Earthworms: One of many indicators of soil health
Earthworms play an important role in soil as they redistribute organic matter, influence soil fertility, and affect soil physical properties. While earthworms are not essential for the formation of well-aggregated soil, their presence can contribute significantly to the formation and stabilization of aggregates and improve soil structure. Earthworm casts -- the excreted mixture of soil and organic matter -- can develop into stable soil aggregates. Depending on the species the cast material may be deposited in the burrow or on the soil surface.
Earthworm-driven improvements in soil structure (Figure 1) can result in positive changes in soil porosity, aeration, water holding capacity, and the rate of water infiltration.
Figure 1. An earthworm and earthworm burrow in a clod of soil in a no-till cover crop research study. Photo by DeAnn Presley, K-State Research and Extension.
The activity of earthworms accelerates decomposition of plant material and mineralization of soil organic matter, increasing the availability of plant available nutrients. A complex relationship exists between earthworms and microorganisms. Bacteria have been found to proliferate in the earthworm gut and be excreted in cast material. Thus, enhanced microbial decomposition of organic matter fueled by the presence of nutrient rich secretions begins in the earthworm gut and continues in earthworm casts.
What effects do cultivation and soil health practices have on earthworm populations?
Earthworm populations generally become reduced in cultivated agricultural fields. Several explanations for the decline and loss of earthworms have been proposed:
1) Tillage implements cause physical injury to earthworms resulting in mortality.
2) Reductions in residue and soil organic matter associated with long-term tillage restrict food supplies.
3) A change in soil temperature resulting from the loss of insulation provided by the vegetation.
4) Increased predation from birds when the soil is turned over.
It is likely a combination of these factors that leads to reduced earthworm populations.
When tillage practices are reduced or eliminated as a result of conversion to a minimal or no-till system, earthworm populations generally begin to increase. Earthworms play an important role in no-till systems as they redistribute organic matter. Earthworms are important in soil fertility, and their burrows play an important role in soil aeration and drainage.
The addition of cover crops can further benefit earthworm populations. In a long-term cover crop study conducted at the former K-State Harvey County Experiment Field in Hesston from 2002 to 2008, more earthworms were counted in the plots with either late-maturing soybean or sunn hemp cover crops (Figure 2).
Improved cumulative water infiltration (Figure 3) was also observed in the cover crop treatments of the study and followed the same pattern as the earthworm populations. The greatest levels of cumulative water infiltration and earthworm populations were in the sunn hemp treatment while the “no cover crop” treatment had the lowest cumulative water infiltration and earthworm populations.
Fig. 2. Earthworm densities in a cropping system with and without cover crops in the rotation.
Fig. 3. Cumulative water infiltration in a cropping system with and without cover crops in the rotation.
How do you assess earthworms as an indicator of changing soil health?
It is important to understand that you will not see changes in earthworm populations immediately after a change in management practices. In fact it will likely take many years to observe a change in earthworm populations. The key is to collect base line population information before you make a management change and then repeat the measurements every few years, being consistent in your sampling date.
The recommended time of year to count earthworms is the late spring (April-May) or mid-fall (Oct-Nov). It is best to identify at least four locations that are representative of the field and which will provide an indication of the spatial variability of the earthworms. Over time you will want to be able to go back to the same general sampling location.
To assess your earthworm population, dig a cubic foot of soil and hand sort the soil, keeping track of how many earthworms you find. The typical range for earthworm numbers in a cubic foot of soil in an agricultural field is 5 to 30 but may be higher depending on tillage and soil organic matter.
Peter Tomlinson, Environmental Quality Specialist