How can I increase the earthworm population in my garden? What can I feed them?

Answer 1

Generally speaking, the best way is to incorporate lots of organic matter in the soil, unless something else is the limiting factor.

Answer 2

Earthworms eat microbes, they are the major soil predator of things like E. coli as well as other bacteria. They feed on material that has begun to decompose including feces as well as decaying plant material. In deeper soil some species feed on larger fauna like nematodes. So the best thing is to keep adding fresh compostable material and water. If the soil dries they will seek deeper damp layers because they need a constantly moist environment.