It would make sense. I grew up in the desert and you can't plant anything in sand. Well, you can try to plant it, but, .......
Not just Dirt. Many soil-like materials can be tilled, last I checked. Edit - You can also till the following materials (aside from Dirt): Ash Brains Clay Cloud Block Corrupt Dirt Dry Dirt Mud Spider Silk Block Wet Dirt However, unlike Dirt, when tilled these blocks will always start off with "wet" tilled soil, an oversight for at least two of them (Ash and Dry Dirt) when both of those would probably logically be lacking moisture.
