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.
1 They're strong against the Fire type because ground or mud is usually used to put off a fire, that's because the ground suffocates the fire, letting it without oxygen.  They're strong against the Poison type because if you drop any venom into the ground, this ends by disappearing.