Oxboy555, "Supposedly these plants can 'fix' nitrogen out of the atmosphere and don't require fertilizer that contains nitrogen."
As I understand it, there is a symbiotic bacterium which fixes the nitrogen in the soil atmosphere and makes it available to the legume; in return for carbohydrates from the legume.
The amount of nitrogen so provided will depend upon the quality of the soil atmosphere, the quantity and distribution of the bacterium and even some environmental conditions.
In the commercial production of black-eye peas, we found it necessary to coat the seed material with the bacterium and provide nitrogen in the fertilizer application.