Answer:
The dust represents his body / his ashes.
Explanation:
The dust in lines 4-5 probably represents the man's body or his "dust" buried in the ground. He describes it as a "richer" dust than the earth, meaning that it's of a higher quality, maybe, or made of a different material, which it is.
He later goes on to describe it as once having been "[a] body of England’s, breathing English air". This is pretty much a clear indication that the "dust" is his body, when he's dead.