The Gemorah indicates that ‘Matunta” cannot be used for Kisui Ha`Dam.
Rashi learns that this is because “Matunta” doesn’t break up into עפר sand [or dust], it doesn’t break into small particles. The Torah reqiures something that is defined as עפר.
The Rem`a however, learns that “Matunta” cannot be used for Kisui Ha`Dam because it is not fertile, nothing can grow in “Matunta”.
The Shach ask on the Rem`a, that our gemorah clearly states that “Matunta“ is fertile. The gemorah then says that when it is “Matunta” wet it is not to be used for Kisui Ha`Dam. Is it not clear that wet earth is no less fertile than dry earth?