Your Soil's Secret Health Indicators

Walk into any healthy field and you'll likely find earthworms hard at work beneath your feet. These underground engineers do more than just wiggle through the dirt—they're reshaping soil structure, cycling nutrients, and leaving behind clues about the overall health of your farming system.

But can earthworms actually tell you if your soil management is working? Research suggests they can. A 15-year agricultural trial near Paris tracked earthworm populations across three different farming systems and found striking differences in how these organisms responded to organic, conventional, and conservation approaches. The findings offer practical insights for anyone managing soil with long-term productivity in mind.

Why Earthworms Matter for Soil Health

Earthworms improve soil in several measurable ways. Through their burrowing, feeding, and casting activities, they enhance soil porosity and aeration, boost water infiltration, stabilize aggregates, and accelerate nutrient cycling—all of which support healthier root growth.

The Food and Agriculture Organization (FAO) recognizes earthworm abundance and diversity as indicators of biological activity and soil health. The logic is straightforward: the more earthworms you have, and the more species present, the better your soil is functioning.

Different earthworm species play different roles:

• Endogeic species creating horizontal burrows and mixing organic matter throughout the soil profile.

• Anecic species create permanent vertical burrows and pull surface residues deep into the soil, building soil structure from the top down.

• Epigeic species live in surface litter and organic matter, breaking down crop residues quickly.

When your management supports all three groups, you're creating conditions for diverse soil functions.

What a 15-Year Trial Revealed

Researchers sampled earthworm communities in wheat fields managed under three systems: a conventional system (annual plowing, synthetic pesticides and fertilizers), an organic system (no synthetic inputs, annual plowing except after legumes, alfalfa in rotation), and a living mulch system (permanent plant cover, no plowing, herbicide use to plant cash crops).

The trial, located southwest of Paris on a deep luvisol soil, tracked earthworms over two periods: 2005–2007 (8–10 years after the trial began) and 2011–2013 (14–16 years in).

Period 1 (2005–2007): Early Differences

After nearly a decade, the living mulch system showed higher numbers of anecic and epigeic earthworms—3.2 to 12.5 times more than the conventional and organic systems. These surface-active species thrive under permanent plant cover and the absence of soil disturbance.

Endogeic earthworms, however, were more abundant in the plowed systems (conventional and organic), likely because tillage disrupts anecic burrows but mixes organic matter into the mineral soil where endogeics feed.

Surprisingly, the organic and conventional systems didn't differ much in total earthworm abundance or biomass at this stage. The researchers suggested that lower yields and available organic matter in the organic system, combined with the relatively low toxicity of pesticides used in the conventional system, kept populations similar.

Period 2 (2011–2013): Long-Term Shifts

After 14–16 years, the picture changed dramatically.

Total abundance increased significantly:

• Organic system: from 122 to 408 earthworms/m²

• Living mulch system: from 121 to 386 earthworms/m²

• Conventional system: from 149 to 216 earthworms/m²

The organic system, which had shown no advantage earlier, now supported 1.6 to 2 times more earthworms than the conventional system. This increase was driven by multiple species: A. caliginosa, L. terrestris, A. longa, and L. castaneus.

What changed? Starting in 2009, both alternative systems increased their use of alfalfa—a legume known to benefit earthworm activity through high-quality residues. The organic system also benefited from over a decade without synthetic pesticides, potentially allowing sensitive species to recover and colonize from field margins.

In the living mulch system, the large anecic species A. longa—absent in the first period—appeared and thrived, representing 25% of total abundance and 40% of total biomass by period 2. This species likely colonized from field edges once conditions (permanent cover, no plowing) became favorable.

The Role of Time and Rainfall

Year-to-year earthworm numbers varied with rainfall. Correlation between precipitation (September–October) and total abundance was high in the living mulch (R² = 0.86) and organic (R² = 0.81) systems, but lower in the conventional system (R² = 0.60). This suggests that the alternative systems retained moisture more effectively, making earthworms less vulnerable to dry conditions.

The trial underscores a critical point: earthworm communities respond slowly to management changes. It took more than nine years for the organic system to show measurable differences from the conventional system—a timeline that matches the biology of earthworms, which disperse slowly and colonize gradually when conditions improve.

Assessing Earthworms in Your Fields

You don't need a research lab to check earthworm populations. The FAO recommends a simple field method:

1. Dig a soil block: Use a trowel to dig a 20 cm × 20 cm × 20 cm block (about 8 inches on each side). Remove any litter layer first.

2. Place on a sheet: Set the block on a plastic sheet or tarp.

3. Count and sort: Collect and count all earthworms. If possible, sort them by type (anecic, endogeic, epigeic) based on size, color, and behavior.

   
   

Interpreting your counts (per 20 × 20 × 20 cm block):

• Poor: 0–4 earthworms, mostly one species

• Moderate: 4–8 earthworms, preferably 2+ species

• Good: 8+ earthworms, preferably 3+ species

  
  

Timing matters. Sample in autumn (or toward the end of the rainy season in Hawaiʻiʻs climates) when earthworms are most active. Avoid sampling when soils are too dry or waterlogged. For year-to-year comparisons, sample at the same time each year and in the same crop to reduce variability.

If you find few or no earthworms, consider what might be limiting them:

• Tillage frequency and intensity: Deep plowing disrupts burrows and fragments larger species.

• Crop residue management: Removing straw or burning residues eliminates food sources.

• Pesticide use: Some insecticides and fungicides are toxic to earthworms, even at field application rates.

• Soil compaction: Hard pans and compacted layers restrict movement and burrowing.

• Soil pH and texture: Earthworms prefer near-neutral pH and soils with adequate moisture-holding capacity.

  
  

Practical Takeaways for Organic and Regenerative Farms

The 15-year trial offers several lessons for farmers working to build soil health:

1. Give it time. Earthworm communities take years—sometimes over a decade—to respond fully to management changes. Short-term assessments (one or two years) may not capture the trajectory of change.

2. Reduce tillage where possible. No-till and reduced-tillage systems support anecic species that create stable, long-lasting burrow networks. If you must plow, consider doing so less frequently or less deeply, especially after legume crops.

3. Keep the soil covered. Cover crops, living mulches, and crop residues provide food and habitat for earthworms while protecting soil from temperature extremes and moisture loss.

4. Use crop rotations that include legumes. Alfalfa and other legumes provide high-quality organic matter that benefits earthworm reproduction and activity. The increase in earthworm populations in the organic and living mulch systems coincided with more intensive alfalfa use.

5. Limit pesticide inputs. While not all pesticides are equally harmful to earthworms, reducing overall pesticide use—particularly insecticides and fungicides—can help sensitive species recover and colonize your fields.

6. Monitor regularly. Earthworm counts provide a low-cost, field-level indicator of soil biological health. Sample the same fields over multiple years to track trends and adjust management accordingly.

Making Earthworms Part of Your Soil Health Assessment

Earthworms won't tell you everything about your soil, but they offer a practical, accessible measure of biological activity. Unlike laboratory tests that require soil samples and lab fees, earthworm counts can be done in the field with a shovel and a few minutes of time.

For organic and regenerative farmers, earthworms serve as indicators of whether your management is building soil health or simply maintaining it. Increasing earthworm abundance and diversity over time suggests your system is moving in the right direction—improving soil structure, nutrient cycling, and resilience.

If your earthworm populations are stagnant or declining, it may be time to revisit your tillage practices, residue management, or crop rotation. Small adjustments—like reducing tillage depth, adding a cover crop, or incorporating more legumes—can create conditions that allow earthworm populations to recover and thrive.

Healthy soils support productive farms. Earthworms, working quietly beneath the surface, help you build both.