this point!).  So what would happen if there was too much nitrogen in the soil?  Any effect on the plants or visible signs?  (read: why would nature limit the amount of nitrogen?)  Also, has anyone observed this similar cycle of N-fixing bacteria?

Too much nitrogen can cause overabundance of soft leafy growth which attracts insects.  Nitrates can build up in plants under stress and make them toxic to people and other animals.

Too much nitrogen (or a lack of P and K can cause a plant to remain in the 'vegative' state too long.  When this happens, frutation and seed production can be delayed to the point that early frosts can shut down the plant before their reproductive systems can function.  No fruit.  No seed.  Just lots of lush, green leaves.  To an annual plant, this means the end of the plants existence.  To a perennial, it means a lost season of production.

I had that problem with some container radishes I was trying this year.  I didn't use any fertilizer, but a nitrogen rich compost.  My radishes produced terrific greens, and no roots before they started going to seed.  The ones in the ground, without the compost came up right on time.  I discovered it too late for me to do anything about it, but there you are.

H Ludi Tyler wrote:
Too much nitrogen can cause overabundance of soft leafy growth which attracts insects.

That sounds like a very effective mechanism for nature to dispose of excess nitrogen in the soil.  Another example of things tending towards the ideal if we don't get in the way.

since you used the word fixation and soybeans in your question it makes me think of the legumes ability to extract nitrogen from the air and this is where legumes grow these tiny potato like root nodules that store the atmospheric
collected nitrogen.

when i am about to plant say sugar snap peas that have been pre soaked to HALF their swelling size
i add the black powder innoculant and shake them up (like shake and bake) and then plant them assurring
that the peas are coated and  have adequate bacteria to affix nitrogen from the atmosphere.

and by the way:  YES, i spell certain words "that i coin"  any way i freaken please  )