In your situation what you could do to introduce the appropriate bacteria is to find a plant (i.e. the Fava beans in your example) that is fixing nitrogen, uproot it, crush the root system and nodules into a bucket with a little bit of water. Use the resulting slurry to soak your seeds in before planting - then water in the seeds with the left over water in the bucket. That's one way of introducing the bacteria to a new sit - but it relies on you having that initial plant that already has a good association.
I've taken a slightly different approach, in that I've been (attempting) to use our native NZ nitrogen fixers as much as possible at my place (I'm in the BoP), however it is hard to get them started due to pest pressures. But by going with the NZ native species it's easier to get the right associations underway as they're already present in our soil. Plus in some cases it's helping some native species that are threatened in the wild. But I would say that overall they're not as all round useful (i.e. they're not edible) as the imports!
Apologies if you know this but below I'll brain dump what I found out about our native Nitrogen fixers etc. These are just my own notes collated from all sorts of places - so treat them as the uninformed gossip they probably are!
I split the Nitrogen fixers here in NZ into two groups (legumes and "other") :
There are four genera of native legumes here:
1. Sophora (Kowhai)
2. Carmichaelia (New Zealand Broom)
3. Clianthus (Kakabeak)
4. Montigena (Scree pea)
There's some interesting research in NZ about our native legumes and their associations, I learnt a lot from this thesis:
Including this gem:
"This study indicates that most rhizobia isolated from New Zealand native legumes are members of Mesorhizobium, and all isolates obtained from the introduced legumes studied are members of Bradyrhizobium."
The three other native non legumes Nitrogen fixing species:
1. Tainui (Pomaderris apetala) - Actinorhizal association
2. Golden Tainui (Pomaderris hamiltonii) - Actinorhizal association
3. Matagouri / Wild Irishman- (Discaria toumatou) - unknown association.
I couldn't get a hold of Scree pea or Matagouri, but have tried the others.
In the end, I've been trying to use Kakabeak as my main nitrogen fixer, mainly because it's endangered in the wild and looks nice
however I'm having a lot of trouble getting it established due to rabbits and slug/snail damage. It's not really vigorous enough to become the sort of "chop and drop" crop I'm ideally wanting. I've also put in some of the Giant flowered broom (Carmichaelia williamsii) too - which does a bit better against predators but isn't what one could call "lush"! I had some Golden Tainui too (it's also known as "Gum-diggers soap" as the flower heads produce a soapy lather!) but that died off - I must get some more in!
. I've been establishing Kakabeak from seed as the seed is widely available and much much cheaper than buying plants.
Anyway, a bit of a long rant - but don't ignore NZ's own native nitrogen fixers - they may not have the all the benefits of the imports (notably edibility) but they're well worth a look at on their own.