I bought a Webster soil auger a few years back. It's features a 1.5hp(?)gas engine on a frame that's like a handtruck. The engine runs up and down on a track sort of like a drill press. There's a cannister that holds your fertilizer, which is dispensed using a small lever that you lift. It wasn't cheap ($500?) and the drill bits aren't the best but it works well for relatively open areas.

A friend recommended finding a 2" rock auger with replaceable screw-in teeth. I've never seen or used one, but he seemed to think they were fairly common.

One word of caution- seek out the locations of underground utilities using utility locates and ask the homeowner about irrigation systems! I found out the hard way about the latter, oh well...