Several species of helpful earthworms will magically appear as you dig compost and other forms of organic matter into your soil. The most important species in terms of soil improvement is the night crawler (Lumbricus terrestris), which specializes in taking organic matter from the soil’s surface and storing it in underground middens, which are combination food supply/trash heaps. As these middens decompose, they become nutrient-rich hot spots in plants’ root zones. In addition, night crawler tunnels create open channels for water and plant roots, which can make a huge difference in tight clay soil.
Using mulches will encourage night crawlers, as will creating grassy mowed pathways, which you might think of as night crawler reservoirs. But the best thing you can do to increase night crawler populations is to place piles of compost and mulch in or near your garden. More night crawler activity goes on around the edges of compost and mulch piles than anywhere else in your garden.
Chances are good that providing habitat for night crawlers will attract them in noticeable numbers, but if you want to import worms to colonize your new garden area, you can use night crawlers that are sold as fishing bait. Mother Earth News
If you put them there, they will do good things.
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