Earthworms' Random Foraging Might Be Tamed by Tillage

By Don Comis
January 6, 1998

In their blind search for food, earthworms play an unseen but critical role in agriculture. Someday, farmers might be able to boost this helpful role with tillage methods that "herd" earthworms towards their food supply--and help guard water quality.

Once a worm stumbles on its food source--decaying crop stubble--it keeps coming back for more, according to a study at the Agricultural Research Service. The implication is, worms can be "taught" how to help soil--and farms--become more productive and environmentally friendly.

Lab studies led by ARS soil scientist Dennis R. Linden in St. Paul, Minn., show that one common earthworm, Aporrectodea tuberculata, excavates 3 feet of tunnel per week. It swallows most of its work into its 2- to 2½-inch-long body. The ten worms per square foot used in the study is equivalent to 435,600 worms an acre--enough to dig almost 250 miles of tunnels in a week.

Earthworm tunnels help loosen soil, create fertile soil clods or aggregates, provide paths for roots, redistribute organic matter and drain and aerate soil. Scientists have long known that the benefits can include better crop yields and less soil erosion.

More recent ARS studies suggest tunneling may also help keep water free of chemicals. Vertical tunnel patterns give water a relatively chemical-free route downward, and the soil in the organic-rich burrows harbors more beneficial microbes to degrade pesticides and fertilizers.

Linden and University of Minnesota graduate student Sabrina M.F. Cook discovered that an earthworm searches randomly until it tastes food. Then it keeps returning for more. The patterns of these return trips depend on the kind of tillage. With reduced tillage, crop stubble is left on the ground. This concentrates organic matter near the surface so the worms form a network of burrows and depressions that funnel water vertically. Plowing, which buries crop residue, leads to more horizontal, meandering burrows that slow down soil water. Linden plans field studies to see how tillage can be designed to encourage vertical tunneling.

Scientific contact: Dennis R. Linden, USDA-ARS Soil and Water Management Unit, St. Paul, Minn., phone (612) 625-6798, fax 649-5175, dlinden@soils.umn.edu.