How Do Earthworms Improve Soil? Eating It and Passing It Through

Earthworms are not interchangeable, and the ecological groups do quite different jobs:

• •Epigeic species live in surface litter, do not burrow, are small and reddish, breed fast and process organic matter rapidly, which is why they dominate compost heaps and wormeries
• •Endogeic species live within the topsoil, make horizontal branching burrows, swallow soil rather than litter and are pale because they never see light, and they do most of the mixing of mineral soil
• •Anecic species, of which the common lobworm is the example, build deep permanent vertical burrows reaching well over a metre, come to the surface at night to drag leaves down, and cast on the surface
• •The anecic burrows are the most consequential single feature, since they are permanent, open to the surface and conduct water rapidly into the subsoil
• •Those burrows also give roots a ready-made path through compacted layers, lined with nutrient-rich material
• •Losing one group does not substitute for another, so a soil with only surface-dwelling species has none of the deep drainage benefit

Worm casts differ measurably from the soil they came from. They are enriched in available nitrogen, phosphorus and potassium, since digestion mineralises organic material and the gut concentrates nutrients, with analyses finding several-fold increases over surrounding soil. They are more stable as aggregates, because mucus and calcium carbonate secreted in the gut bind particles together and the cast then hardens, which improves structure and resists erosion. They carry a distinct and highly active microbial community, since the gut is a selective environment and the cast is a bacterial hotspot afterwards. Worms also transport material vertically, bringing subsoil to the surface and taking organic matter down, which mixes horizons and buries surface material steadily, an effect Darwin demonstrated by measuring how stones on a field surface sank over decades and by explaining the burial of archaeological remains, which is why the technique of earthworm-driven burial is still considered in interpreting shallow sites.

Earthworm populations respond strongly and quickly to management. Ploughing kills them directly, destroys the permanent burrows and removes the surface residue they feed on, with no-till fields consistently supporting several times the population of ploughed ones. Loss of organic matter starves them, since they need a food supply and a field receiving no residue or manure cannot support them. Compaction restricts movement and reduces oxygen. Certain pesticides are directly toxic, and some veterinary wormers pass through livestock in dung and kill the worms that would otherwise break it down, which has measurable effects on pasture. Acidity limits distribution, since most species are intolerant of strongly acid soil. Drought and waterlogging both reduce numbers. Because the benefits accumulate over years and the damage can be done in one pass, the practical implication is that worm populations are an indicator of management history rather than of current conditions, which is why counting them in a spade sample is a recommended simple soil health assessment.

Earthworms are not universally beneficial, and the clearest case is in northern North American forests. Large areas were left without native earthworms by the last glaciation, and the forest ecosystems that developed there depend on a thick undecomposed litter layer, in which tree seedlings, wildflowers and ground-nesting birds live. European earthworms introduced through agriculture, soil in plant pots and discarded fishing bait have invaded those forests and consume the litter layer rapidly, which removes the seedbed, exposes roots, changes nutrient cycling and has been associated with declines in understorey plants and in the animals depending on them. The invasion front moves slowly and is effectively irreversible. Similar concerns exist elsewhere. The lesson is the general one about introduced species: an organism that performs a valuable function in a system that evolved with it can be destructive in one that did not, and earthworms are a useful corrective to the assumption that soil life is uniformly good.

Earthworms swallow soil, digest its organic content and excrete stable nutrient-enriched crumbs, and their three ecological groups do different jobs, with deep-burrowing species creating permanent vertical channels that conduct water and give roots a path through compaction. Ploughing kills them and destroys those burrows, so no-till fields support several times the population. Darwin calculated their reworking of English soil and explained how they bury stones and archaeology. In North American forests without native worms, introduced ones are destructive.