During the 2003 Crop Protection Technology Conference, Eileen Kladivko, professor in the Department of Agronomy at Purdue University, presented some very interesting facts concerning the influence of agricultural practices on earthworm populations. We frequently receive many inquiries each year about this topic, so I will attempt to summarize some of Kladivko's key points on this subject.
Typically, one to five "shallow-dwelling" species and one "deep-burrowing" species of earthworms exist in agricultural fields. Nightcrawlers belong to the deep burrower category. They create vertical burrows that may be 5 to 6 feet in depth. Nightcrawlers collect plant residue and pull it into their burrows. Lumbricus terrestris is the common nightcrawler species in the north region of the United States and may reach a length of 4 to 8 inches. Nightcrawlers are generally more active in the spring and fall when the soil temperatures are cooler and the soil moisture is more favorable to their survival. As soil temperatures increase and the soil becomes drier, nightcrawlers can move to greater depths in their soil burrows.
Other earthworms that inhabit agricultural fields include the shallow-dwelling worms (redworms, grayworms, fishworms). These species are found most often in the upper 12 inches of the soil. They are smaller than nightcrawlers, reaching lengths of 3 to 5 inches. Unlike nightcrawlers, they do not build permanent burrows. Instead they tunnel throughout the upper 1 foot of soil ingesting plant debris. They do not require plant residue at the surface of the soil, and in this regard they also differ from nightcrawlers. Peak activity for the shallow-dwelling worms is spring and fall. During the summer and winter, they enter into a resting state. This is accomplished by moving more deeply into the soil (1 1/2 feet) and encasing themselves in a mucous secretion.
Kladivko indicated that the importance of earthworms in the maintenance of soil productivity varies according to circumstance. Some soils are considered very productive even without earthworms. However, earthworms are generally believed to improve both soil structure and tilth. Because of their tunneling activities, they create soil channels that can improve water and airflow movement throughout the soil profile, enhancing root development. In addition, earthworm casts (worm feces) have elevated available nutrients, as compared with the surrounding soil. The mixing of organic matter and soil nutrients by earthworms is likely to be especially beneficial in no-till production systems.
Kladivko measured earthworm densities on silty clay loam fields near West Lafayette, Indiana, that had been devoted to various agricultural production systems for at least 10 years. The densities of earthworm populations were as follows:
Continuous corn, plowed--10 earthworms per square meter
Continuous corn, no-till--20 earthworms per square meter
Continuous soybean, plowed--60 earthworms per square meter
Continuous soybean, no-till--140 earthworms per square meter
Bluegrass-clover--400 earthworms per square meter
Dairy pasture, manure--340 earthworms per square meter
Dairy pasture, manure (heavy)--1,300 earthworms per square meter
These results substantiate that continuous production of a crop and tillage had negative impacts on earthworm densities. Kladivko reported that other surveys indicate that no-till production systems most often lead to increases in earthworm densities. She reported that 8 of 14 sites had greater densities of earthworms in no-till production systems. In some instances, densities of earthworms in no-till fields were 10 times as great.
What effect do agricultural chemicals have on earthworm populations? The following quotes are taken directly from Kladivko's paper that she delivered at the Crop Protection Technology Conference in January 2003.
Fertilizer applications. "Inorganic nitrogen fertilizers promote greater plant production than in unfertilized fields and therefore higher earthworm populations. Although anhydrous ammonia kills a few worms in the narrow band where injected, field effects are probably minimal due to the small area affected. There is little information on other nitrogen sources commonly used in the Midwest, but effects are probably small when used at typical field rates."
Herbicide applications. "Most herbicides used in crop production in the Midwest are harmless or only slightly toxic to worms and should not be a great concern."
Insecticide applications. " some corn rootworm insecticides are toxic to worms, but their effects can be reduced by keeping the application band as narrow as possible. In general, the organophosphate and pyrethroid insecticides are harmless to moderately toxic, whereas the carbamate insecticides and fungicides are highly toxic. Nematicides in general are also highly toxic."
Hopefully, this summary will provide answers for many of the questions we receive each growing season regarding the impact of agricultural practices on earthworm populations. Kladivko's complete proceedings paper can be found on pages 56-61 in the Proceedings of the Crop Protection Technology Conference, January 7-8, 2003, University of Illinois.--Mike Gray