Not only have I planted alliums and legumes next to each other in the same bed many times over the years but I have also succession planted bush beans after a garlic or shallot harvest and have never noticed any lack of production from the beans.

Companion planting guides are pretty much useless, in my experience. As long as two crops don't compete for rootspace, water, nutrients, or sun then I've found you can pretty much plant any crop next to another.

Rodney

dan's comment is 100% correct. Alliums - especially garlic - emit a chemical that interferes with the bacteria necessary for legumes to fix nitrogen. Legumes tend to grow stunted and less productive if in very close proximity (eg, side by side). Nearby is fine - immediately adjacent is not.

Not all companion planting advice is scientifically sound.....this is!!

https://www.gardenweb.com/discussions/6320567/building-a-garden-what-should-garlic-not-go-next-to#n=1

Alliums do produce allicin and that allicin can have a negative effect on the rhizobia bacteria that legumes use to fix notrogen. That is a scientific fact. But how much allicin is actually needed to cause a problem in a garden setting, and do growing (or decaying) allium roots produce enough during a season to affect legumes planted closeby? Grown in a dead soil, grown together in a container, or if the allium crop is heavily stressed from environmental conditions or pest and disease (which would increase allicin production) there very well could be significant issues for the legumes. However growing the two together in a healthy soil with a good amount of organic matter you're not likely to have a problem. Like I said, I have grown the two crops directly next to each other many times and have succession planted legumes after allium with no noticeable stunting or drop in productivity.

Rodney

Rodney makes a fair point. It's commonly said that alliums interfere with nitrogen-fixing bacteria. but curiously, I can find NO REFERENCE to that in extension literature or research literature in terms of crop productivity. But maybe someone knows more? If this was considered a real ag problem, the professional ag literature would be all over it. There have been research articles pointing out that allicin doesn't get along well with nitrogen-fixing bacteria, but it isn't entirely clear that allicin-containing plants seriously pollute the surrounding soil with allicin. No, you probably shouldn't grind up onions and garlic and throw it on legumes, though allicin biodegrades pretty quickly, so allium compost probably won't hurt anything. This may have gotten a bit mythologized and extrapolated.

That being said, there is a wealth of at least research articles that make the case that legumes are somewhat allelopathic.

Let me add an interesting fact to all this. Turns out that allicin doesn't just interfere with nitrogenating bacteria in legumes, but it is a well-understood urease inhibitor. Urease is the microbially produced enzyme that that breaks down urea into ammonia and CO2, nitrogenating the soil. So urea and manure-based fertilizers are not going to effectively nitrogenate soil if allicin is around. Then again, as noted, just having onions and garlic growing doesn't necessarily pollute the soil with allicin. Also, allicin biodegrades pretty rapidly, so it just means that urea-based fertilizer may not work well immediately if you've ground up some onions and garlic and spread it around.