To elaborate on this, @Jon, because the elements are derived from the regions of the world and not material and phenomena that exist in nature, you don't NEED to differentiate Onua and Pohatu's powersets in this way. Onua gets things like earthquakes and lil' mountains, and perhaps crystals. Pohatu gets sand and boulders. Those are different enough in terms of practicality.
Plants aren't necessary to distinguish Onua's element at this point. Combining Earth and Plantlife is only as valid as combining Air and plantlife, which is what JtO apparently did. Plantlife belongs as its own element if it's going to be added at all, not a subset of a totally different element. Also, bear in mind that Onua's region is located in underground caves, which aren't really known for their foliage. Perhaps if he was on the surface, it'd make more sense; but he's not. He's down below, where plants don't grow a whole lot.
If you wanna add another G1 element to Onua's powerset, add Iron. Metallic ore can be found in the ground, so that gives Onua an excuse to control metal. He doesn't get to control the metal in a Matoran's body because it's not from the ground in any capacity.