Earthworms prefer pH levels of 6.5 to 7.5
Earthworms do not like acid soils with pH (CaCl2))* less than 4.5. The addition of lime raises pH and also adds calcium. Earthworms need a continuous supply of calcium, so are absent in soils low in this element. South Australian research found that earthworm numbers doubled when pH(CaCl2) rose from 4.1 to 6.7.