Are earthworms a guaranteed sign of healthy soil?
Written by
Paul Reynolds
Reviewed by
Prof. Charles Hartman, Ph.D.Earthworms indicate lively soil biology, but I have dug in many of the worm-filled gardens that still did not yield crops to eat. A client who had a sustainable garden had many active earthworms, yet the soil was saturated with lead from old plumbing. Just because you see life, does not mean that it is safe for food crops.
What Worms Indicate
- Adequate moisture and organic matter
- Active microbial communities
- Moderate pH (5.0-8.0)
What They Don't Reveal
- Heavy metal contamination
- Nutrient imbalances (excess phosphorus)
- Soluble salt toxicity
Contextual factors matter. Urban clients of mine frequently mistake worms for a clean bill of health; city soils can have worms in them and also have petroleum hydrocarbons, or arsenic. Rural gardens may be receiving relatively high copper concentrations due to decades of fungicides applied to those gardens. Always confirm worm observations with heavy metals lab tests.
Low organic matter may sometimes keep worms going in compost piles on top, but it can starve deeper layers of soil. I've noticed active topsoil teeming with worms over a clay subsoil that felt lifeless. I used a core sampler to find out for sure. Plants usually struggle when their roots hit that subsoil layer, which means we must sample at multiple depths to get the full picture.
The key principle is balance! Give your worms time to establish in your compost system, but it's still appropriate to test your soils each year. My rule of thumb is to test every two years if growing food, regardless of your worm population, and to test ornamentals if you have worms and a pH meter unless your plants are showing signs of needing nourishment. Remember, you can still trust the data rather than just dirt wigglers!
Read the full article: How to Test Soil: 7 Essential Steps for Healthy Gardens
