The general answer is that wool is much more agreeable to the felting process than the plant fibers are. Wool is easier to work with and it's fibers just want to cling together and are more easily manipulated. Does that mean you can't felt Bamboo or Soy Silk? Well, no. It can be done. You can blend them with wool to felt them. Still, will they felt on their own? I have sold both Bamboo and Soy to felters on Etsy, but the thing is, I can't really speak from personal experience so here is my plan.
Over this weekend, I will take a sample of Merino, Soy Silk and Bamboo rovings and try to felt them two different ways. First I will take a pencil of each roving and loosely crochet a chain and then try to wet felt the samples. I will also try needle felting all three. I'll take pictures along the way and share my results next week.
While I work on my experiment, I'd love to hear comments from anyone who has felted or tried to felt Bamboo or Soy Silk. That way we can all learn from our shared experience. I did a little research before writing this post, and I really didn't find much. This is our chance to help each other and put something out in the blogosphere for rising young felters everywhere!