Extension Ag Update
January/February 2003
Articles Research Resources Internet Links Ag Facts Education

Assessing Soil Quality: Counting Earthworms

Ellen Phillips, Extension Educator – Crop Systems, Countryside Extension Center, 708-352-0109, ephillps@uiuc.edu

Earthworms are most active at the soil surface during the spring and fall.  The best time to observe their activity is when soil temperatures are below 65 °F.    If you are looking for earthworms, you may need to sample in different locations since their habitats tend to be patchy within a field.  Avoid places where their populations might be affected, such as very wet spots in the field. Count earthworms several times during a season and use the average to gauge changes from year to year.

To consistently count earthworms, measure a square-foot area and dig out the soil down 12 inches with a hand trowel or shovel.  Remove as much soil as possible with each full shovel to avoid hurting the earthworms.  Count the number of earthworms.  Keep in mind some species of earthworms can be red, white or gray and vary in size from very small to the very large eight inch long nightcrawlers.  To count those deep burrowing nightcrawler earthworms, you may have to pour a mustard solution (2 tablespoons mustard powder in one gallon of tap water) down any burrows you see.  The mustard solution should not harm the worms.  In about five minutes, the worms will begin to appear.  Make certain to give them a bath to rinse off the mustard solution with water, before returning them to the soil.

So how many earthworms did you find?  You probably did not find all the deep burrowing or fast moving earthworms.  Ten earthworms per square foot of soil is considered a good population in agricultural systems. In cultivated soils, it is rare to find more than 20 earthworms per square foot of soil. Tillage can kill around 25% of the earthworm population. Earthworm populations are often larger under no-till than under conventional tillage. In grassland systems, populations can reach as high as 50 earthworms per square foot of soil.  It can take up to five years of providing a good soil environment for earthworm populations to increase.
Many things can affect earthworm populations. Earthworms prefer loamy textured soils more than sandy or clay soils. The majority of earthworms live best in soils with a pH between 5.0 and 7.4. Soil moisture, either too much or too little, dramatically impact earthworm distribution and their activity.  The optimum temperature range for earthworms is between 50 and 70 °F. Temperatures below 32°F or higher than 80 °F can begin to kill earthworms.  Most earthworm species have physiological or behavioral adaptations that help them survive unfavorable conditions for a while.

Many of our management decisions can dramatically affect earthworm populations.  Cropping systems high in organic amendments and residues provide food for earthworms and will increase the distribution and populations of earthworms.  Ammonium sulfate, anhydrous ammonia, and sulfate coated urea have all been shown to decrease earthworm populations.  Some pesticides are toxic to earthworms. Triazine herbicides are slightly toxic. Carbamate-based fungicides are very toxic.   Organophosphates and many carbamate-based insecticides are toxic. Most of the nematicides are also toxic to earthworms.  To increase earthworm populations, provide food for them by adding organic amendments and residues while decreasing tillage and choosing pesticides that are less toxic to earthworms.