How much soil do earthworms turn over in a year?
― A stone on the ground sinks by about 20cm in a century
The soil a single earthworm swallows at once is barely a pinch. But multiply that by the size of a field and a few decades of time, and the picture changes completely. Darwin, famous for the theory of evolution, spent over 40 years obsessed with this "multiplication of small numbers," and confirmed that the ground itself is slowly turning over.
Have you ever seen little mounds of soil, shaped like soft-serve ice cream, piled up on the lawn of a park after rain?
Those are earthworm castings. Earthworms eat their way through soil and pile up what's passed through their bodies on the surface.
Each mound alone is about the size of a fingertip. But if that happens all over a lawn, every year, for decades, how much soil does it add up to?
Only two reasons small mounds add up to "moving the ground"
In rich grassland, a single square metre of soil is said to hold tens to hundreds of earthworms. That works out to dozens living in an area about the size of half a tatami mat under your feet.
Earthworms eat their way through the soil and keep carrying castings up to the surface. In one year that's a thin film, but after a hundred years it's thick enough to bury something left sitting on the ground.
The work of one worm is tiny, but multiply "numbers" by "time" and the scale jumps by several orders of magnitude. Let's look at it step by step.
Darwin's measurement: "10 tons a year"
Darwin collected the soil particles that appeared in his garden and pasture and weighed them. From this he estimated that, across much of England, more than 10 tons of soil per acre (about 0.4 hectares) passed through earthworms' bodies and was carried to the surface every year.
10 tons sounds like a lot. But spread across an area about half the size of a football pitch, it's only about 2 millimetres thick per year. No wonder nobody notices.
Yet those 2 millimetres build up every year. And as worms pile soil on the surface, they're simultaneously eating away the soil below, hollowing it out. So the ground level barely changes, while a stone left on the surface gradually sinks. Try moving the dial in Figure 1 to change the number of years.
Even the floors of ruins got buried by earthworms
Darwin scattered white chalk powder on a field near his home, and dug up the soil years later. The chalk that should have been on the surface turned up as a thin white layer inside the soil. After about 30 years, it was said to be at a depth of about 18 centimetres.
He also examined the floors of Roman-era ruins and the base of fallen megaliths. Old floors ending up buried under soil isn't only because soil rained down from above. Darwin thought a large part of it was earthworms carrying soil up from below, gradually "swallowing" the floors.
These observations were gathered into a book, The Formation of Vegetable Mould through the Action of Worms, published in 1881. Darwin died about six months after its publication, in the final research of his life.
Soil that has passed through an earthworm's gut has its fine particles bound together by a sticky substance, forming clusters of tiny clumps. Because there are so many gaps between them, water soaks in easily and roots grow more freely. The narrow tunnels earthworms dig also act like drainage pipes, channelling rainwater underground.
Summary
The soil earthworms carry to the surface each year amounts to about 2 millimetres in thickness. But when tens to hundreds of them keep working the same patch of ground for decades, the maths works out to the entire surface layer of a field passing through an earthworm's body roughly once every hundred years. Their unassuming work — turning fallen leaves into soil, creating channels for water — keeps building the ground beneath our feet.
One worm: just a pinch.
But over a century, the ground itself turns over.
For why earthworms come up to the surface on rainy days, see "Why do earthworms come up to the surface on rainy days?", and for where fallen leaves disappear to, see "Why doesn't fallen leaf litter in forests just keep piling up?".
- After rain, look for earthworm casting mounds on a lawn or flowerbed, and count how many appear within a 50cm square.
- Place a marker — a flat pebble or a pinch of white sand — in that spot and take a photo.
- Check the same spot every few weeks and record how the casting mounds gradually cover the marker.
Casting mounds are most often seen after rain in the warm, damp seasons of spring and autumn. If you find an earthworm, please put it back in the soil after observing it.
Want to know more? ― Terms, formulas, and textbook connectionsWe've marked which level each part belongs to, from lower-secondary science to university-level specialist subjects
- JHSCovered in lower-secondary school science
- HSCovered in "Basic Biology" / "Basic Earth Science" in upper-secondary school
- HS+Advanced upper-secondary content, or treated as a textbook sidebar
- UnivNot taught in upper-secondary school — content from university specialist subjects (soil science, geomorphology)
- ResearchNot yet settled as "established theory" even at university — an active area of current research
JHSTerms: this phenomenon has names
- Decomposer: an organism that eats fallen leaves, dead matter, and the like and returns them to inorganic form. Earthworms, woodlice, and fungi are examples.
- Cast: a lump of soil excreted by an earthworm, on the surface or within the soil.
- Soil aggregate structure: a state in which fine soil particles cluster into small clumps, giving the soil many gaps.
JHSHSChecking with a formula: how many millimetres build up in a year
Let's convert Darwin's estimate — "about 10 tons per acre per year" — into a thickness. Dividing weight by density and area gives the thickness.
| In symbols | h = M ÷ ( ρ × A ) |
| In words | Thickness deposited per year = weight of soil carried up ÷ ( soil density × area ) |
| Where it comes from | A combination of "weight = density × volume" and "volume = area × thickness." It uses the fact that the weight of the soil carried doesn't change (is conserved) when it's spread out. |
| Symbol | Meaning and unit |
| h | Thickness of soil deposited per year (m) |
| M | Weight of soil carried to the surface per year (kg) |
| ρ | Density of the deposited soil (kg/m³) |
| A | Area (m²) |
| Weight of soil carried (Darwin's estimate) | about 10000 kg |
| Area of 1 acre | about 4047 m² |
| Density of deposited soil (assumed in this article) | about 1200 kg/m³ |
| Density × area | 1200 × 4047 = 4856400 |
| Thickness deposited per year (m) | 10000 ÷ 4856400 ≈ 0.0021 |
| Converted to millimetres | 0.0021 × 1000 = 2.1 |
| Over 100 years (mm) | 2.1 × 100 = 210 |
That's about 2.1mm a year, or about 21cm over 100 years — roughly the depth a spade reaches when digging a field. In other words, the surface soil of a field is estimated to pass through earthworms' bodies once about every 100 years. Darwin's chalk experiment (about 18cm in around 30 years) was faster than this, which shows how much the rate varies by location.
HSHS+Nutrient cycling in soil
HSIn Basic Biology, students learn that decomposers return organic matter to inorganic form within an ecosystem, cycling carbon and nitrogen. Earthworms pull fallen leaves down into the soil and mix them in, making conditions easier for fungi and bacteria to decompose them.
HS+The soil aggregate structure formed by earthworm castings increases the proportion of gaps in the soil (porosity). Large gaps handle drainage and aeration, while small gaps retain water, which is thought to give soil good drainage and good water retention at the same time.
UnivBioturbation, and the speed of soil "mixing"
The action of organisms mixing soil or sediment is called bioturbation. In geomorphology, this mixing is sometimes expressed using the same kind of equation used for heat diffusion (a diffusion equation), to estimate how fast soil on a slope creeps slowly downhill. Some research uses optically stimulated luminescence dating, which measures the time since a soil grain last saw sunlight, to study how quickly soil turns over vertically.
📖 For the derivation of the equation and further reading: Bioturbation (Japanese Wikipedia) ／ Earthworm (Japanese Wikipedia)
ResearchWhat isn't fully understood yet
- Do earthworms accelerate warming, or slow it down? They both lock organic matter deep in the soil and speed up decomposition, releasing carbon dioxide and other gases — research on the net balance is still ongoing.
- Introduced earthworms are changing forests. In parts of North America, non-native earthworms have spread into places that historically had none, thinning the leaf-litter layer, and reports link this to effects on wildflowers and other wildlife.
- How fast does soil actually turn over? The rate is said to vary by orders of magnitude depending on location, climate, and earthworm species, and measurements using dating methods are ongoing.
In other words, this article too reflects "what is understood at present." Treat the figure of 2mm a year as a rough guide based on Darwin's estimate.
Textbook connections (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science, "Balance in the natural world" (decomposers) / "Density" | Earthworms' role decomposing leaves and soil; checking with a formula |
| HS | Basic Biology, "Ecosystems and material cycles" | Carbon and nitrogen cycling in soil |
| HS+ | Advanced Basic Earth Science / agricultural subjects (soil) | Aggregate structure and porosity |
| Univ | Soil science, geomorphology | Bioturbation, diffusion equations, optically stimulated luminescence dating |
| Research | Soil ecology, climate change | Earthworms and warming, invasive earthworms |
| ― | Everyday connections | Building healthy garden/field soil, the value of leaving fallen leaves, why ruins end up buried |
- Charles Darwin, The Formation of Vegetable Mould through the Action of Worms (original 1881; includes observations on the quantity of soil in pastures and the chalk layer)
- C. A. Edwards, P. J. Bohlen, Biology and Ecology of Earthworms, 3rd ed., Chapman & Hall, 1996
- Yoshio Nakamura, ミミズと土と有機農業 (Earthworms, Soil, and Organic Farming), Soshinsha (創森社)
- Wikipedia (Japanese): "Bioturbation" (生物擾乱)
- Wikipedia (Japanese): "Earthworm" (ミミズ)
※ This article is a popular-science explainer for a general audience. The figures given are rough estimates meant to aid understanding of the underlying mechanism. The number of earthworms and the amount of soil they carry vary greatly by location, climate, and season.