When the beans are grown in the ground the roots penetrate quite a way down and help to keep the soil nitrogen rich.
But if you have grown them in pots, the best thing you can do with the soil is throw it on your compost heap. The soil or compost you have used will have had all it's nutrients drawn out of it and will be spent.
If you want a good result this year change your soil or compost for some fresh stuff.
Not long now to start sowing your beans ... about a weeks time for me !