Count earthworms to assess soil biology

When soil is moist (spring or fall, not frozen or baked dry), mark a 12x12 inch square, dig out the soil to 12 inches deep onto a tarp, and hand-sort through it counting every earthworm. Per the USDA NRCS, 10 or more per cubic foot indicates good soil health.

Why It Works

Earthworms require adequate organic matter, near-neutral pH (5.5-7.5), sufficient moisture, and minimal chemical contamination to thrive. Their presence is a reliable proxy for overall soil biological activity. Kansas State University Extension identifies earthworm counts as a standard soil health indicator. MSU Extension calls 10 per cubic foot a "good target number."

Tips

• 0-4 worms: Poor soil health — likely low organic matter, possible compaction or chemical issues
• 5-9 worms: Moderate — functional soil that could be improved
• 10+ worms: Good biological activity
• Sandy, acidic, very dry, or recently tilled soils naturally have fewer earthworms
• Best tested in late spring (April-May) or mid-fall (October-November) when worms are most active near the surface