How to Do an Earthworm Count to Check Your Farm Soil

A lab test tells you pH and phosphorus, but it will not tell you whether your soil is alive and working. An earthworm count does, and all it costs is a spade, a tarp and about 20 minutes per spot.

I started doing worm counts on our five acres after a soil report came back "fine" on a bed that grew stunted beans two years running. The lab numbers were fine. The soil was not. A single spadeful turned up zero worms, and that told me more than the paperwork. Here is how to run the count properly, what the numbers mean, and where people misread them.

Why an Earthworm Count Says So Much About Soil Health

Earthworms are a summary of several things at once. They need organic matter to eat, moisture that does not swing between bone dry and waterlogged, a pH somewhere between roughly 5.5 and 8, and soil that has not been tilled to dust every spring. When all of that lines up, worms show up. When one piece is missing, they thin out fast.

Their burrows also do real work. Deep burrowing species open channels that let rain soak in instead of running off, and their castings are richer in available nutrients than the soil around them. A field with lots of worms usually drains better after a storm and holds water longer in a dry spell.

How to Do the Count, Step by Step

Timing matters more than technique. Count in spring or fall when the soil is moist and the temperature at 4 inches is somewhere between 50 and 70 degrees Fahrenheit. In midsummer heat or a hard frost, worms go deep or go dormant and you will get a false low.

1. Pick your spots. Choose at least 5 locations per field or bed block, spread out, not all near the gate. Mark each with a flag so you can return next year.
2. Dig a cube. Cut a block 12 inches by 12 inches and about 12 inches deep. Lift it onto a tarp or a feed sack in one piece if you can.
3. Break it apart by hand. Crumble the soil slowly. Worms are fast, and the big ones pull back into clods.
4. Count and sort. Tally every worm. Separate them roughly into small surface dwellers (reddish, quick, near the top) and large deep burrowers (gray or pinkish, thick, often with a flattened tail end).
5. Check the hole. Look at the walls and floor of the hole. Count open burrows wider than a pencil lead. Those are deep burrower homes even if the owner escaped.
6. Put it back. Return the worms and the soil to the hole in roughly the same order.
7. Write it down. Date, spot number, soil moisture (dry, moist, wet), worm count and burrow count.

Five spots take about an hour and a half with a sharp spade. If your spade is dull, fix that first; our guide on sharpening farm hand tools with a mill file covers it in about ten minutes of work.

Reading the Numbers

A cube of 12 by 12 by 12 inches is one cubic foot. That is the standard most extension services use, which makes your counts comparable over time.

Average your five spots. One great spot and four empty ones tell a different story from five middling ones, so keep the individual numbers too.

Where Worm Counts Mislead People

A lot of soil health advice treats more worms as always better, and treats a low count as proof that your soil is failing. I disagree with both.

First, sandy soils carry fewer worms even when they are well managed. A sandy loam in good shape might give you 6 or 7 per cube, and that is fine for that soil. Compare your fields to themselves year over year, not to a clay pasture in another state.

Second, a high count in one spot can point to trouble. I once found 30 worms in a cube right where the downspout from the barn emptied. That spot was wet all year and the rest of the field was too dry. The worms were showing me a water problem, not a fertility win.

Third, a single count is a snapshot. Rain two days before you dig can double the number at the surface. Count on roughly the same week each year, after similar weather, and watch the trend over three or four years before you change anything big.

Raising a Low Count

If your average comes back under 4, the fixes are usually simple and slow:

• Feed them. Worms eat dead plant matter. Leave crop residue on the surface, mulch beds with straw or leaves, and add finished compost. A pile that actually reaches temperature makes better food; see building a compost pile that actually heats up.
• Cut tillage. Every pass with a rototiller chops worms and wrecks deep burrows. Even dropping from two passes a year to one shows up in counts within a season or two.
• Loosen compacted layers. Worms avoid hard pans. If your spade hits a wall at 6 inches, deal with that before anything else, and our guide to finding soil compaction on a small farm shows how to map it.
• Correct very acid soil. Below about pH 5, worm numbers crash. Lime gradually rather than all at once.
• Keep roots in the ground. A winter cover of rye or oats gives worms food and cover through the cold months.

Do not buy worms to "seed" a field. If conditions are right, the local population will grow on its own. If conditions are wrong, bought worms die or leave.

FAQ

What is a good earthworm count for garden soil?

Ten or more worms in a 12 inch cube is a healthy result for most loam and clay soils. Sandy soils can be in good shape with 5 to 8. The trend over several years matters more than any single number.

When is the best time of year to count earthworms?

Spring and fall, when the soil is moist and cool. Avoid hot dry spells and frozen ground, because worms move deeper and your count will come out falsely low.

Why are there no earthworms in my soil?

The usual causes are frequent tillage, very low organic matter, compaction, strongly acid soil or long dry periods. Fix one at a time, starting with whatever a spade test shows most clearly.

Do earthworms mean my soil does not need fertilizer?

No. A good worm count shows active biology and decent structure, but it does not measure nutrient levels. Pair it with a lab test every few years for the full picture.