Earthworm populations can be surveyed using the hot mustard extraction technique.
- About 4 hours before sampling in the field (or alternatively, the day before sampling), weigh 53 grams of hot mustard powder into a glass jar or plastic container with a leak proof lid. Add about125 ml of water (1/2 cup), 50 ml (about 1/5 cup) at a time, stirring constantly with a spatula until a paste forms that is thick but pourable and free from lumps. Place the lid on the container. The hot mustard paste must stand for at least 2 hours so that the "hotness" can develop.
- At the field site, select a suitable sampling area and place a wooden frame 0.5 m x 0.5 m square on the ground surface. Use a shovel to bank the soil up against the outside of the frame - this will prevent the water-mustard mixture from escaping.
- Add about 3 L (2.5 qts) of water to a household plastic bucket. Add the mustard paste to the water in the bucket. Stir vigorously to remove any lumps. Add water to the bucket up to the 7 L (6 qt) mark. Stir vigorously.
- Gradually pour the water-mustard mixture to the soil surface. Worms should appear within 1 to 2 minutes and all activity will end in about 20 minutes. Wait until each worm completely emerges or is nearly finished emerging before picking it up with the tweezers. If picked up too soon, the worm will pull back into its burrow and any tugging or squeezing will result in a ripped worm.
- If the worms are to be returned to the field, the worms should be rinsed in clean tap water before releasing.
Note: Some people's skin may be sensitive to the hot mustard, so it is advised to wear latex or rubber gloves during the mixing and field preparation.