It could be soil from a battlefield - the Wikipedia article on the subject of the statue says there "are monuments to him around the world ... bringing earth from the battlefields where he had fought"[0]
But I liked the anticlimactic way the BBC article on the subject[1] ended:
"'I am sure some in our audience have questions for our various experts up on stage?' Ms Voigtschild said after the mud was found.
There were none."