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Earthworm Casts Reflect Soil

The earthworms on the sidewalk after a summer storm are more than a fisherman's dream. Lumbricus terrestris—the common night crawler—is also a valuable tool for determining the health and viability of crop-producing soils. By studying the organic matter of earthworm fecal matter, or casts, scientists in the ARS Soil and Water Management Research Unit at the University of Minnesota have discovered a simple way to determine soil health in agricultural fields.

The casts contain soil, organic matter, microbes, and carbon, all of which give clues about the condition of the field. Researchers have found that reduced tillage farming and certain crops encourage carbon deposition, an indicator of high soil quality.

Earthworms ingest and mix fresh residues from crops like corn and soybeans with soil in a moist, microbe-rich environment. They also feed on microbes growing on residues. The carbon encapsulated by earthworms is a key building block in a well-structured soil.

"Earthworms are an important link in decomposing the previous year's crop residue," says Dennis Linden, an ARS soil scientist. "They are the primary consumer of organic material in the soil."

Scientists found that fields that had been tilled less had higher populations of earthworms. Another discovery: Corn residues produce a higher concentration of carbon deposits than soybean residues do.

"It appears earthworms need to consume more material from cornfields to get the same quantity of protein and nitrogen as they do from soybean residues," Linden says. "The earthworm doesn't need as much material when it eats soybeans."

Earthworms are being studied as one way to measure carbon sequestration, which is the capture and use of carbon in the soil.

Carbon is important because it improves soil's ability to support crop growth, water infiltration rate, and other functions. Linden says earthworms can greatly affect carbon sequestration.   -- By Dawn Lyons Johnson, ARS.