As part of our feedback survey we asked if there were any comments or questions. One question given was why Earthworm Watch uses mustard, if it harms the earthworms and if there are less disruptive methods that could be used. This blog post aims to answer this.

Many scientific studies on earthworms over the years have found that sorting through soil does not find all the earthworms. In particular, deep-living earthworms may occur too deep in the soil, or hide in their burrows when they sense the soil is being disturbed. To count these types of earthworms a solution, called a vermifuge, is poured onto the ground, or into a hole which has already been sorted for earthworms. In the past scientists used harmful chemicals such as potassium permanganate or formalin to bring the earthworms up to the surface, nowadays, we use a mustard solution. Mustard powder contains allyl isothiocyanate which makes mustard taste 'hot'. As the mixture soaks in the soil the earthworms crawl up to avoid it but it is not harmful to them or the soil – especially as we then wash the earthworms with water.

Studies have found that mustard water is just as good as the old harmful chemicals for finding earthworms – if we don’t use mustard water we would miss nearly half the deep-living earthworms! Deep-living earthworms are particularly important for healthy soils as they make deep burrows which help water drainage so we need to have accurate numbers.

But my mustard water didn’t work 

You may have been disappointed not to find any earthworms after using mustard water – don’t worry! Whether the mustard works depends on what type of soil you have and how wet the soil is – if the water table high the solution will not sink into the soil so will not work as well. Of course, there could be no earthworms there to begin with! To try and minimise differences between participants we still would like people to use the mustard and let us know if none come up – this is still a result.

Do you have any any questions on Earthworm Watch? Please email us at