Harmful to the soil or garden is a little hard to understand from a permaculture perspective. Poisonous to humans is quite understandable, and there are many. A few are even deadly. Harmful to the ecology doesn't make sense when you think about the overall balance of nature, as Rachel Carson would say.
I love a good mentalist. And so does this tiny ad:
Eat Your Dirt - a free online summit with more than 30 speakers