Vol. 7, Issue 3, Part E (2021)

The digestive system of earthworms consists of a pharynx, oesophagus and gizzard (‘reception zone’) followed by an anterior intestine that secretes enzymes and a posterior intestine that absorbs nutrients. During progress through this digestive system there is a dramatic increase in numbers of micro-organisms of up to 1000 times. There is experimental evidence that microorganisms provide food for earthworms. Bacteria are of minor importance in the diet, algae are of moderate importance; protozoa and fungi are major sources of nutrients. Worms, produced under sterile conditions, could live on individual cultures of certain bacteria, fungi and protozoa, but grew best on various mixtures of microorganisms.

Symbiotic interactions between earthworms and microorganisms break down and fragment organic matter progressively, finally incorporating it into water-stable aggregates. The mineral nutrients in earthworm casts and lining earthworm burrows are in a form readily available to plants. There is evidence that interactions between earthworms and microorganisms not only provide these available nutrients, but stimulate plant growth indirectly.