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That depends on what you're doing and what kind of yield you're expecting. It's not going to completely replace the huge quantities of fertilizer we apply to conventional fields, but it helps. That's part of the reason we do crop rotation - beans/legumes fix nitrogen as well.

Not at commercial scale, but it helps. You can try it yourself in your lawn or garden by planting clover.

No. Potassium and phosphorus are two other macro nutrients. There are also some micro ones. (Going on the basis of what I read long ago or learned in school, maybe there is more current science now.)

But all the nutrients can be got from organic manure and compost.