To those who still don't like the "ground = earth" argument lets look at another very popular Conjuration (Creation) spell, Evard's Black Tentacles, which reads in relevant part:
This spell is quite explicit- any surface underfoot can emit a tentacle, which contrasts sharply with the spell that is the main topic of debate in this thread, which states that the spike grows from the ground. EVT even distinguishes between earth and floor.
PHB p228
...These waving members seem to spring forth from the earth, floor, or whatever surface is underfoot- including water...
This spell is quite explicit- any surface underfoot can emit a tentacle, which contrasts sharply with the spell that is the main topic of debate in this thread, which states that the spike grows from the ground. EVT even distinguishes between earth and floor.