Broad Beans

Mine have just finished.  I think I read somewhere to leave the bottom bit in the ground as it helps to fix nitrogen.  Is this true or am I thinking of something else??

Thanks

Yes this is correct.

Somebody correct me if I'm wrong but you can put the tops on your compost heap as these don't fix nitrogen. However digging in the roots will provide a source of nitrogen to your next crop.

Both peas and beans have nitrogen-fixing nodules that can be dug into the soil for next year's nitrogen-hungry brassicas.

Broad beans are a legume so they do fix nitrogen, and I've often wondered just how much the surrounding soil benefits from it.

This article (a PDF file) suggests that the answer is "not much" because most of it is used by the plant itself (if the plant is actually well fed it apparently shuts down the nitrogen-fixing nodules because they absorb energy from the plant which prefers an easy life - how clever is that?!)
This is an extract from the above link if you don't want to read it all:

The stalks, leaves and roots of grain legumes, such as soybeans and beans contain about the same concentration of nitrogen as found in non-legume crop residue. In fact, the residue from a corn crop contains more nitrogen than the residue from a bean crop, simply because the corn crop has more residue.
 
A perennial or forage legume crop only adds significant nitrogen for the following crop if the entire biomass (stems, leaves, roots) is incorporated into the soil. If a forage is cut and removed from the field, most of the nitrogen fixed by the forage is removed. Roots and crowns add little soil nitrogen compared with the aboveground biomass.

My runner beans will still go on the compost heap though, roots'n'all!