I think there is nitrogen in all parts of the bean plant because there is in all green plants. Eating the beans will remove some of the nitrogen but it seems a good trade-off to me.

I like your idea of using grocery store beans as seed for the cover crop. The plants will provide biomass as well as nitrogen. Consider fava beans for a winter cover crop.

Jim

Harvest of the beans will typically remove 10-16% of available nitrogen. The residue and roots contribute at least 10lb/acre after a seed harvest.

Available inoculation bacteria and the growing conditions influence fixation. Read the pdf linked below.

Here is a link that might be useful: Nitrogen Fixation info

Although it is only one consideration in choosing a bean, different kinds of beans have different nitrogen fixing ability. Among the best are cowpeas (e.g. black eye peas) and soy beans. Common beans (e.g. pinto beans) are among the less effective nitrogen fixers.

Jim

I have read that the best time to turn the cover crop under (legumes) is just as the plants begin to bloom, as once they bloom, the plants begin to convert the nitrogen they stored in the roots into flowers and fruit. I observed this summer that cowpeas that I had harvested and were turning into the soil did not have the little round nodules on the roots that the ones I turned in before flowering occurred. If, indeed, the nitrogen is in those nodules, it would appear that what I read is true.

how did you go about doing this? How did you prep your soil, how far apart did you plant? Any info would help. Thinking of trying this... thanks, Marv