Earth control pairs nicely with psi because of the -def in multiple skills, allowing drain psyche to more reliably hit mobs. Quicksand is a great power to take for that purpose, as well for the large -run speed debuff on mobs that helps to keep mobs from surrounding your dom in melee. With earth also having a notably tough pet that has a taunt, it's a particularly survivable pairing.
That said, as tex points out fire / psi is the most damaging "single target damage" pairing for a psi assault character. And, bonfire is very helpful for staying alive in melee in most situations.
