Is easy to understand even without bibliography. Soy is an annual herb. It grows all the summer, invest everything on producing fruits in fall and then die in winter.

Beans are the crop. Everything else must wait until they are ready, and by then it will be basically dry matter. Therefore all except the beans is a residual.

Can still be recycled into more human food by herbivores, but we couldn't do it directly. We can't feed on dry stems, withered flowers or brown leaves.

I would highly recommend the book Sacred Cow. It has links to hundreds of studies within for further research. The authors, Diana rogers and Robb Wolf have been on many podcasts since it's release to discuss the topic.

If you want specific studies I can go look up in the book but it sounds like your asking for further reading, not cited sources.