Earthworms are natural invertebrates of agro ecosystem belonging to the Phylum Annelida, Class – Chaetopoda and Order Oligochaeta and dominant in the temperate and tropical soils. They are the first group of multicellular eucoelomate invertebrates to have succeeded to inhabit terrestrial environment (Kale and Karmegam 2010). Being hermaphrodites, both male and female reproductive organs are present in every single earthworm but self-fertilization does not generally occur. Earthworms, as ecosystem engineers, play an important role in many soil ecosystems and are one of the numerous ranges of burrowing organisms, which improve soils (Lavelle 1997). Due to their relatively large size and characteristic feeding behaviour, certain species have significant impacts on soil structure, soil fertility, plant growth and crop yields.

Earthworms affect soil physical structure by their tunneling activity which improves soil aeration, porosity and permeability, increase the moisture absorption and availability of moisture to plants. The main processes affected by feeding, casting and burrowing activities of earthworms are microbial activity (Lavelle et al., 1983;  Scheu 1990; Pashanasi et al., 1992), dynamics of organic matter (Lavelle and Martin 1992), dynamics of chemical processes (Edwards and Bohlen 1996), nutrient release (Sharpley and Syers 1976; Bouche et al., 1987; Scheu 1987), and physical properties such as aggregation or infiltration (Bostrom 1986).They produce plant growth stimulants and increase the mineralization of soil by mixing soil minerals with organic matter.  Earthworm activity changes the microbial community structure of soil. It has been proposed that earthworms have a mutualistic relationship with microorganisms (Barois and Lavelle 1986; Lavelle et al., 1995) and may contribute to the maintenance of soil fertility and soil microbial diversity.

In 1881, Charles Darwin was the first to actually document increased plant litter decomposition due to earthworm activity in his book ‘The formation of Vegetable Mould through the action of Worms’. Since Darwin’s observations, the role of earthworms in litter decomposition and fertility of soil has been well studied. They are not essential to have in the soil, but their presence can be an indicator of good soil quality. The effects of earthworms on soil processes differ between ecological categories and species. Earthworm can affect soil populations either directly or indirectly viz; comminution, burrowing, casting, grazing or dispersal. Soils with low earthworm populations often have a layer of undecomposed litter on the surface, with a sharp demarcation from the underlying soil, which usually has a poor crumb structure. The presence of earthworms in soils is generally indicative of the presence of other faunal representatives contributing to soil fertility.

Earthworms are generally more active in moist soils and contribute to maintain soil fertility in at least three ways: (i) they build and maintain a soil structure based on resistant macro-aggregates; (ii) they release nutrients from the plant residues to the soil organic matter; and (iii) they physically protect humus inside compact globular casts (Lavelle et al., 1994). Earthworm activity creates structures, globular and granular casts, and galleries that modify soil aggregation and porosity, as well as the communication between pores.