Common earthworms. There are microscopic worms called beneficial nematodes, which eat harmful insect larvae, but finding the right kind for your conditions can be difficult. Encouraging earthworms is easy, though--just add organic matter. Compost, composted manure, compost tea, and/or "humates" added a couple-three times a year will draw in earthworms from hundreds of meters away, and allow the local ones to multiply.