I've always considered nitrogen fixing plants such as legumes to be a good idea. However, recent reading suggests that the case is a bit more complicated. A recent thread here suggests that pinto peanuts planted to fix nitrogen are becoming an invasive pest. Behavior of the nitrogen fixing plants in the local environment should be an issue in choosing the appropriate plants. A research paper suggests that a long term influence of Lupin in an untended environment lessens biodiversity (https://scholar.google.com.mx/schol...=0ahUKEwj0urz8mZDTAhVhslQKHRTMDRoQgAMIGigBMAA). I'm not suggesting nitrogen fixing plants are always bad, simply attempting to provide more information to assist informed choices. Mixing a nitrogen fixing crop with other crops introduces competition for resources other than nitrogen and may do more harm than good in some circumstances. Rhizobium fixation depends on many factors including host species, temperature and existing nitrogen content in the soil. A well tended and fertilized garden may already have the necessary nitrogen available and it is recognised that plants preferably uptake nitrogen from soil rather than rhizobium (https://overton.tamu.edu/faculty-st...eason-legumes/nitrogen-fixation/#.WOZjWnpMFJ8).