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.
Fire is strong against grass, ice, bug, and steel. Grass is strong against water, ground, and rock. Steel is strong against ice and rock. Ice is strong against grass, ground, flying and dragon. Ground is strong against fire, electric, poison, rock and steel. Fighting is strong against normal, ice, rock, dark and steel. Ghost is strong against psychic and ghost.