Earthworms are an indicator of biological life in the soil, large and small. Easily visible and easy to count, higher numbers can be an indicator of improved soil health.
It is best to count earthworms in early Autumn or late Spring. Avoid carrying-out during extreme weather conditions eg: very dry, hot or cold, or in the following few weeks after a manure/compost application.
Want to learn to identify the 3 different types of earthworms?
Here you can learn about the 3 types, why counting different types is important and take the brilliant quiz from soil scientist Jackie Stroud.
This earthworm guide from AHDB is also helpful.