Earthworms Play Vital Role in Soil Health
When it comes to soil health, conversations around biodiversity for the most part focus on microbial life forms. Despite their presence being a well-known indicator of healthy soils, the humble earthworm and it's more than 7,000 species globally, rarely receive the credit they deserve.
"Earthworms are known as ecosystem engineers because they affect not only the soil, but ultimately, how plants grow and even bird abundance," says Dr. Jackie Stroud, a soil scientist at Rothamsted Research in the United Kingdom.
According to Stroud, the presence of earthworms also plays a large part in plant productivity. Tunnels created by deep burrowing earthworms help with plant rooting depth, drainage, and soil structure. Further up in the soil profile, topsoil earthworm species improve the availability of nutrients for plant uptake through their composting of surface debris and litter.
Based approximately 30 miles northwest of London, Stroud's soil research consists of a combination of lab studies, field trials, and on-farm research. Her work examines earthworm numbers and presence in fields, as well as classifying each species into their respective ecological groups.
"This is so we can get a feeling for what the earthworms are doing in that environment," says Stroud. "I have found some soils that are overworked or underfed lose various ecological groups. They become locally extinct within the field."
Additionally, Stroud is looking at how earthworms affect soil properties and comparing the effects of different types of above-ground management practices (e.g., tillage vs. no-till) on earthworm populations.
Stroud explains there are currently about 28 different species of native earthworms present in the UK. Of those 28 species, only about ten are found in arable soils. They can be divided into the following three ecological groups:
- Deep burrowing earthworms - These earthworms are classified as "night crawlers" and inhabit burrow systems that may extend several meters into the soil.
- Topsoil earthworms - These worms are quite distinctive and usually brightly colored or quite small. They live in the upper soil strata and eat primarily soil and other organic matter.
- Surface earthworms - These are usually smaller red worms, says Stroud. They live near or in surface plant litter and are adapted to highly variable moisture and temperature conditions.
Measuring earthworm impact
To create a better way to determine earthworms' impact on soil health Stroud developed the #60minworms survey. Featuring soil sampling methods used extensively in her research, the aim of this traffic-light based tracker study is to indicate the likelihood of earthworm populations in-field, the type of each earthworm, and whether the worms are present in high numbers.
"I've been hugely humbled by the interest in the pilot worm census on farms," says Stroud. "It's taken off nationally. I've got farmers all the way from Cornwall up to northern England, Scotland, and Wales taking part."
Stroud notes, beyond having farmers volunteer their time and sending in soil samples, she's also found the conversations she's been having with them about earthworms very rewarding.
"Research papers and citations are nice," says Stroud. "But, when you're speaking to people and getting useful feedback, you're able to take action. I feel like that's been my biggest research success so far."
While Stroud's research is on-going, she points out she has come to a few early conclusions. Her work to date shows there is an obvious depletion in the surface and deep-burrowing ecological groups of earthworms in the soils she's studied, particularly in intensively-managed (tilled) soils. In addition, Stroud says, when management practices change and focus more on reduced tillage and building organic matter on the surface, this helps build earthworm populations.
"Earthworms seem to play an important role in soil structure and I'm investigating this into more detail with lab studies and more," says Stroud. "The thing is, I think everything takes quite a long time. I have a field trial that is only two years in now and we're not seeing big differences. But, in another trial that's been running five years I'm starting to see real trends and changes in soil structure and properties."
Despite these early findings, Stroud says she needs to conduct more long-term studies across a larger variety of fields before she can make any hard-line conclusions.
Creating earthworm-friendly soils
"Creating and maintaining healthy soils is very much about knowing what you have and then trying to make it the best it can be," says Stroud. "A healthy soil is one that adds value; it's not just a medium to grow plants. It's teaming with life and good at storing and moving water in the system. Ultimately, it adds value to the environment."
A great way to form a baseline on what the status of a soil's health is, Stroud describes, is to grab a shovel and head out to the field to dig a shovel-sized pit.
"This practice can tell you if you have compaction, you can look at soil structure, disturbance, see earthworms, and even determine which species you have," says Stroud. "Then go to a hedgerow or grassy marsh and compare it, see what differences there are. This will give you a good indication at where you're sitting."
All of these factors and more can tell farmers about their soils, says Stroud. Using the examples given, she explains, it is possible to compare the best case scenario, being the hedge that's never been touched, to the field or area being investigated, and determine how management practices are influencing the soil.
Once a baseline for current soil health has been established, Stroud recommends farmers look at tillage frequency and intensity in the fields they want to increase earthworm populations. While there is no rule of thumb that fits every situation, it's the habitat quality that matters. Along with tillage, farmers should be aware of the organic matter inputs (i.e., manure, fertilizer, plant litter) going back into the soil system. If not enough compostable material is being returned, rotation strategies and the types of crops grown may need to be diversified.
On the flipside, in a pasture scenario, pH can be quite important.
"Many pastures trend towards more acidic conditions over time," says Stroud. "Keep an eye on soil pH, because earthworms are more sensitive to acidic environments."
As her research advances, Stroud plans to continue to look at ways to help farmers improve soil health, reduce tillage intensity, and increase earthworm populations across the UK. She also hopes to collaborate with soil microbiologists to study the crossover between earthworms and microbial life in the soil profile.
"Earthworms eat through the soil completely changing the soil microbial community and casting it throughout the soil profile," says Stroud. "You can't do much with soil biology without an earthworm in the system. They're super important."
Learn more about Stroud's research and earthworm science at www.wormscience.org and follow her on Twitter (@wormscience).