Leave the roots in the soil. Also, when planting beans in heavy soil, cover the seeds with a mixture of soil and compost, or sand, so that the seeds can break through the surface.

There are several answers to this question. I suppose it depends upon what you hope to accomplish, besides the soil enrichment. Do you want snap beans, dry beans, or just to till the crop under & plant something else?

Can't find a reference for it now, although I know Jimster & I have discussed this previously... but common beans (Phaseolus vulgaris) are some of the poorest nodule-formers. They will add nitrogen when turned under, just not as much as other species. Peas (both English & Southern) will add more, and it's even possible to plant them both in one season.

You will get the best soil enrichment if you harvest peas or beans in the green stage, either snap or shell - then turn the plants under immediately, while they are still green. But you will still get nitrogen if you let the plants go to dry seed, as long as the roots are not pulled.

If food production is not a top priority, there are other cover crops that will not only add more nitrogen, but will provide a lot of organic matter to help break up the clay. Clover, vetch, and alfalfa are some good choices. You could, perhaps, follow an early crop of peas or beans with a leguminous crop which will winter over & be cut/turned under in the Spring... your local Extension could probably advise you on varieties & planting times.

zeedman thanks for sharing this information about nitrogen enrichment differences. So glad to hear about the peas being better for N enrichment, I have P. sativum growing all over the place and being harvested as pods ;-)

If you do find the reference/link I would be interested in reading about differences in N enrichment, but I will still keep growing P vulgaris of course. Sometimes though I am planting an 'in between' crop of bush legumes of some sort, and limas, soy beans and southern peas will also all work weather wise. Since I like all legumes, it is nice to know the choices from a nitrogen enriching perspective.

Fava beans are a great source of nitrogen and can be planted early in the season as they tolerate cool temperatures like peas. They make a substantial plant quickly, and many but not all like the beans, which resemble lima beans with a tough seed coat.

Here is a link that might be useful: favas as cover crop

If you have pole beans, on a E-W trellis, you can plant anything on the S side and it will grow well without any N. Even N-hungry things like brassicas.

If you have large greens, such as chard or cabbage, you can plant white clover in between plants. The plants will stay above the clover canopy. Again no N needed.

Where you planted beans last year, N-thrifty veggies like carrots will grow without fertilization. I have never tried to plant cabbage in a place where beans grew the previous year.