What can worms tell us about our soils?

Worm © Alan Price

Earthworms are one of the most overlooked components of a healthy ecosystem - there is far more to the humble earthworm than most people imagine!

Earthworms are ‘ecosystem engineers’ – their actions on the physical environment around them has a direct effect on its structure and the other organisms that inhabit it. They break down plant matter and release nutrients into the soil, making these available for soil micro-organisms and fungi to utilise. Their burrows allow oxygen and water to penetrate the soil, improving drainage after heavy rain and feeding plant root-systems. They mix soil layers and bring organic matter down into the soil, improving fertility.

Their behaviour fascinated Charles Darwin who called them ‘nature’s ploughs’. His 40 year study culminated in the publication of a book: ‘The Formation of Vegetable Mould through the Action of Worms’ which was more popular than the more famous ‘Origin of Species’ in his lifetime!

A handful of small earthworms

©Mariko Whyte

All this means that earthworms are excellent indicators of soil health, and the importance of their ecological role is a rapidly growing area of research as our ever growing global population relies on continued agricultural productivity.

A project I am working on this year aims to look at the differences in earthworm abundance and diversity in arable farmland and relating this to differences in management and crop type. This study is being conducted at one of Vitacress Ltd.’s farms using a protocol developed with Sparsholt College and the Trust.

Simply speaking, the survey work consists of digging five sample holes in each of 10 arable fields and counting the earthworms present in the soil. The adults are further separated into four functional groups, with any juvenile or damaged earthworms counted separately.

A soil pit dug in an agricultural field

©Mariko Whyte

There are 31 species of earthworm in the UK which vary greatly in size, appearance and habitat preference. The functional groups relate to the ecological role and general behaviour of the worms and can be summarised as follows:

Anecic worms: Usually very large, these worms live in deep permanent vertical burrows in the soil and pull leaves and organic debris from the surface down into the earth. They are the ones that leave casts – mounds of excreted earth – at the surface and when you see a blackbird or thrush really struggling to pull a worm from the ground, chances are it’s one of these!

Apporectodea longa - an anecic earthworm

©Trustees of the Natural History Museum

Apporectodea longa - an anecic earthworm

Endogeic worms: These tend to be pale in colour and live below the soil surface in horizontal burrows. They rarely come to the surface and eat and extract their nutrients from the soil. One of the most easily recognisable species in the UK is the green worm – which, as its name suggests, is often greenish in colour.

Allolobophora chlorotica - an endogeic earthworm

©Trustees of the Natural History Museum

Allolobophora chlorotica - an endogeic earthworm

Epigeic worms: These are surface dwelling and found under rotting leaves and in logs where they feed directly on organic matter. They don’t tend to burrow and are usually a dark red colour. Adults of different species can be between 1 cm and 18 cm in size.

Lumbricus castaneus - an epigeic earthworm

©Trustees of the Natural History Museum

Lumbricus castaneus - an epigeic earthworm

Compost worms: These are bright red worms, often with a stripy appearance giving them the nickname ‘tiger worms’. They prefer areas of really high organic content and are frequently found in compost heaps or under cow pats in fields. If you have a compost bin, these are the ones you’ll find in it.

Dendrobaena veneta - a compost earthworm

©Trustees of the Natural History Museum

Dendrobaena veneta - a compost earthworm

And how can you tell if a worm is an adult? – surprisingly easily!

Adult worms have a saddle – a thickened band – closer to the head end of the body which is used during reproduction. In some worms this is very clearly a different colour to the rest of the body. In others it is harder to spot and may just appear to be a slightly different texture to the rest of the body surface so you need to look carefully. In juvenile worms the saddle is completely absent.

Over the summer, I will be counting worms in each of the 10 fields over three different survey periods to see what the changes are as the season progresses and the crops and cover crops develop.

The aim of the work is to be able to compare earthworm abundance and diversity with past cropping and management data, as well as chemical measurements of soil fertility to find out what the worms can tell us about overall soil health. It will be exciting to see what we find out, and I will post an update later in the year about the project and the results we are getting. 

In the meantime, if you are interested in finding out more about worms and their importance to us, take a look at the website for the Earthworm Society of Britain.

Worms are also the focus of this year’s Wild About Gardens challenge from the Wildlife Trusts and Royal Horticultural Society so visit the website for more information and to find out what you can do to go #wildforworms in your garden.

And next time you unearth a worm, take a closer look and see if you can tell whether it is an adult or a juvenile, and which end is its head!