In this article, Lynette Abbott from the University of Western Australia highlights the importance of earthworms to soil health
Earthworms are common soil organisms and have been noticed by many of us as they occur in gardens, parks, agricultural regions and natural ecosystems. They can be grouped by their feeding habits and life cycles, which affects where they occur in the soil profile and how they contribute to soil health.
Some earthworms live mainly in the litter layer and feed mainly on organic plant material before it is broken down into finer particles. They may prefer plant material that has a relatively high nitrogen content. Ingestion of these plant residue fragments contributes to its breakdown into smaller particles, which are colonized by other soil organisms, including fungi and bacteria.
Another group of earthworms feed on organic material from surface plants and transfer it to the soil through their underground burrows. This group of earthworms usually lays small piles of worms on the soil surface as they move back and forth from their burrows.
Other earthworms live in deeper soil layers, in more permanent burrows. These earthworms also ingest organic matter combined with soil particles and deposit worm castings in burrows, which line their walls.
In all cases, worm castings can improve soil health and increase the rate of decomposition of organic matter, due in part to enzyme activity in the earthworm’s gut.
Charles Darwin had a great interest in earthworms
Darwin’s book “The Formation of Vegetable Mould: Through the Action of Worms with Observations on their Habits” is a seminal text based on his detailed observations of earthworms in his own garden and on farms. The first edition of this book was published in 1881, and the record of information in this text is extraordinary.
Darwin lived in Kent, England and placed a “worm stone” on the lawn of his garden. He noted the time it took for the stone to be buried due to earthworm activity. Earthworms ingest and redistribute soil in the profile. Darwin’s book describes the habits of earthworms, the amount of fine soil they bring to the surface, their role in the burial of ancient buildings, and their role in developing soil profiles that lack larger rock particles.
This latter process is facilitated in part by finer soil particles (such as worm castings) being deposited above rocks and rock fragments, which effectively sink deeper into the soil profile. While earthworms are clearly more abundant and active in soils in humid environments, they have a role in the distribution of soil organic matter and in improving soil conditions across climate zones and soil types.
Earthworms in agricultural, horticultural and horticultural soils
Earthworms are common in gardens due to the high load of organic matter present after the application of compost or manure. Similarly, earthworms are commonly found in agricultural soils, which experience relatively high rainfall (and are therefore moist for much of the time) and in soils containing high levels of soil carbon, including more clayey soils with high plant densities under grassland or pasture. .
Earthworms are found worldwide but vary in abundance, primarily in response to moisture and organic matter. Their abundance in surface soil layers may decrease during the drier season as some species retreat deeper into the soil and aestivate, and others form egg capsules. Aestivation is a period of reduced metabolic activity or dormancy; when conditions improve, they become active again or recolonize the soil from egg capsules.
Disturbance of the soil through cultivation or the addition of additives such as lime or fertilizers can alter the physical, chemical and biological environment to which earthworms are exposed. Cultivating soil can reduce earthworm numbers by increasing the rate of decomposition of organic matter, reducing their food supply.
Although there may be some physical damage to earthworms caused by tillage, the indirect impact of tillage on their food source is likely to have a greater impact on their abundance. Changes in soil pH can alter soil suitability for some groups of earthworms, but not others.
The abundance of earthworms changes seasonally with the availability of different types of organic matter, and some species prefer organic resources with higher nitrogen content than others. A few species usually dominate earthworm communities in a particular location; the distribution and occurrence of the species largely depends on the plant types present, the amount of organic material, soil structure and soil moisture.
Earthworms can have a significant impact on the condition of the soil and its health for plant production. By ingesting soil and organic matter, earthworms act as natural “ploughs” and aerate the soil, redistributing soil particles, including organic matter, throughout the soil profile and providing channels for roots to penetrate deeper layers. This can facilitate root access to groundwater, or channel water deeper into the soil.
Earthworms in natural ecosystems
Although earthworms are most noticeable in well-maintained ground environments such as gardens and farms, they also occur in natural ecosystems. In fact, there is a great diversity of earthworm species in forests and heaths where moisture levels and plant debris support their activities at least some of the year. The species of earthworms that predominate in disturbed environments such as home gardens, compost piles, and agricultural soils are generally different from those that predominate in natural ecosystems.
For example, in southwestern Australia, earthworm species commonly found in agricultural soils have been accidentally introduced from Europe and end up in Cape Town, South Africa, on the early sailing ships transporting pots of citrus trees and other horticultural plants. The points of origin for the introduction of these exotic earthworm species into agricultural lands in south-western Australia were likely farmyards.
Consequently, the dominant earthworms in this region do not come from the naturally occurring earthworm community. However, there is one notable exception to this: a very large earthworm (Megascolex imparicystis, average length 21.5 cm) (1) found in agricultural soils around Dandaragan and Lancelin, north of Perth, Western Australia, has managed to survive in soils under agricultural production. This is very unusual. In contrast, another even larger earthworm (Megascolides australis, average length 75 cm) (2)a species native to South Gippsland, in southeastern Australia, does not occur in soils converted to agricultural production.
The role of earthworms in soil health processes
Earthworms play an important role in improving soil structure so that roots can penetrate the soil. They facilitate the mineralization of organic matter and release nutrients for plants. They support the development of cohesion between soil particles as they ingest soil and organic matter before releasing worm castings into the soil with improved soil health properties.
Earthworms interact with other soil organisms, improving their environment and redistributing fungi and bacteria in the soil habitat.
Soils with high earthworm abundance may have increased soil aggregation, improved bulk density, increased soil organic carbon, greater brittleness and ability to avoid erosion, improved moisture holding capacity, and improved water infiltration.
Earthworms have roles at the interface between soil physical, biological and chemical processes that support soil health and help retain soil carbon. They may occur to varying extents in different climates and soil environments, but when soils are managed to support their survival and reproduction, they contribute significantly to the health of agricultural ecosystems.
References
- Ian Abbott (1982) The distribution of earthworms in the Perth Metropolitan Area. Records of the Western Australian Museum 10: 11-34.
- Beverley Van Praagh (1992) The biology and conservation of the giant Gippsland earthworm Megascolides australis McCoy, 1878. Soil Biology and Biochemistry 24: 1363-1367.
