The power allows you to target an object; the ground is an object. No problem, so long as you actually do what the instructions say: You can't detonate it at somebody's feet and still hit them. You make a secondary against each enemy adjacent to the primary target, so if you're targeting a square, you need to make an attack against each enemy in squares adjacent to that target square, but not IN the target square.
It's not a big deal, balance wise; you're effectively giving up an average of four damage against the primary target, if you can even manage to catch him in the blast. You still have to make an attack roll and all. This strategy is really not a big deal, honestly; against normal enemies you're not getting much of a benefit from it, and if you're dealing with minions, you'd be better off with a Scorching Burst than this strategy.
Judging by Disintegrate, you don't have to make attack rolls if you target an unattended object. (The ground is always unattended.)