posted 3 years ago
There are many way, way smarter people on here than I, but since no one has answered, I will give my simplistic understanding of this.
The inoculant will be special bacteria (so not fungi) that then colonizes the beans' roots and fixes nitrogen for the beans (or any legumes). Different kinds of legumes may partner with different strains of bacteria, so different inoculants may be used for different kinds of legumes. Most inoculants will contain multiple strains to cover multiple kinds of legumes, though, and it will tell you on the package or the website what you can use them for. The one I bought covers all the legumes I am growing, from pole beans to hairy vetch to sunn hemp.
If you have used inoculant in the past and are growing beans again in the same bed, there is no need to inoculate your beans every year, because the proper bacteria will already be there in the soil. Or even if no beans have been grown there before, you might get lucky and the correct nitrogen-fixing bacteria may just be present anyway. Someone smarter/ more educated than I could probably tell you if a healthy, living, bacteria-rich soil can replace inoculant altogether.
Beans can certainly grow without inoculant, but if they do not find that bacteria to partner with, then they won't be fixing nitrogen, and will be just taking it from the soil like any other plant.
I always inoculate legumes, because why not? Inoculant is not expensive and if I am doing all the work of preparing beds, planting seeds, etc., I may as well take that extra step to help ensure success. It's little of a hassle, but I do it anyway.