As farmers begin preparing for the upcoming growing season, one question they should ask themselves is, “How is my earthworm population?” 

It is easy to determine whether you have an adequate population of earthworms in your soil. 

Look for their casts in the forms of little piles of soil, mineral particles, or organic matter at the soil surface.

You can also, dump a spade full of moist soil into a bucket or onto a sheet of plastic, and sort through for earthworms. To find the deep burrowing species, pour a dilute mustard solution onto the soil. 

Many will quickly come to the soil surface in response to this irritant.

Scientists estimate that there are approximately 3,000 species of earthworms worldwide. 

The number of species in North America is estimated to be 187, of which 35% are non-native, introduced species. Distribution of earthworms across the globe is wide and ultimately controlled by properties such as soil temperature, moisture, pH, and texture. 

At the field or landscape scale, their distribution is extremely patchy or highly variable as a result of the significant influence of these soil properties.

While the relationship between earthworms and soils has been studied for many years, advances in science and technology have only recently allowed us to understand the potential magnitude of their impacts on soil–plant systems. 

Because they actively change the soil environment in which they live, through burrowing and the excretion of casts (spherical aggregates containing soil particles, mucilage, and organic matter), earthworms influence the numbers and activities of other soil organisms and important processes such as the conversion of organically bound nitrogen to plant-available forms. 

The ability to restructure and modify the soil environment has led to the widespread acknowledgement of earthworms as “ecosystem engineers.”

Through feeding, burrowing, and casting, earthworms can significantly alter soil physical, chemical, and biological properties. Earthworms ingest soil particles as they burrow through the soil. 

As ingested soil aggregates move through an earthworm’s gut, they are largely destroyed, mixed with mucilage and gut bacteria and excreted as relatively stable. 

Earthworm casts generally have higher water storage, aggregate stability, and nitrogen and carbon mineralization rates than do non-cast aggregates. 

Casts can make up a significant portion of the soil when high densities of earthworms are present and strongly influence soil behavior, including creating hot spots for nutrient cycling. 

Earthworms can ingest up to 25% of the soil in an A horizon soil (organic matter enriched-mineral soil or topsoil) in a year, and casts have been reported to make up more than 90% of A horizon material in soils with very high earthworm density.

Thus, over time, the accumulation of casts can lead to surface soil with greater nutrient-cycling capacity, water-holding capacity, and structure favorable to crop growth.

Agricultural practices directly influence earthworm habitats, primarily through the types of inputs used, the crops grown, and mechanical operations. 

Overall, agricultural practices influence two main factors:

(1) the quality and quantity of food available to earthworm populations and

 (2) the amount of disturbance caused to their habitat. In general, reducing tillage disturbance or adopting no-till practices results in larger earthworm population density. In a side-by-side comparison, the density of earthworms ranged from 22–42 earthworms (per one square meter) in chisel-plowed soils compared with 62–111 earthworms (per one square meter) under no-till after three years. 

This study, suggests that reduced tillage can nearly double earthworm density in as little as three years. This result is likely due to differences in direct mortality, environmental conditions, or crop residue (food source) between the two tillage treatments.

Returning high levels of plant biomass to the soil (cash crop residue, cover crop biomass, etc.) or using organic-based fertilizers (i.e., manure) provides good sources of food for earthworms. 

Research has shown that earthworms are more resilient to tillage disturbance when abundant, high quality food sources are available. 

Earthworms also tend to be more resilient to disturbance if the tillage is shallow and if the disturbance occurs while populations are dormant, this might be in the fall when soils are either dry or too cold for earthworms to be active. Avoiding tillage during the spring and after fall rains will also help limit damage to earthworm populations.

Another important management practice to consider is the use of pesticides and their associated impacts on earthworm populations.

Much of the research suggests that pesticides can decrease earthworm population density and impact their physiology and behavior. However, the impact that pesticides have on earthworms depends on several factors, including the specific chemical compound, concentration of the compound, earthworm ecological class, activity levels of the earthworm, and the vertical distribution of earthworms during application. 

Although research on the direct and indirect effects of different classes of pesticides on earthworms is somewhat limited, insecticides appear to effect mortality most strongly, whereas fungicides appear to impact reproduction to a greater degree.

The pesticide families reported to be most harmful to earthworms include nicotinoide, carbamate, and oganophosphate insecticides; strobilurin and triazol fungicides; and sulfonylureas herbicides. However, studies showed that glyphosate (Roundup) did not significantly harm earthworms.

Earthworms are important to the proper function of soils and in the cycling of organic matter to release nutrients for plant growth.

An understanding of earthworm population size, number of years that active populations have existed, and types of earthworm species present in a field can help agricultural producers estimate potential impacts to the soil environment. 

Reducing tillage, returning crop biomass to the soil, and minimizing the use of pesticides known to be harmful are management practices that can help support healthy populations of earthworms.

Sources:

Earthworms and Their Importance to Agricultural Soils in the Inland Pacific Northwest, by Kendall B. Kahl, Jodi L. Johnson-Maynard.

Earthworms, by Clive A. Edwards, The Ohio State University.