What they do
Darwin gave earthworms 40 years of his life. His last book, 1881, argued no other animal had shaped the world more.
He was right.
Worms burrow, feed, cast, and die. Each act rebuilds the soil.
Channels. A single anecic worm drills a vertical burrow a meter deep. It lasts years after the worm dies. Healthy worm populations absorb rainfall 4 to 10 times faster than depleted soils.
Casts. Worms eat leaf litter, dead roots, and manure mixed with mineral grit. They excrete casts richer in nitrogen, phosphorus, potassium, and calcium than the surrounding soil. A continuous, slow-release fertiliser made by billions of animals.
Casting pulls surface organic matter down. It mixes the profile. It feeds the soil food web.
Count them
Worm abundance is the simplest soil health test you can run.
Healthy temperate soil under pasture or mature woodland: 300 to 500 worms per square meter. Roughly 1 to 3 million per hectare. Old grassland can exceed 1,000 per square meter. The biomass of worms beneath a healthy pasture can outweigh the cattle grazing above it.
Tilled, sprayed, or compacted soils: fewer than 50. Sometimes zero.
The test is simple. Dig a 30 cm cube. Sort the soil onto a tarp. Count.
Do it once at the start of a project. Repeat yearly. Rising numbers mean organic matter is building and structure is improving. This is your core monitoring metric in no-dig and regenerative work.
The three groups
Not all worms do the same job. Soil ecologists split them into three functional groups. Healthy soil needs all three.
Epigeic. Surface dwellers. 3 to 8 cm long, reddish, fast-breeding. They live in the leaf litter and the top few centimeters. The compost worm (Eisenia fetida) is epigeic, which is why it thrives in vermicompost bins. Vulnerable to sun, drying, and surface disturbance.
Endogeic. Topsoil workers. 5 to 15 cm, pale grey or pink, living in the upper 10 to 30 cm. They build the crumb structure of healthy topsoil through horizontal burrowing. They take the worst hit from tillage. Every plough pass shreds their networks.
Anecic. The engineers. 15 to 30 cm long, with a flat tail to anchor them in permanent vertical burrows. Lumbricus terrestris is the classic. They emerge at night, drag leaves down, and leave small dark middens around the burrow mouth. Their deep channels are what carry rain into the subsoil.
What kills them
Tillage first. One pass of a plough or rotary cultivator kills 10 to 30 percent of the worms directly and wrecks the burrow network the survivors depend on. Over years, arable populations crash to a fraction of those in adjacent pasture. This is the core reason no-dig works.
Pesticides next. Neonicotinoid seed coatings reduce worm reproduction and burrowing at field-rate concentrations. Copper-based fungicides accumulate in vineyard and orchard topsoil over decades until they hit toxic levels. Integrated pest management buys back ground.
Acidity. Most species prefer pH above 5.5. Populations crash below 4.5. Liming an acid soil produces visible worm gains inside one season. On mining sites or worn-out farmland, correcting pH is often the prerequisite for any biological recovery.
Build the conditions
You do not need to import worms. You need to feed and shelter the ones already there.
The recipe is short. Organic matter as food. Stable soil structure. Adequate moisture. Moderate pH. No toxic chemicals.
Feed them. Mulch with compost, leaf litter, straw, or woodchip. Surface food for the epigeic, dragged food for the anecic, mixed food for the endogeic. Cover crops between cash crops supply root exudates year-round. Leave residues on the surface. Do not burn them.
Stop disturbing them. Convert ploughed arable to no-dig and you get measurable gains within 2 to 3 years. Anecic species, the slowest to recover, return to pre-disturbance levels in 5 to 10 years. Keep ground covered always, with living plants or mulch.
Plant trees. In orchards, food forests, and restoration plantings, worm populations build as the canopy closes and litter accumulates. Diverse litter from multiple species feeds a more diverse worm community than any monoculture. Over decades, the soil engineering compounds into the deep, dark, structured ground you find under old forest.