When conditions are favorable, earthworms are the dominant animal life in the soil—at such times, their weight equals or exceeds that of all the other soil-dwelling animals combined. Earthworms number several million to an acre in favorable soils, and their total weight per acre may be as high as one-half ton.
Earthworms flourish in well-drained soils that contain abundant organic matter and a continuous supply of available calcium. They are susceptible to drought, cold, waterlogging, and extremes of acidity or alkalinity.
Earthworms are important agents in mixing surface organic residues with the underlying soil. The earthworms in an acre can bring to the surface 20 tons of soil a year. Their burrowing activity is most intense in the top 6 inches, although some tunnels extend to depths of 6 feet. Earthworm channeling improves soil aeration and increases movement of water into and through soils.
Earthworms contribute to the formation of soil structure by their cast-forming activities. In feeding, the earthworm ingests soil and organic wastes. The residue, combined with the calcium carbonate and mucus secreted from the gut wall, is ejected as a granular cast.
Although earthworms help the soil’s structure, they don’t aid its fertility. Earthworms digest and help decompose organic matter to release its constituent nutrients, but they don’t add to the nutrients. The beneficial effects of earthworms on plant growth are largely those associated with improved aeration and improved tilth. In the final analysis, earthworms should be considered an indication of good soil fertility rather than its cause.