This weeks blog takes a look at the insights from a research article published in 2023 by Schon et. al, titled ‘Earthworms for inclusion as an indicator of soil biological health in New Zealand pastures’. The research was undertaken by AgResearch and Plant & Food Research, with funding from Beef and Lamb’s Hill Country Futures programme. The focus of this research was to produce a review that ‘collated the knowledge of earthworms in New Zealand, to contribute towards the development of a biological indicator of soil health for pasture soils that could be used by farmers and their consultants’.
Why is this sort of research important?
Agriculture in New Zealand is quite literally built on the foundations of soil. The health of that soil is therefore central to the success of farming businesses, the sector itself, and the provision of ecosystem services for people and planet.
As Schon et al. (2023) state, soil health has three components – physical, chemical and biological. All three of these components are essential for ensuring healthy soil, and therefore are also all essential for resilient and efficient production systems. In New Zealand most on-farm assessment of soil health and quality is undertaken through a chemical lens, which involves measuring components such as soil fertility, organic resources, and pH. There has also been an increase in the use of Visual Soil Assessments (VSA’s) to monitor the physical components of soil health. While understanding the chemical and physical components of soil health is important, there has been a lack of accessibility to techniques to provide insights into the biological health of farmers soils. In part the challenge for this has been contributed to by the continued need to understand how soil biology responds to management and environmental conditions, which will allow better understanding of how indicators could be developed. Despite this, it is clear that it is necessary to be able to monitor the biological components of soil health.
Schon et. al (2023) identify that microbial biomass, nematode abundance, and earthworm abundance and diversity are the common biological indicators for soil health. Commercial tests for biological soil health in New Zealand are very limited, can be very expensive, and some may not necessarily be suitable for New Zealand soils. However, earthworms provide an indicator for soil health that is inexpensive, and accessible for farmers to learn about and monitor themselves. It is hoped that the use of a biological indicator, such as earthworm abundance and diversity, could be used in conjunction with physical and chemical indicators of soil health to produce a more complete understanding of soil health on farm.
What is the role of earthworms in soils?
Earthworms have great importance within soil, and contribute in ways such as:
- playing a role in soil formation (microaggregates, soil porosity),
- facilitating nutrient cycling through decomposition and increasing nutrient availability,
- feeding on organic matter and redistributing it throughout the soil profile through worm casts,
- nutrient supply associated with burrowing encourages root growth,
- increasing soil porosity, and
- improving water infiltration rates and water holding capacity.
As stated by Schon et al. (2023), the overall result of earthworm activity commonly leads to increased plant growth, with benefits increasing alongside increasing earthworm abundance.
There are three ecological groups of earthworms:
Dung earthworms (Epigeic): these earthworms live and feed on organic matter near the soil surface, playing an important role in organic matter decomposition (Schon et al. 2023). They contribute to the ecosystem services of plant growth, nutrient supply, and greenhouse gas regulation (Figure 1).
Topsoil earthworms (Endogeic): these earthworms burrow extensively through the topsoil, forming semi-permanent burrows. They feed on organic matter within the soil, which enhances nutrient availability for plants (Schon et al. 2023). They contribute to the ecosystem services of plant growth, water and air movement, flood mitigation, and physical support (Figure 1).
Deep-burrowing earthworms (Anecic): these earthworms are larger and form semi-permanent burrows deeper into the soil. As they burrow deeper they incorporate organic matter deeper into the soil profile, as well as aid water infiltration (Schon et al. 2023). They contribute to the ecosystem services of plant growth, water and air movement, flood mitigation, physical support, nutrient supply, and greenhouse gas regulation (Figure 1).
Figure 1 explains the key roles of the three ecological groups of earthworms, as well as identifying the species of worms within each of the three ecological groups.
In a New Zealand agricultural context, earthworms are mostly exotic (introduced) species. Their abundance varies widely from zero to over 1000 earthworms per m2. While New Zealand also has nearly 200 species of native earthworms, these are generally confined to our native forests and other undisturbed areas.
Absence of earthworms from pastoral land can be a result of two factors: 1.) previous land use, and 2.) the accidental nature of the introduction of earthworm species to New Zealand. Schon et al. (2023) emphasise that areas of North Island hill country, and much of the South Island, only have dung and topsoil earthworms. In an ideal world, pastoral soils would have the presence of dung, topsoil, and deep-burrowing earthworms to fully optimise the role of earthworms in soil function and health, however, this isn’t currently achievable in many parts of the country due to the absence of the deep-burrowing earthworm group.
How can earthworms be used as an indicator of soil biological health?
Assessing the diversity and abundance of earthworm populations on farm provides a way to gain insights into their role in farm soil health. The great thing about collecting earthworm data is that it can fit in with your transect sampling for soil fertility and/or Visual Soil Assessments. Please note: this methodology, and the information it provides, is intended for permanent pastures.
There are some key considerations for sampling earthworms:
- To allow accurate monitoring and comparisons over time, sampling needs to be undertaken during the same month each year. As earthworms are seasonally active, it is also important that sampling is undertaken during their active period which is generally June – September.
- It is important to account for spatial variability of earthworm populations on farm. This can be done by sampling in each major land management unit on the farm, which takes into account factors such as differing soil types, vegetation, livestock types, and grazing practices. This is fits in well with both soil fertility and VSA methods.
Earthworm populations can be sampled through hand-sorting. This involves digging a soil turf sample that is 20 x 20 x 20 cm using a spade. The size of the soil turf sample important to know as it is used to infer what the earthworm abundance will be per square metre. If you’re already undertaking visual soil assessments, this works in great with that technique that also assesses earthworm presence and size.
When hand-sorting, carefully collect the earthworms from your soil turf sample and put them in water. When you’ve finished collecting all the worms, you can then assess the abundance and diversity.
- Earthworm abundance = the total number of all immature and adult earthworms collected from the soil turf sample. Record the total number of earthworms.
- Earthworm diversity = assess the adult earthworms (those with a saddle). Look at their size and colour to identify which of the three ecological groups they belong to. Record how the total number of earthworms for each of the three ecological groups.
The flow chart below (Figure 2) shows how you can assess what the abundance and diversity of earthworms in your soil sample indicates. In order to calculate the number of earthworms present per m2 , multiply the value by 25.
Schon et al. (2023) recommend that target earthworm abundance is greater than 400 earthworms per m2 . When it comes to diversity, they state it would be desirable that all three ecological groups of earthworms are present at abundances greater than 25 earthworms per m2 , however as stated above deep-burrowing earthworms are not present right across New Zealand.
For more detailed instructions on undertaking earthworm abundance and diversity, you can download the pdf instructions via the following link: https://www.agresearch.co.nz/our-research/the-great-kiwi-earthworm-survey/
Enhancing Earthworm populations in permanent pastures
While earthworm abundance and diversity is impacted by a range of factors, it is possible to enhance the abundance of earthworm populations on farm through management practices. This can be done through approaches such as:
- enhancing organic matter through increasing soil fertility and pasture production,
- utilising grazing practices that minimise the impact of animals on the physical soil environment
- reducing moisture limitations through irrigation,
- reducing both nutrient and moisture limitations through effluent application,
- improve drainage of water-logged soils,
- ensuring that pH is in the optimum range for pastures (pH 5.8-6.0).
The absence of all three earthworm ecological groups is not always a sign of an impact of farm management practices. A good way to tell if you have all three groups on farm is to compare your earthworm diversity results from your different land management units and samples.
In summary...
Soil health is of importance to farmers and agriculture. We have been accustomed to looking at soil health through a chemical lens, however physical and biological components of soil health are equally as important. While physical components are beginning to be assessed more using Visual Soil Assessments, there has been a lack of biological indicators that farmers can use to assess pastoral soil health.
Earthworms have great importance within soil and contribute in many ways such as nutrient cycling, soil formation, increasing infiltration and porosity, and more. There are three ecological groups of earthworms which all provide benefits to the soil environment.
Assessing the abundance and diversity of earthworms on farm provides a way for farmers to gain insights into the biological health of their pastoral soils. This technique fits in well with other sampling methods used for assessing chemical and physical components of pastoral soils.
Management practices can enhance the abundance of earthworms in our pastoral soils. They can also lead to a reduction in abundance.
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