Earthworms are key indicators of soil health and ecosystem function so we’ve been checking on the numbers of earthworms in our soil at HCF for several years now. What have our surveys revealed? Andrew Ross explains.
Earthworm sampling is valuable at Highbridge Community Farm because earthworms are key indicators of soil health and ecosystem function. Their presence and diversity reflects soil fertility and the overall health of the ecosystem.
Earthworm activity, including their burrowing and castings, improves soil structure, aeration, and water infiltration, all of which are crucial for plant growth and overall soil fertility. The earthworms helps to create and maintain soil structure, which is essential for healthy root growth and water retention.
Earthworm sampling is one of a few ways we monitor the health of our soil. We also have a nutrient analysis conducted every two years or so, which tells us the levels of organic matter.
Three types of worm
We identify three types of worms, described below, and juveniles which have not yet developed a saddle. To find out more about how we take the sample, see one of our previous blogs, Worming our way into soil health.
Epigeic worms (surface worms)
- These worms live in the leaf litter at the surface.
- Dark, red-headed worms. They are of small size (less than 8cm).
- Often fast-moving (good for escaping from birds and most likely to escape from the worm pot!)
- Sensitive to: digging (which is detrimental) and a lack of organic matter on the surface. They are prey for native birds.
Endogeic (topsoil earthworms)
- Pale-coloured and green worms (not red)
- Small to medium size
- Often curl up when handled. These are the most common earthworm group in our plots.
- Sensitive to: Increased organic matter (beneficial)
- Roles: Nutrient mobilisation for plants
Anecic (deep burrowing earthworms)
- Milky-coloured worms, with increasing red or black pigmentation towards the head.
- Large size (over 8 cm), typically similar size to a pencil. Make deep vertical tunnels, up to 2m.
- Often found below surface earthworm casts or midden residue piles. Feed at night, foraging the soil surface around their burrow for litter.
- Often absent from ploughed fields (ploughing destroys their burrows) and where there is no surface litter.
- Roles: Deep burrows that improve aeration, water infiltration and root development.
A traffic light system to assess the results
A traffic light system to give an indication of the results has been developed by the Agriculture and Horticulture Development board (AHDB) for the sampling technique that we have used.
Comparing results over time
We have been counting the numbers of earthworms on our plots since 2019 to gain an impression of soil health. This table compares the results from March 2019, March 2022, April 2023, and March 2024 compared, with the plots on the left. The colours in the first column of the data come from the Agriculture and Horticulture Development Board’s traffic light system.
The 2025 results suggest that there are a surprising number of plots where the sample revealed very low numbers of worms (identified by the red or orange boxes).
Other plots (identified in green) seem to be doing well with several plots recording worm counts of 14-18 worms per sample.
Summary of results across the Farm
What do the results suggest?
Estimates of the total numbers of worms went down from 2019 until 2022 but they have been rising slowly since. We are not sure why. Is it changes in the weather? Or changes in the soil? Or have numbers been affected by more moles moving into the site? (Moles only move into rich areas of soil which is an encouraging sign of soil quality.) However, an ecological principle is that predators (the moles) do not determine the population size of the prey (the worms) so the presence of moles is unlikely to be the cause of the decline in worm numbers.
As you can see from the graph below, there have been changes in the percentages of each of the three types of worms and the percentage of juvenile worms, but because of the relative inexperience of some people undertaking the identification, there are likely to be a significant proportion of worm misidentifications. It is good to see a small rise in average numbers of epigeic worms per plot, which suggests that more organic matter is being left on the surface for these worms to hide in and feed on. It is also good to see a small rise in average numbers of anecic worms per plot and this may be because an increasing number of plots are adopting a “no-dig” approach and so their burrows are not being damaged by digging and remain intact. It is disappointing to see a decline in the numbers of endogeic worms.
We still cannot account for the very low total worm numbers on certain plots. In future years, we suggest that plots with very low numbers which bear a red flag are repeat sampled (perhaps by more experienced samplers) to try to determine whether this is a random sampling error or if this is uniform across the plot and the low worm numbers are the result of an unhelpful growing practice.
Earthworm sampling continues to give us valuable insights into the health of the soil that we need for successful crops. Please let Andrew Ross know of any thoughts you have.