Doesn't that sound nasty? Here's an infected root on one of one my fava plants.
All the plants were infected, you can see clusters of nodules on a lot of the roots of the plants that I just removed today. Many of the nodules remained in the soil when I pulled the plants out.
I have to say that I was really pleased to see all those nodules. Happy to see infected plant roots?!! Absolutely, I intentionally infected the plants myself. It's a bacteria that has a symbiotic relationship with its host plant. The bacteria invades the plant roots and creates those nodules. The rhizobium within the nodules fix nitrogen from the atmosphere into a form that can be used by the plant. The plant in return supplies the bacteria with food and oxygen. The bacteria usually fix more nitrogen than the host plant can use and the excess remains in the soil and can be used by plants that are grown in that area after the host plant dies or is removed.
Most legumes have symbiotic relationships with nitrogen fixing rhizobium. Each type of legume hosts a different strain of the bacteria, so fava beans (Vicia faba) host a different strain than do peas (Pisum sativum), and soybeans (Glycine max) and green beans (Phaseolus vulgaris) each host their own strain as well. Other familiar legumes that host rhizobium are clover, alfalfa, lupines, and peanuts. I've even pulled up weedy burr clover that has had root nodules. The rhizobium dwell in the soil and can detect and infect their appropriate host when it starts to grow. If you inoculated your beans or peas when they were sown in a particular location in the past than you shouldn't have to inoculate them again when you plant them in the same area, the bacteria should still be present in the soil. I've used an inoculant each time I've sown legumes in my garden since I've been rotating them around the garden and I haven't planted each type in each bed yet. Once I've rotated each type of legume through each bed then I shouldn't have to inoculate my seeds anymore, but it wouldn't hurt if I did.
If you've not been happy with the performance of the legumes that you've been growing in your garden you might be able to improve the productivity of your next crop by trying an inoculant. Inoculants can be purchased in a powdered form from various seed companies. The packaged inoculant is a living thing so it is highly perishable. Buy only what you need for one season and be sure to buy the correct strain for the legume that you are sowing. Most of the products that I've seen are a mix of strains to inoculate beans, peas, and favas, but soy rhizobium are generally not included in the mixes, you probably need to buy that separately.