Elemental logic, book 2 : Earth logicLaurie J. Marks2004

Shaftal has a ruler again, a woman with enough power to heal the war-torn land and expel the invading Sainnites from Shaftal. Or it would have a ruler if the earth witch Karis G'deon consented to rule. Instead, she lives in obscurity with the fractious family of elemental talents who gathered around her in Fire Logic. She is waiting for some sign, but no one, least of all Karis herself, knows what it is.

Then the Sainnite garrison at Watford is attacked by a troop of zealots claiming to speak for the Lost G'deon, and a mysterious and deadly plague attacks the land, killing both Sainnites and Shaftali. Karis must act or watch her beloved country fall into famine and chaos. And when Karis acts, the very stones of the earth sit up and take notice.