From time to time we hear from a yardener who considers earthworms in their lawn to be a serious problem. The complaint comes from the fact that earthworms are coming out of the soil and leaving their castings on the surface. Those castings (worm poop) become very hard making it very uncomfortable for walking in bare feet. In this instance, we agree that the earthworm is a problem, but the cause of the problem is compacted soil. Soil often becomes so compacted in a lawn that even the tough earthworm can’t work its way through it. Consequently they operate fairly near the surface, creating those hard castings on top of the surface.
The solution to the problem is to get the earthworms down deeper in the soil and the way to do that is add some organic material and some soil microbes to help break up that compaction. Compost is the best material, buy Canadian sphagnum peat moss is cheaper and almost as good. Just adding the organic material will get the microbe population already present in the soil starting to work. However, that population is probably low already, so by giving the soil a shot of fresh microbes every few weeks for a few months will solve the earthworm castings problem much faster.
For more information in this area see the file Caring For Soil Under The Lawn