Hi Den, you may want to link your question to the legume forum too (below). I've never pruned my pole beans but I would assume that topping them at 6" would force side shoots to make them bushier. But the bean brains would know best ;-)

https://www.houzz.com/discussions/beans-peas-and-legumes

Red kidney beans should be bush habit. They should not require pruning unless they are grown in pots & become too large for the space. If I may ask, what is the nature of this project? Is it related to a course, or some type of research? Is a rhizobial inoculant being used?

Yeah, I am doing this for a course. I do need to use a rhizobial inoculant. Is this the same as a mycorrhizal inoculant?

Rhizobial inoculant consists of Rhizobium bacteria, mycorrhizal inoculant consists of mycorrhizal fungi.

Ditto on the explanation by @windberry zone5a BCCanada. The rhizobial inoculant provides the symbiotic bacteria that the beans need to form nitrogen-fixing nodules. However, the nodules will be few (or none) if sufficient N for the plant's needs is already present in the soil. Beans are not the best at nodule formation either, compared to some other legumes (such as clover, and cowpeas). You would probably get the most observable results if the beans are grown in sand, which would also be easy to remove to expose the root structure. What is the goal of the project?

Goal of the project is to grow a legume (I picked beans), and focus on the roots’ nitrogen fixating nodules. I need to use a rhizobium inoculant however I cannot find it anywhere as it is a seasonal product at every garden center i called, so will have to ask the instructor on what to do about that. Thank you for all the info, learned some new things thanks to you and the sand recommendation will be of much help later when im taking the roots apart for inspection.

Plants need not only nitrogen in a soluble form to grow. They need other nutrients as well. I believe there are not much nutrients in sand.

BTW it is not nodules fixing atmospheric nitrogen for plants to use, but the Rhizobium bacteria living inside these nodules.

Rhizobia from Amazon

some nurseries may have it about now, it is seasonal now as it is seed buying season.

Garden inoculants are often sold in combinations to cover the most commonly-grown species. I realize time may be a factor, but if you are unable to find inoculant locally, a couple mail-order sources are linked below.

True-leaf market inoculant

Johnny's inoculant

True Leaf shipped an order to me in 4 days; Johnny's is experiencing delays. There are other sources too, which might be nearer, depending upon your location.

"Plants need not only nitrogen in a soluble form to grow. They need other nutrients as well. I believe there are not much nutrients in sand."

Which goes without saying. The fact that sand is almost empty of nutrients means that liquid feeding would be required. It also means that the grower has absolute control over what nutrients are - or are not - being added. I suggested sand because nodules can be broken loose if handled roughly (especially with beans) and sand is easy to rinse off with minimal damage to root structure. I have performed this experiment myself both with sand, and with soil-less seed starter. For extensive root systems, the sand is much easier to remove.

I have purchased inoculant from the Burpee display in early spring at Tractor Supply. It may also be at Home Depot.

It's not entirely clear what it has to do with the project, but the OP asked about how to make pole beans "bushier". No responses to that yet. The idea that topping them, like for peppers, might make them fill out more is interesting. I think I've vaguely heard of that - as in "pole bean pinching". Does it really work? Does it increase yield? There is some disagreement about whether it increases yield for peppers.

When deer have 'topped' my pole beans it didn't make them grow bushier, it just made them not grow.

Pole beans need a pretty long season before producing (~65-75 days) and any set back early in the growing season, especially in short season climates, can be the difference between a good harvest and little to none at all.

That being said, the OP seems more interested in growing the root nodules and not the beans themselves.

I'd go with Zeedman's suggestion to grow in sand and use an inoculant. He has grown more different bean varieties over the years then I ever even knew existed.

I've not used an inoculant on pole beans and they still grow and produce like crazy. Just sayin'...

The bacteria that the legume innoculant innoculates into the soil is often already there. Rhizobium is very common in soil. You innoculate your soil with it just in case, for some reason, your soil doesn't have any. If you have it, you're good to go. It would certainly save some trouble if it were easy to test for it.

Most soils do have the rhizobium already especially if you have grown legumes in it previously. Sterilized sand would not have any though. I've used inoculant a few times and not used it many other times and really didn't notice a difference in the root nodules at the end of the season. That is no noticeable nodules either way. But maybe they break off when I pull the plants out like Zeedman mentioned upthread.