Why are earthworms called 'farmer's friend'?
They help in vermicomposting
The type of earthworms called red worm are used for composting. This method of preparing compost with the help of red worms is called vermicomposting. Red worms have a special structure called gizzard in their bodies with which they grind food material and convert it into manure. Thus they help in increasing the fertility of the soil and are called as 'farmer's friend'.