Earthworms play an important role in the health of soil. Their burrows improve drainage and enable oxygen to reach the roots of plants. They also take plant material into their burrows where it decays and improves the organic matter content of the soil. Earthworms are an important food for many animals, from beetles to birds.

Assessing earthworm populations

Take a garden spade and measure the breadth and depth of the blade in cm. Push the blade vertically into the soil to two-thirds of its depth. Do this three more times in a square configuration and then remove the soil, aiming for a neat cube. This is usually very difficult because the soil crumbles (unless it is very compacted). In either case, put the soil sample on a sheet of plastic or newspaper, tear it apart with your fingers and collect all the earthworms that you see. Put these on a tray or on plastic or paper as above.

Count the earthworms

Divide them into adults and juveniles and count the numbers in each category. This is easy because adults have a slightly swollen pinkish ‘saddle’ one-third from the head end, while juveniles are uniform along their length. You may see light and dark patches along the earthworms’ body, this is soil passing through their gut. The saddle contains the adult’s reproductive organs. Earthworms are hermaphrodite so the saddle contains male and female reproductive organs.

Next, brush as much soil from the worms’ bodies as possible and weigh them all together. If your kitchen scales are not sensitive enough, combine several worm samples into one batch and divide the combined weight by the total number of spade-fulls taken.

One hectare of land contains around 4.5 million spade-fulls of soil. If your sample contained, for example, ten earthworms then that is the equivalent of around 45 million earthworms/ha. One gram of earthworm makes one gram of new soil per year, so ten earthworms per spade-full in your garden make the equivalent of around 4.5 tonnes of soil/ha. We can buy topsoil for around $60/tonne so the earthworms in your garden make the equivalent of $270/ha/year.