Tunneling is accomplished by forcing themselves into small cracks, swallowing soil and debris that get in the way. Feeding is accomplished at the same time as tunneling. The maze of tunnels allows water and air to penetrate the soil. The other way earthworms improve soil is through their feeding. Soil and plant debris are bound together into crumbs as they pass out of earthworms, improving soil structure.
Soil structure refers to the arrangement of soil particles.
Fine particles of clay soils pack close together, impeding water and air penetration. The larger soil crumbs produced by earthworm feeding increase the space available for air and water between soil particles. The plant debris passed through earthworms is broken down into the original nutrients used to build the now-dead plant. Many nutrients, such as nitrates, phosphates and potash, are immediately released into the soil while other nutrients are released gradually. The earthworm's activity is like a miniature composter that mixes and conditions plant wastes into fertilizer for new plants.