Earthworms improve plant productivity, are principally responsible for engineering the soil environment and are an important food source for native birds such as the song thrush. There are up to 10 common earthworm species in agricultural soils and these can be grouped into three ecological types: epigeic, endogeic and anecic earthworms – each group having a unique and important function. Earthworms are an indicator of soil health, being impacted by pH, waterlogging, compaction, tillage, rotation and organic matter management.
How to identify earthworms
Epigeic (litter-dwelling earthworms)
• Dark red-headed worms
• Small (<8cm) in size, typically about the length of a matchstick
• Often fast-moving (most likely to escape from the worm pot!)
Sensitive to: Tillage (detrimental) and organic matter management such as manure applications (beneficial)
Roles: Carbon cycling and prey for native birds
Endogeic (topsoil earthworms)
• Pale-coloured and green worms (not red)
• Small to medium size
• Often curl up when handled, and green worms may emit a yellow fluid
• The most common earthworm group found in arable fields
Sensitive to: Organic matter management (beneficial)
Roles: Soil aggregation and nutrient mobilisation for plants
Anecic (deep burrowing earthworms)
• Dark red or black-headed worms
• Large size (>8cm), typically similar size to a pencil
• Make deep vertical tunnels, up to 2m
• Often found below surface earthworm casts or midden residue piles
• Feed at night, foraging the soil surface around their burrow for litter
• Commonly found in grassland but often absent from ploughed fields and where there is no surface litter
Sensitive to: Tillage (detrimental) and organic matter management such as manure applications and straw return (beneficial)
Roles: Deep burrows that improve aeration, water infiltration and root development
Identifying adults and juveniles
Adult earthworms have a clearly developed saddle (reproductive ring) and juveniles do not. You may need to rinse worms with water to determine if a saddle is present. Size is not a good indicator of maturity as adult earthworms typically range in size from 2cm to 15cm, depending on species.
When is it best to count earthworms?
Spring and autumn are the best times to carry out earthworm assessments. Timing the sampling after warm, wet conditions often provides the best earthworm population estimates.
How to assess the earthworm populations
Tools: Spade, pot, bottle of water, mat and a record sheet
1. Dig out a soil pit (20cm x 20cm x 20cm) and place soil on mat
2. Hand-sort the soil (aiming to spend 5 minutes sorting), placing each whole earthworm into the pot
3. Count and record the total number of earthworms
4. Separate earthworms into adults and juveniles (see above)
5. Return juveniles to the soil pit 6. Count and record the number of each type of adult earthworm (see overleaf)
6. Count and record the number of each type of adult earthworm (see overleaf)
7. Repeat steps 1–7, until 10 soil pits per field have been assessed following a standard W-shape field-sampling pattern.
What does this mean?
• RED = If 3 or fewer of the ten pits have 16 or more worms, this suggests suboptimal earthworm populations, which can indicate problems with the soil’s physical or chemical properties
• YELLOW = If 16 or more worms are found in 4 to 6 pits, this suggests a patchy presence of earthworms. With this number, you could make improvements in the parts of the field where earthworms are not currently present
• GREEN = If you have 7-10 pits containing 16 or more earthworms (of any type). The most significant benefits to plant productivity are more likely in fields where you find high numbers of earthworms
• If you are unlikely to find epigeic, endogeic or anecic earthworms, you are unlikely to be benefiting from their specific actions