Earthworms are a vital part of soil ecosystems and can be thought of as the ‘canary in the coal mine’ as they provide an indication of soil health.
Beneath our feet the earth is teeming with these burrowing invertebrates and their role in forming healthy soils is strongly documented. Charles Darwin, a world renowned English naturalist, geologist and biologist, understood the vital importance of earthworms and studied them throughout his life.
Earthworms are hermaphrodites having both male and female reproductive organs. Some smaller earthworms can reproduce after a few months, and once fully mature, they can continue to produce several egg cocoons per day until their lifespan ends, usually after a year or two.
The old tale is that if you cut an earthworm in half both ends will survive. Earthworms do have an enormous power to repair damage to themselves, however, both parts of the earthworm will only survive depending upon where the cut has been made on its body.
Earthworms enjoy dark and damp places because their skin is permeable and allows water to pass through. They are most active in the spring and autumn when the soil is moist. When soil is dry during the summer months, earthworms go deeper down into the profile and some smaller species may even die out. However, their egg cocoons remain and are ready to hatch baby earthworms when moisture is present again.
Earthworms play a vital role in the biological health of our soil. Their activities of feeding, burrowing, digesting organic matter, casting decomposed plant matter and converting that plant matter into humus (decomposed organic matter) improves soil structure and makes nutrients plant available. Castings contain plant available nitrogen (N), phosphate (P), potassium (K), calcium (Ca), and magnesium (Mg). The earthworm’s natural activities bring mineral soil to the surface and takes organic matter down into the soil as food. This promotes pasture growth by encouraging root growth down into the nutrient rich channels made by the earthworm.
Counting the number of earthworms is an easy activity and an indicator of your soil’s health. For year-to-year comparisons, count earthworms at the same time and place each year (preferably in late winter or early spring when soil moisture is adequate) and avoid dry soil conditions. Use a 20 cm wide spade and remove a 20 cm cube of soil. Count the earthworms in this cube and then multiply by 25 to convert the number of earthworms per square metre.
According to the Visual Soil Assessment guidelines by Graham Shepherd1, greater than 45 worms is a good score while fewer than 15 is a poor score, though this depends on soil type and its organic matter content1.
For more information on assessing your soil health, contact your local PGG Wrightson Technical Field Representative.
¹Shepherd, Graham. (2009). Visual Soil Assessment. Pastoral grazing and cropping on flat to rolling country. Second Edition.