earthworms

It might as long as your soil has enough organic matter to support them.

Well, its not a desert if that's what you mean, its on my lawn with the usual stuff you find at soil level (dead grass, leaf bits etc). The lawn is about average but the soil is quite hard and the layer of top soil is thin, sitting on hard pan or something close to that - I'm looking to increase the depth of the top soil over time, making it more porous and easier for lawn to grow. Seems like year or two of earthworms might improve it - I just dont know Eric

And if you do have enough organic matter, the worms will already be there. Get your soil analyzed first.

Earthworms require several inches of organic sandy loam which will hold moisture. If you don't already have these conditions, then dig an area in part of your lawn (yard), make the necessary soil preparations and add the worms. Earthworms also do well in soil that has coffee grounds sprinkled over it. Their castings help to fertilize the soil around the grass roots. Try these websites for help.