Influence of Earthworms on Soil Fertility and Productivity

Earthworms are probably the most important macro animals in soils. They are egg-laying hermaphrodites that eat detritus, soil organic matter and micro organisms found on these materials. They do not eat living plants or their roots and so do not act as pests to crops.
Influence of earthworms on soil fertility and productivity are:

Burrows

Earthworms literally eat their way through soil. They may ingest a weight of soil equal to 2 to 30 times their own weight in a single day. In a year, the earthworm population of 1 hectare land may ingest between 50 and 1000 Mg of soil, the higher figure occurring in moist, tropical climates. In so doing, they create extensive systems of burrows.

Earthworms burrow - Soil Science - Soil Ecology

Casts

After passing through the earthworm gut, ingested soil is expelled as globular soil aggregates called casts, some of which can be seen. During the passage through the earthworm's gut, organic materials are thoroughly shredded and mixed with mineral soil materials. Probably because of enhanced bacterial activity, earthworm casts are usually high in polysaccharides, which are credited with stabilizing the granular structure. The casting behavior of earthworms, therefore, generally enhances the aggregate stability of the soil. The casts are deposited within the soil profile or on the soil surface, depending on the species of earthworm.

Casts of Earthworms - Soil Ecology - Benefits of Earthworms

Nutrients

The  activities of earthworms greatly enhance soil fertility and productivity by altering both physical and chemical conditions in the soil, especially in the upper 15 to 35 cm of soil. Earthworms increase the availability of mineral nutrients to plants in two ways. First, as soil and organic materials pass through an earthworm's body, they are ground up physically as well as attacked chemically by the digestive enzymes of the earthworm and its gut microflora. Second, as earthworms ingest detritus and soil organic matter of relatively low nitrogen, phosphorus and sulfur concentrations, they assimilate part of this material into their own body tissues.

Compared to the bulk soil, the casts are significantly higher in bacteria and organic matter and available plant nutrients. Plant roots commonly grow down earthworm channels. In compacted soils, the burrows, whether empty or filled with casts, may offer a path of little resistance through an otherwise impenetrable soil layer.

Physical incorporation of surface residues into the soil by earthworms and other soil fauna reduces the loss of nutrients, especially nitrogen, by erosion.

Beneficial Physical Effects

Earthworms are important on other ways. The holes left in the soil serve to increase aeration and drainage, an important consideration in plant productivity and soil development.The mixing activity of earthworms can reduce compaction problems and nearly eliminate the formation of an undesirable thatch layer. Under conditions of heavy rainfall, earthworm burrows may greatly increase the infiltration of water into the soil. Moreover, stable macro aggregate formed from earthworm casts may form as much as 50 to 60% of the surface soil in some cases.