I have a special interest in this since I recycle my gray water for my garden. Out of all the soaps I've seen on the market, it seems a homemade one, from natural oils that would biodegrade well on their own, would be safest. There are no petrollates and no carcinogens like manufactured "soaps" have. I found this interesting piece on the chemicals found in a lot of our beauty products.
http://blogs.dickinson.edu/ecofeminism/tag/softsoap/
Oils that biodegrade anyway, plus an alkaline product that is close to what we find in nature, chemically changed to a salt. I think it's better than most things you could buy. I'm not going to recycle the water for my dogs, but I feel it's fine to use in moderation around the plants.
Please correct me if I'm wrong.