The size of the earthworm population depends very much on the availability of usable organic material such as leaves or harvest residues. Earthworms are picky about their food. For example, a soft poplar leaf is clearly preferred to a hard, tannic acid-rich beech or oak leaf.
Fungi and bacteria replace teeth
The earthworms use the carbohydrates and proteins of the dead plant residues and the microorganisms living on them for their nutrition. In addition, bacteria, algae, single-cell organisms and fungal threads are grazed on the surface around the tube or taken in and digested when the soil is rummaged through with the earth.
In order for the toothless earthworms to be able to eat the organic material at all, it must first be broken down by fungi and bacteria. For this purpose, leaves and crop residues are drawn into the living tube and composted in the uppermost area of the tube.
The worms are real master composters: They stick their food to the wall of the tube, cover it with excrement and thus offer ideal living conditions for the pre-digesting microorganisms.
When eating rotted, organic material, larger amounts of mineral earth are also absorbed and mixed with the microorganisms living in the intestine. Worms eat up to half their own weight every day.
Gourmets but poor feed converters
The earthworms have adapted their diet to the conditions of their habitat. Species living on mineral soil prefer dead plant roots and already heavily rotten organic material with the microorganisms living on them. The litter dwellers mostly feed on the fall deaf. Some species specialize in soil algae, manure, compost or rotten wood. Attempts to eat large, vertically digging earthworms on various types of foliage have shown that nitrogen-rich and low-tannic acid leaves such as black alder, ash or elm are preferred. Other leaf types are only eaten at an advanced stage of degradation. At the very bottom of the menu is the needle litter. Charles Darwin (1881) attested earthworms a well-developed "sense of taste".
Earthworms are not good eaters because a large part of the ingested organic matter is excreted undigested. The well-known dewworm (Lumbricus terrestris) eats around half its own weight in food every day. The well-mixed excrement enriched with nutrients is definitely a food for the other soil organisms.