Dirt with grass on top.
Home ground advantage should be renamed to plain,"Ground Advantage".
It's to do with the shape and size of a ground that determine whether the ground will suit the teams playing style.
The only Home Advantage that a team gets is the number of supporters that rock up to influence the umpires decisions and perhaps support team moral.

I look at the situation this way:
If Melbourne were playing Geelong at the MCG as a Home game they would get a bonus of 6 points.4 points for ground and 2 points for supporters.

If Geelong were playing Melbourne at the MCG as a Home game then neither team gets a bonus.Maybe 4 points for supporters to Geelong cancelled by 4 points ground for Melbourne.