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.
To name a few, Land Umber (the earth beast) is weak against fire as well as wind, and Mispolm (the tree beast) is weak against wind/thunder (though this is only relevant for Thunder Saber, since its Counter Magic skill reflects all non-Shade, non-ultimate magic).