Q." The first question that came up was: Which area does a nitrogen fixing plant actually affect? For example: an alfarrobeira (carob) tree. Will it fix nitrogen only on the area of its roots? Or a bit more? What about bushes or plants like fava beans and clover? The other questions were: -do non evergreen nitrogen fixing plants fix nitrogen all year? Or only when they leaf? -Should we put clover around our trees and bushes? -can fava beans be used as a perennial N fixer? -should we remove the weeds around the trees and bushes?" By Constanca
A.Now to try and briefly answer your questions, which isn't the simplest to answer definitively. In my opinion, from what I know all nitrogen fixing plants achieve their fixation in different ways. Some mostly through their roots e.g acacia; some mostly through their pods e.g. carob; and some still through both their root nodules and N rich leaves and pods e.g. fava beans. As a general rule, with those that fix through their roots it is better to cut them back before podding out but not when it is too dry. With those that mostly fix through dropping of pods (like carob), it is better just to gather this N rich compost from around the base of these trees to use elsewhere in the garden, otherwise it will mostly only fix N for itself. There are many other N fixers all with their own habits and unique microrizonal bacteria that is associated to each. But don't over think it, just plant lots of them at the beginning to make way and support other more productive plants. In the future, you can phase many of them out, as indeed they will naturally do so as the top soil becomes deeper, richer and more full of diverse life, creating niches for other less hardy/ more hungry species. Remember a forest only grows off of a fallen forest. N. fixers, fast bio mass producers and other dynamic nutrient accumulator plants are all the sources of that fallen forest.