Is this the same sign as 'dirt'?
Yes, this specific sign is used interchangeably for 'ground,' 'dirt,' and 'soil.' The context of your conversation will easily help the person you are signing with understand which specific English concept you mean.
Can I use this sign for the floor inside a building?
No, this sign specifically refers to the earth, soil, or outdoor ground. If you want to talk about an indoor floor, you would use the sign for 'floor,' which uses flat hands with palms facing down moving apart from each other.
Do I need to use both hands for this sign?
Yes, the standard and most clear way to sign 'ground' uses both hands simultaneously. While you might occasionally see native signers use one hand in very casual, rapid conversation, beginners should always practice using both hands.