Black dirt also comes from the mainland and is basically fertile top soil. The hand filled bags have less gravel in it than the machine loads. The Mayans used it for balast way back on their trips to the island and probably also used it for farming here. Large mounds of it can be found all about the island. The little park that is for sale is an example of this (and has soil stolen from it). Mixed with horse manure, the stuff is awesome for growing all "kind" of good stuff.