River Kelpie

River Kelpie doesn't give you the ability to cast spells from graveyards. Its second ability merely triggers whenever a spell is cast this way (by using Memory Plunder, for example).

If you play a land card from a graveyard (by using Crucible of Worlds, for example), only the first ability triggers. That's because the land (which isn't a spell) is put directly onto the battlefield from the graveyard.

If you cast another artifact, creature, enchantment, or planeswalker spell from a graveyard, only the second ability triggers. That's because the card is put onto the stack, not onto the battlefield.