Well, I have no real basis for what I'm about to say, just vague impressions of things I've read over the years, so, I could be wrong on this.

I seem to recall, though, that 1) the host-bacteria relationship is pretty specific and specialized, and only a narrow range of Rhizobium will innoculate any one kind. This is why there are several innoculant strains sold. 2) However, I think that almost all legumes will develop at least a few of the nodules that host the bacteria. It seems that the bacteria probably travel with the seeds themselves, or on the wind, and they somehow find each other. Also, if your soil has been used in years past to grow legumes, some of the bacteria probably survives, at least for a while.

It has also been my observation that the roots of wild legumes, like vetch and clover, almost always have the nodules on the roots when I pull them up as weeds.

I do know that innoculated seeds will grow into plants that have a lot more nodules on their roots, but plants which are NOT innoculated always seem to have some anyway.

The answer above is basically correct. What is missing is that legumes have to be heavily nodulated to effectively fix nitrogen. The amounts of bacteria in soil are not normally enough unless the same type legumes have been grown in the soil for at least 2 of the preceding 5 years. In addition, some soil types retain the innoculum for shorter periods, sandy soil for example.

Common legume innoculants are prepared with a range of different bacteria so that a single package can innoculate several varieties of legumes. Look on the package to see what it works on. It is common that an innoculum that works on green beans does not work on green peas.

So as a rule of thumb, always innoculate beans and peas for best effect.

DarJones

This kind of segues into the fact that many gardeners do not grow enough variety of legumes nor often enough in the rotations. It shouldn't be a question, IMO, as to whether a garden has the necessary bacteria presuming it has been an active garden for a few years. I inoculate every 4 or 5 years, just to be safe though my various gardens must be well-infested by now.

N fixation for legumes is 100% dependent upon the proper bacteria being present, but also a significant enough population of them being present.

I guess the question is what are you looking to accomplish?

Are you looking to get the best growth possible from your current crop or are you looking for a natural way to increase N in the soil for the next crop?

I find this very interesting. I have to say that until I came onto these forums I had never heard of innoculation for legumes. Can someone explain why it is never even mentioned in UK gardening books and yet we all grow beans and peas without any problems? Are the appropriate bacteria already there in our soils? And if so why/how do we have have them and you don't? How much of the concern about innoculation do you think mighht be motivated by marketing rather than gardening reasons? What happens if you don't innoculate your legumes? At the school where I work the kids have grown runner beans and peas this year on part of a sports field which has not been gardened in living memory, and probably not ever. The plants are fantastic. Lots of questions - I don't know any of the answers.

I'd be willing to imagine that heavier chalky and clayey soils are more likely to be already containing the correct rhizobacteria. Legumes grown in very sandy soils whether in the UK or elsewhere probably would be more in need of inoculation if the soil has not had legumes ever or for some time.

Flora, the soccer field already had mixed clovers (those white flowers and tiny yellow flowers) in there, and therefore a substantial population of at least some species of bacteria. I have no idea whether clover bacteria will work with other pulses.

I have to say that until I came onto these forums I had never heard of innoculation for legumes. Can someone explain why it is never even mentioned in UK gardening books and yet we all grow beans and peas without any problems? Are the appropriate bacteria already there in our soils?

Probably not. I think in large part the idea of innoculating the soil with N fixers is marketing. Probably folks in the US buy into the idea more than folks in the UK ;)

It should be noted that there is no requirement for N fixing to occur in order for legumes to grow well. They are perfectly able to take up N via their roots the same as any other plant.

Under ideal conditions though, legumes can grow well without N being added due to their ability to use N from the air with the aid of bacteria.

I guess the way I look at it providing an N source is cheaper and more reliable than trying to get the appropriate bacteria and maintain an ample population.

After the crop is done I remove the plants by cutting them at soil level to leave the roots in the soil though. I figure whatever N fixing that may have occurred will only help the next crop.

justaguy2, glib and flora, I 2 wondered about the innoculents as I had never heard of them before I came to the USA either, I just planted my runner beans and peas in to the ground and the runner beans are in rude health in a garden area where even weeds won't grow!
I have however brought some for my fall peas, because I am hoping that the claim that it helps fix more nitrogen is true, so that I can slowly improve this area of soil with this and compost.
there a lot of differences between gardening in the UK and States, things like Tomato cages? I had heard of them but never needed more than a bamboo cane to train them up, here I am looking at these monster plants in my garden, and although my 5 foot canes are holding up, the tomato plants are about to head up over the top! I had only seen this in Greenhouses before!
And drum roll please, I have been amazed at how much stuff that I have brought for the garden here in the USA working on my budget (so therefore as cheap as it can be whilst still doing the job) as turned out to be made in England! I didn't think we made anything anymore!