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.