Earthworms are a great indicator of soil health and are easy to look for in the field. In North Dakota, we have about a dozen species of earthworms with the main genus being Aporrectodea, or grey garden worm. Rod Utter, a research technician who has done decades worth of earthworm research at North Dakota State University tells me, "if you call a worm an Aporrectodea in North Dakota, you'll be right over 90 percent of the time."
The cool thing about a grey garden worm is that they move laterally in the top two feet of the soil, creating channels that are rich in nutrients leaked out of the earthworms' skin as it moves through the soil. This is a perfect place for roots to grow, right? Think of all the soil roots exploring in the top two feet of soil just by following the path of least resistance in nutrient-dense earthworm channels. We also have nightcrawlers in our soils. I've found a few, and those move vertically in the soil sometimes reaching a depth of eight or nine feet.
But for the most part, the grey garden worm dominates the soils here. We saw this visually with our earthworm latex molds that soil health technician, Luke Ressler, made last year. The molds didn't go very deep into the soil because earthworm channels were concentrated in the top couple feet of soil.
Here's what all this means for management - especially in the top two feet of soil. We often hear that reducing tillage and adding diversity are the primary ways to maintain and grow earthworm populations. Those soil health building practices are important, but not always from the "do not disturb" and "feed your soils" angles. There's another way to think about the system and this might help explain why you find worms where you don't sometimes expect to find them.
Almost 70 percent of why you find an earthworm is related to the moisture and temperature of the soil. Worms like conditions similar to what we like as humans. If it's too hot or too cold, we tend to stay inside. Worms will move deeper into the soil or find patches of residue where it's cooler or better insulated from cooler temperatures. If it's too dry, we slow down to accommodate for being thirsty and find water to drink. Worms do the same thing. They will ball up or limit movement when conditions are too dry and find the moisture patches in the soil, often under residue. The other 30 percent of earthworm presence is related to food sources amongst some other things that maybe we can't quite identify.
Reducing tillage to build residue and leaving standing corn stalks or stripped wheat stubble to catch snow and insulate the soil helps to regulate temperature year-round. Standing stalks and stripped wheat stubble also helps the soil breath because there isn't a mat on the surface, and that helps manage moisture conditions.
Diversifying rotations and/or including cover crops can also help manage moisture through plant transpiration to remove moisture and reducing wind speeds at the soil surface to maintain moisture. So, even though reducing tillage "does not disturb" a worm's home and crop diversity and cover crops "feed the soil," their benefits for moderating moisture and temperature should not be overlooked.