Depends on what you're growing and how much is "fixed". I think you would want a higher nitrogen soil for grasses, but most vegetables you would want less. Some trees also need less nitrogen, because they become more prone to diseases and pests despite the additional growth.

Nitrogen fixation refers to the microbial process of converting gaseous atmospheric nitrogen into plant usable forms. Only a few microorganisms can do this. The most effective grow in association with the roots of legumes (beans, peas, etc.). People have used legume/bacteria associations for thousands of years to supply nitrogen to crop plants and this process still provides about a quarter of the nitrogen used by crop plants in the US.

Nitrogen is required by all plants, and nitrogen fixation is how NEARLY ALL of it gets into soil systems (a small amount comes down in rainfall). The alternative to microbial biological nitrogen fixation is an industrial process called the Haber-Bosch process that does the same thing. This is where nitrogen in fertilizers comes from.

'Is nitrogen fixed in the soil good or bad?'

It's a good thing.

All plants, as far as I can think, need nitrogen. Most of them cannot use the stuff that is in the air.

There are certain plants that can use the stuff in the air, and these are legumes. Beans, for example, peas, wattles (Acacia), lucerne (alfalfa) - anything that produces a pod-like fruit.

They are not working alone. There are greeblies in the soil (various types of bacteria) which help this happen.

It is a lovely balanced system where legumes help the non-legumes.

Nitrogen is also available to plants from rain. Rain picks up nitrogen in the atmosphere along the way down.That is why after rain, things are green, whereas after a sprinkler session, even one that lasts an aeon, it isn't the same.

(There is 70% nitrogen in the atmosphere. If there was a lot more oxygen that what there is, things would spontaneously combust, which would be most inconvient for plant lovers or even plant haters, I suppose.)

I doubt that a whole lot of nitrogen fixed in the soil would harm anything. When we apply nitrogen, that's a different thing, as humans tend to be so heavy-handed with anything. You know the feeling, if a little is a little good then a whole lot must surely be great. But that's when you get a lot of growth at the expense of the fruit. And, these forced plants are probably weak and more susceptible to insect attack.

In any case, the only 'fixed nitrogen' is the stuff for which the bacteria and legumes are responsible. I've raved on a bit.

Plants that "fix" nitrogen" in soil are good and all legumes take the Nitrogen in air and store it in soils provided the proper bacteria are in the soil to assist that operation and is why an inoculant is usually necessary when preparing these legumes for planting.

http://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/nitrogen-nodules-and-nitrogen-fixing-plants.htm

kimmq is kimmsr