Soils & Earthworms

Soil & Earthworms

Jackie Stroud

Research Fellow/Senior Lecturer in Farming Systems at SRUC (Leaf Innovation Centre)

jacqueline.stroud@sruc.ac.uk or @wormscience on Twitter

Earthworms are obviously involved in soil biological health, so I wondered why people aren’t already earthworm surveying. What’s stopping them? Worms are common, known for their roles in soil fertility, and associated with good soils management – what are the main obstacles?

The answer came back: <60 minutes method and dynamic interpretation support was needed. I continue to focus on removing the obstacles and to date our science-farming partnership has led to 31, 930 worms being studied. This month alone I’ve been helping people from NZ, Australia and the USA who want to know how we’ve had this unprecedented success. It has certainly been a learning curve!

Spring and Autumn worm sampling

It is not a lecture, nor a marketing campaign – the aim is to stimulate small group discussions. Scientists and farmers don’t normally interact much, but the seasonal survey creates opportunities to tackle questions as they arise. To leverage the knowledge of each other then we need to record our observations in the same way (same method, timepoint). It is impossible to be helpful when people know earthworms in terms of bird numbers when ploughing! Digging five holes per field is a small ask, considering that it puts us all on the same page.

The first event (#60minworms) was all about the method – moving away from meaningless worm counts - to checking for the three types of earthworms (earthworms are grouped by feeding and burrowing behaviours) and that worms are found in each soil pit (widespread). We collectively identified the need for midden counting, otherwise, deep burrowing earthworms (also called anecic worms) would go unreported. Middens are made by Lumbricus terrestris - they are not worm casts - but the piles of surface organic matter collected at night which overly their permanent vertical burrow. The burrows can extend over a metre deep – so this type of worm is not easily captured when digging holes! Their middens are easy to spot, and this year I’ve had reports that they are covering up to 30 % of the soil surface. These middened patches are biological hotspots - with fundamentally different carbon and nitrogen dynamics, water drainage and crop rooting patterns – the survey suggests these patches are present in 4 out of 5 arable fields. From my perspective, it was curious that 1 in 5 fields people found no evidence for the presence of this type of earthworm (or their middens). Unexplained absences have also been detected in no-tillage fields in the USA - which surprised the researchers because this type of worm is tillage sensitive (tillage disturbs their habitat and removes their surface food supply). Therefore, I reviewed the invertebrate pathology literature (spanning 1880 – 1990’s; before earthworm research was replaced by biotechnology) which revealed the vulnerability of these earthworms to castrating parasites – with high infection levels found in earthworms living in soils impacted by air pollution. I published this literature review because it is important to know parasites and pollution are a component of soil health.

Is soil health a commodity? Or is soil health supporting expert behaviours that maintain or enhance the life-sustaining components of the ecosystems that we are dependent upon?

To me, soil health is about fostering expertise through dynamic, locally adapted research. For example, the first event revealed a mutual interest in knowing more about deep burrowers, which empowered me to highlight the knowledge gaps in pollution-parasite dynamics. The second event (#30minworms) stimulated a lot of questions about pesticides – enabling me to do some interesting lab and field slug pellet and glyphosate research and resulted in good feedback which was useful to management decisions. The curious anecdote that earthworms eat slug eggs remains on the ‘to do’ list! The third event (#worldwormweek) addressed the perception that earthworm identification is difficult and unsupported – so I curated an online results portal to make it easier to share/compare results. People took part from all over the world – investing their time in digging 5 soil pits and sharing their observations – including worms living in delightfully exotic banana plantations and rice paddies.

Then I stopped

It has been very tough trying to achieve peer acceptance. To better understand the hostility from soil scientists I asked thirty of them based in the UK and Europe: what is the obstacle? There is 100 years of research on earthworms, they are known ecosystem engineers, people freely invest their time in laborious surveys which are revealing information gaps – what is the barrier to developing science-farming partnerships? The obstacle is simply negative perceptions about farmland data quality and its usefulness to science. Science management is a ‘publish or perish’ culture, so concerns about data quality means the data outputs are unpublishable. However, the scientists revealed just one change is needed for peer acceptance: a photograph of the earthworms. Most scientists would be confident in farmland data quality with photo verification, an obstacle which we can easily resolve together using the smartphones in our pockets.

As the saying goes, you only really understand something when you try to change it

People are more proficient at earthworm identification, we are leveraging the knowledge of each other and building the capacity for co-ordinated actions in soil biological health. There is decent scientific support now, for example, the dry spring has been largely unsuitable for reliable surveys, so the collective decision is that we should survey within three weeks of a 25 mm/day rainfall event in the Spring and Autumn, so I’ll be updating the sampling booklet.

Earthworm species that are found in farmlands have a lifespan of a few years to over a decade. Earthworm activities have a broad influence on agro-ecosystems including water, gas and root movement in soils, predator populations, nutrient cycling, disease persistence, plant productivity, pollution transfer and parasite transmission. This is why they have been called ecosystem engineers.

Differences in earthworm communities associated with tillage intensity was first documented in the 1930’s (absence of ‘red’ worms) but it wasn’t until the 1970’s when earthworms were grouped by their feeding and burrowing behaviours. There are three groups: epigeic earthworms which are surface dwelling, plant-litter feeding earthworms; endogeic earthworms which are topsoil burrowing, soil-feeding earthworms, and anecic earthworms, which live in deep vertical burrows, and drag plantlitter at the soil surface to plug the top of their burrow (midden).

Conventional tillage e.g. ploughing/powerharrowing is detrimental to the plant litter-feeding epigeic and anecic earthworm populations. This is because it disrupts their habitat and food supply. In contrast, tillage can increase topsoil dwelling endogeic earthworm numbers by increasing their food supply through soil mixing. This is why worm numbers are a confusing metric – because it provides no information as to the types of earthworm present, that is, if the system is functionally intact.

Invitation to survey earthworms this Autumn

We need a photographic template, so there is an upcoming competition for young farmers with creative flair to design the digital photography templates for us all to use (and win soils books in the process).