Your concerns are legit.
Tilling can kill the hardworking beneficial earthworms in your garden.
Even to shallow depths, tilling can greatly reduce the earthworm population and changes their diversity in the soil. Anecic and epigeic earthworms are most affected because tilling can injure them, remove their food source, and change the conditions of their habitat (soil temperature, moisture, organic matter content).
In this article, we will examine the controversial question of tilling your soil from the perspective of earthworms, how the practice affects them, and which types are most affected.
Let’s get started.
How is tilling done?
Tilling involves lifting and turning the soil over and breaking down compacted soil to a finer texture ready for sowing, for controlling weed growth, and for incorporating fertilizer or plant residue after a harvest. It can be done by hand, animal, or machine.
Conventional tilling done by a machine can be as deep as 36 inches (90 cm), depending on the desired results and the tool used.
Tilling differs from digging which involves making trenches 8 – 12 inches (20 – 30 cm) deep by removing portions of the soil using a spade or pitchfork. The removed soil and the base of the hole are further broken up with a pitchfork, and then the hole/trench is backfilled with the removed soil.
Of the three types of earthworms, tilling affects the anecic species (deep burrowers) and the epigeic species (litter dwellers) more profoundly than the endogeic ones (soil dwellers).
1. Anecic earthworms
Anecic earthworms are the species most affected by tilling because they feed on decaying organic plant matter and animal manure in the top layer of the soil.
Known as “deep burrowers”, they dig vertical channels or burrows as they pull the food from the topsoil back to where they live 4 to 6 feet (1.2-1.8 meters)below the surface. Tilling, however, destroys these channels.
They are also the slowest to reproduce or replenish their declining population, compared to the other earthworm species.
Anecic worms are perhaps the most commonly recognized earthworm species. They are reddish-brown and quite large, with adults measuring 4 to 6 inches (10 to 15 cm) long.
An example of anecic worms is the Lumbricus terrestris (commonly known as “garden worms” or “nightcrawlers”).
2. Epigeic earthworms
Epigeic earthworms are also affected by tilling because turning the soil alters the conditions (moisture, temperature) of the topsoil where these earthworms reside in.
They differ from anecic earthworms that they rarely burrow deep into the soil and reside in the top layer where the decaying organic matter is. They also feed in the topsoil. This is why they’re also called litter dwellers.
Although their population is affected by tilling, these small earthworms can reproduce quickly.
Epigeic earthworms are responsible for composting organic litter on the soil surface, as they ingest and excrete organic matter to speed up the decomposition. Adults are generally between 0.5 and 2.8 inches (1 to 7 cm) long. They have a dark red-brown coloring, which is believed to offer some protection from UV rays.
Common epigeic earthworms include red wigglers (Eisenia fetida) and red earthworm (Lumbricus rubellus).
3. Endogeic earthworms
Compared to the other two types of earthworms, endogeic earthworms are the least affected by tilling practices because their food source is mineral soil, rather than decaying organic matter in the top layer.
Also, they live at a deeper level of the soil, about 10 to 20 inches (25 to 50cm) below the soil surface in the mineral layers of the soil. They make horizontal tunnels as they travel through the mineral soil for feeding.
Even though their population may be affected, they reproduce relatively quickly.
They have a pale yellow, gray, or white color because they do not move to the topsoil and see sunlight. And they take on the color of the soil when they ingest it.
They are medium-sized, with adults measuring between 1.5 and 4 inches (4 to 10 cm).
Common endogenic species include Aporrectodea caliginosa and Octolasion tyrtaeum.
Tilling the soil definitely reduces the earthworm population.
According to an analysis of 165 field studies of earthworm populations around the world, there is an average increase of 196% in earthworm biomass in no-tillage fields, compared with fields with conventional tillage (Briones and Schmidt, 2017).
Other studies show a similar conclusion that no-till fields had at least 2 times as many earthworms (Pennsylvania State University, 2013), or even up to 9 times (Chan, 2001), compared to fields with tillage practices.
Even when conventional tillage is stopped, earthworm populations would on average take about 10 years of no-till or reduced tilling for earthworm populations to be restored to optimum levels although this can be sped up if plant residue is retained in the soil.
Tilling the soil, even in shallow depths, can lead to a decline of the earthworm population because turning the soil eliminates soil cover, thereby reducing soil moisture, increasing soil temperature, exposing them to predators, and taking away their food source.
A study on the earthworm biomass in the top 2 feet (60cm) layer of soil over three years found that shallow tillage and herbicide use decrease the earthworm population because removing ground cover exposes the soil to high fluctuations in temperature and moisture (Vrsic et al., 2021).
In contrast, soils with mulch and green cover maintain the highest earthworm population because they can better maintain a rather constant level of moisture and soil temperature, and the plant residue provides a source of food for earthworms.
For conventional tilling which can reach depths of 8 to 12 inches (20 to 30 cm), the mechanical operation of a moldboard plow or similar tilling equipment can directly injure and kill any earthworms in its path, particularly surface dwellers and those that move vertically between soil layers. It can also destroy their cocoons before new worms hatch.
Additionally, the earthworms that are brought to the surface by the plows can be easily picked up by predators like birds and rodents.
Furthermore, tillage removes the food sources of litter feeding earthworms.
In the case of anecic worms, tilling also disrupts the soil structure, destroying their homes/burrows. And as these worms are typically weaker than others, they may not have the energy to form new burrows to their food source, causing them to starve (USDA, 2001).
Tilling changes the diversity of earthworms
Prolonged tillage can change the composition of the earthworm population in the soil, resulting in a disproportionate number of one type of earthworms compared to the other types.
Tilling equipment mostly harms epigeic and anecic earthworms and destroy their cocoons. On the other hand, endogeic earthworms have little contact with the surface and their horizontal burrows are found in the deeper mineral layers about 10 to 20 inches below (25 to 50cm). Such depths are usually below the reach of the tilling equipment which generally digs to depths not exceeding 12 inches (30 cm).
Endogeic earthworms are also less affected because they feed on soil particles. In fact, their population even increases under conventional tilling.
An imbalance in the population of the three types of earthworms can affect soil drainage, as the deep vertical burrows of anecic worms help increase water penetration, promote root growth, and improve soil drainage.
The absence of anecic worms also affects the incorporation of organic matter down into the soil profile as they pull surface litter into their burrows. There will also be less decomposition of organic matter and humus formation, leading to a reduction in microbial activities (Briones and Schmidt, 2017).
Even after tilling is stopped, earthworm population diversity will take some time before balancing out again. This is because epigeic worms are short-lived, while anecic worms live longer but reproduce more slowly.
So now there is another reason why we should avoid tilling our soil, or at least do it when the soil is dry and under warm temperatures if you really need to. That’s because you can be disturbing or even destroying their food source and habitat, causing a decline in their population and diversity in the long run.
Briones, M. J. I., & Schmidt, O. (2017). Conventional tillage decreases the abundance and biomass of earthworms and alters their community structure in a global meta-analysis. Global Change Biology, 23(10), 4396–4419.
Butt, K. R., & Lowe, C. N. (2010b). Controlled Cultivation of Endogeic and Anecic Earthworms. Biology of Earthworms, 107–121.
Chan, K. (2001). An overview of some tillage impacts on earthworm population abundance and diversity — implications for functioning in soils. Soil and Tillage Research, 57(4), 179–191.
Edwards, C. A. & USDA. (n.d.). Earthworms. United States Department of Agriculture.
Faber, F., Wachter, E. & Zaller, J.G. (2017). Earthworms are little affected by reduced soil tillage methods in vineyards. Plant Soil Environ., 63, 257–263.
Pennsylvania State University. (2013, September 5). Why are Earthworms Important? PennState Extension.
University of Minnesota. (2021). Tillage implements. UMN Extension.
University of Minnesota. (n.d.). Earthworm Ecological Groups. UMN Duluth.
USDA. (2001, June). Agricultural Management Effects on Earthworm Populations. United States Department of Agriculture.
Nieminen, M., Ketoja, E., Mikola, J., Terhivuo, J., Sirén, T., & Nuutinen, V. (2011). Local land use effects and regional environmental limits on earthworm communities in Finnish arable landscapes. Ecological Applications, 21(8), 3162–3177.
Vršič, S., Breznik, M., Pulko, B. & Rodrigo-Comino, J. (2021). Earthworm Abundance Changes Depending on Soil Management Practices in Slovenian Vineyards. Agronomy, 11 (6), 1241