The Collaborative International Dictionary of English v.0.48:

Chthonophagia \Chthon`o*pha"gi*a\, Chthonophagy
\Chtho*noph"a*gy\, n. [NL. chthonophagia; Gr. ?, ?, earth + ? to
   eat.]
   A disease characterized by an irresistible desire to eat
   earth, observed in some parts of the southern United States,
   the West Indies, etc.
   [1913 Webster]