Like bonesii said, dirt/soil - Onua's element - is/has decayed plant matter, an organic component. Stone is entirely inorganic (and dirt can, over time and through pressure and/or heat, become stone, but I can't think of any way to turn stone into dirt). The difference between Onua and Pohatu, I think, is that Pohatu could control sand. Pohatu could probably turn a pile of sand into a solid rock by basically putting it back together using his control over Stone. Onua couldn't, except maybe by squeezing it hard enough, but that requires Onua has his Pakari on so it's not part of his elemental powers.There are also more obvious things: Pohatu kicks boulders around that didn't exist a second ago, Onua has to find them first. Onua seems in tune with seismic tremors and such (the earth as in that thing beneath our feet) and can I think cause earthquakes, while Pohatu is more finding weaknesses in rocks to bust them to sand. I think Onua's Nova Blast would be something like a serious earthquake, and Pohatu's would more likely be something like rocks fall everyone dies.