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.
Earth is strong against water because earth naturally contains water or absorbs water to the point when it becomes muddy then dries up but weak against lightning because it can easily travel through and break the ground apart. In the anime, Earth Release was shown being used in conjunction with Fire Release.