The iconic evidence of healthy soil is the presence of earthworms. And earthworm populations generally become reduced in cultivated agricultural fields, says Peter Tomlinson, K-State Research and Extension environmental quality specialist.
No research provides concrete evidence of why that happens, but there are several possible explanations for the decline and loss of earthworms, he says.
"It could be that tillage implements cause physical injury to earthworms, resulting in mortality. Also, reductions in residue and soil organic matter associated with long-term tillage restrict the earthworms' food supplies," Tomlinson says.
"A change in soil temperature resulting from the loss of insulation provided by the vegetation could also be reducing earthworm populations. Another possibility is increased predation from birds when the soil is turned over," he suggests.
It is likely a combination of these factors leads to reduced earthworm populations, the K-State agronomist says.
In studies comparing adjacent cultivated and uncultivated soils, population reductions are wide-ranging, he said. In one five-year study, the population was reduced by 70 percent by tillage but in a different series of 25-year studies the population was only reduced between 11 and 16 percent, he says.
When tillage practices are reduced or eliminated as a result of conversion to a minimal or no-till system, earthworm populations generally begin to increase, Tomlinson says.
"Earthworms play an important role in no-till systems as they redistribute organic matter. They are important in soil fertility, and their burrows play an important role in soil aeration and drainage," he says.
Worms move organic matter from one place to another. It's estimated that an acre of good garden or farm soil will contain 2 million to 3 million earthworms or 10 to 50 per square foot.
Compared to soil that has not been through an earthworm, worm castings have seven times the phosphate; 10 times the potash; five times the nitrogen; three times the usable magnesium; and 1.5 times the calcium.