How does the balsam plant and the earthworm help each other in the same habitat?

Not knowing which of the common balsam plants you are talking about, and having quickly went over the list of them in a number of resource books (I wouldn't want to miss anything, and the thought of there being a special relationship between a plant and it's companion worm would be really interesting), regardless of the plant, in any soil environment where one finds earthworms, they are a benefit, though I found no special relationship. They consume the organic materials of the soil and in the process help in breaking down the products to form humus. Their manure, which is called "castings" is a really excellent form of organic nitrogen, and so beneficial that I have spent money to purchase worms to add to my garden every time I open up a new area. Along with the castings, their tunneling also provides a means for air (oxygen) to better reach the root zone of plants, the roots needing gas exchange (O2 in, CO2 out) as much as the visible plant tops. Castings are actually so rich in nitrogen that, they have to be used with care to keep from "burning" the plant. This was shown by way of a lab demonstration back in my university daze, and I was impressed by the little worm. Aside from that, other benefits can be gained from them in that they will attract birds to a garden and they will eat bugs that prey on your crops, while searching for the elusive worm.