induration. 1. the process of hardening or being hardened.
1 They're weak against the Ice and Water types in reference of erosion, which cause this elements to the ground. 2  They're immune against the Electric type because in real life, the ground is a natural sink of electrons.  The Bug type resists them because many bugs like worms live inside the ground and many of them eat it.
Flying Type Good against: Grass, Fighting, Bug Bad against: Electric, Rock, Steel. Ghost Type Good against: Ghost, Psychic Bad against: Dark, Steel. Grass Type Good against: Water, Ground, Rock Bad against: Fire, Grass, Poison, Flying, Bug, Dragon, Steel. Ground Type Good against: Fire, Poison, Electric, Rock, Steel Bad against: Grass, Flying, Bug. Ice Type Good against: Grass, Ground, Flying, Dragon Bad against: Water, Ice, Fire, Steel. Normal Type Bad against: Fighting, Ghost, Rock, Steel. Poison Good against: Grass Bad against: Poison, Ground, Rock, Ghost, Steel. Psychic Type Good against: Fighting, Poison Bad against: Psychic, Dark, Steel. Rock Type Good against: Ice, Fire, Flying, Bug Bad against: Fighting, Ground, Steel. Steel Type Good against: Ice, Rock Bad against: Water, Fire, Water, Steel. Water Type Good against: Fire, Ground, Rock