why are earthoworm's called ploughman of nature?

Earthworms carry out decomposition of detritus (dead plant parts, animal remains and excretions). Earthworms feed on larger pieces of detritus. Pulverisation occurs in the digestive tract of earthworms as a part of detritus comes out undigested. Due to fragmentation, left-over detritus comes to have large surface surface area. In this way, earthworms are called ploughman of nature because they help in fragmentation of detritus and loosening of soil.  The decomposition of detritus gives rise to two products-humus and inorganic nutrients.