My brother really wanted to know why Toph couldn't just bend the earth beneath the wooden cell in episode 307, The Runaway. I, of course, had to argue that.
I do not remember a single point in the series where Toph was able to bend earth without being in contact with earth. This isn't to say she can't eliminate flying rocks. I mean, if she is not in contact with any earth, she cannot bend anything. So, in this case, she was only touching wood. There was no substantial earth around her to bend.
What do you guys think?