SOIL QUALITY – AGRONOMY TECHNICAL NOTE No. 11

United States Department of Agriculture Agricultural Management Effects on

Natural Earthworm Populations Resources Conservation Service Introduction: Earthworm habits and their effects on soil Soil Quality Many people consider earthworms to be an indicator of soil quality because they Institute respond to and contribute to healthy soil. For earthworms to be abundant, a field

must meet several conditions that are also associated with soil quality and agricultural sustainability: moderate pH, surface residue for food and protection, and soil that is not waterlogged, compacted, droughty, or excessively sandy. Not

all healthy soils will have earthworms. Worms are not common in sandy soils, in drier regions of the southern and western United States, and in local areas where earthworms have not yet migrated or been introduced by human activities.

Earthworm species vary in how they get food, and thus inhabit different parts of the soil, and have somewhat different effects on the soil environment. They fall into three distinct ecological groups based on feeding and burrowing habits.

Epigeic (litter dwelling) earthworms live and feed in surface litter. They move horizontally through leaf litter or compost with little ingestion of or burrowing Technical Note No. 11 into the soil. These worms are characteristically small and are not found in low organic matter soils. Lumbricus rubellus is an example of epigeic species. June 2001 Endogeic (shallow dwelling) earthworms are active in mineral topsoil layers and associated organic matter. They create a three-dimensional maze of burrows while This is the eleventh note in a series of consuming large quantities of soil. The genuses Diplocardia and Aporrectodea Soil Quality- have endogeic life habits. Anecic (deep burrowing) earthworms live in permanent, Agronomy technical notes on the effects nearly vertical burrows that may extend several feet into the soil. They feed on of land management surface residues and pull them into their burrows. Lumbricus terrestris is an on soil quality. example of an anecic species (Coleman and Crossley, 1996). Once established,

earthworms contribute to soil function. They: Series written by: • Shred residues, stimulating microbial decomposition and nutrient release; Soil Quality Institute 411 S. Donahue Dr. • Produce casts rich in N, P, K, and other nutrients; Auburn, AL 36832 • 334-844-4741 x177 Improve soil stability, air porosity and moisture holding capacity by burrowing and aggregating soil; http://soils.usda.gov/sqi • Turn soil over and may reduce the incidence of disease by bringing deeper soil to the surface and burying organic matter;

• Improve water infiltration by forming channels and promoting soil aggregation; and • Improve root growth by creating channels lined with nutrients for plant roots to follow.

Both endogeic and anecic species are tainted residue before it has a chance to important in contributing to these functions leach down through burrows. Earthworms in agricultural systems. The shallow may act as plugs in their own burrows, and, dwellers improve topsoil porosity and the in the case of the herbicide atrazine, deep burrowing earthworms improve earthworm-feeding activity may actually infiltration and drainage. change the chemical to reduce its mobility (Farenhorst et al., 2000). In addition, Earthworms may have undesirable impacts earthworm activity improves soil structure if they remove too much surface residue and therefore reduces runoff of chemicals and leave the soil surface unprotected, or if into surface water. Another concern is that their burrows open into surface irrigation fresh earthworm casts on the surface are furrows. There is also some concern that unstable and may lead to higher soil earthworms may enhance the “preferential erosion and nutrients in runoff. Earthworm flow” of herbicides and other pollutants casts stabilize as they age so that the risk of down burrows and into groundwater, erosion is greatly reduced. In summary, causing water to pass too rapidly through earthworms affect soil function in multiple the soil matrix. When their total effect is ways. In specific situations they may have considered, however, earthworms are undesirable effects, but predominately they unlikely to have a significant negative contribute to improved soil quality and are impact on water quality. Where they are a sign of a healthy, properly functioning active, earthworms may bury herbicide- soil.

What determines earthworm abundance?

The number of earthworms in an • Moisture holding capacity and internal agricultural field is influenced by the drainage – Earthworms need moist but intensity and number of soil disturbance well-aerated soil. events like tillage and traffic, the • Rainfall and temperature – Climate abundance and quality of food sources, the affects the soil environment and food chemical environment of the soil, and the sources (plant biomass) for soil microclimate. Important factors of the earthworms. soil environment include: • Predation and parasitism. • Organic matter (food sources) – Higher inputs of fresh organic matter • Earthworm introduction – Even where are associated with greater earthworm environmental factors are favorable, populations. earthworms may not have migrated and established populations. • Soil type – Populations are highest in medium textured soil. When all other factors are equal, the availability of plant litter and organic • Depth to a restrictive layer – matter is usually the most important in Earthworms prefer deeper soil. determining earthworm abundance, but any • Soil pH – In general, earthworms will of the other factors may override the not thrive in a soil with a pH below 5 influence of organic matter. (Edwards and Lofty, 1977).

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Many management practices affect of management: (1) tillage, (2) crop earthworm populations because they rotations and cover crops, (3) fertilizers, (4) change one or more of the environmental pesticides, (5) irrigation and drainage, and factors listed above. This technical note (6) worm seeding (inoculation). will examine the effects of six components

Tillage

As the number and intensity of tillage Research has found the following: operations increase, so does the physical destruction of burrows, cocoons, and the • Earthworms were reduced by 70% earthworm bodies themselves. Less compared to previously undisturbed sod after five years of plowing intensive tillage systems that leave residues (Edwards and Bohlen, 1996). on the surface throughout the year improve the environment for earthworms. The • After 25 years of conventional tillage residues provide food, insulate earthworms crop production earthworm populations from weather conditions, provide cover to were only 11-16% of what existed in protect them from birds and other surface the original grass field (Edwards and predators, and protect their burrows. Bohlen, 1996). Decreased tillage disturbances particularly • Edwards et al. (1995) reported up to 30 benefit night crawlers (L. terrestris), which times more earthworms in no-till move in the same burrow between deeper systems compared to plowed fields. soil layers and the soil surface in search of food. When tillage destroys the burrow, • In Nigeria, researchers found 2400 some earthworms will not have the energy earthworm casts/m2 in no-till plots 2 reserve to form a new burrow to their food compared to 100 casts/m under source. Endogeic (shallow dwelling) conventional tillage (Edwards and earthworms will tolerate annual tillage Lofty, 1977). because they continually form new burrows • In a Georgia experiment, no-till fields and acquire a greater proportion of their had an average of 967 earthworms/m2 food from the soil rather than surface litter. compared to 149 /m2 in conventionally No-till and other methods of conservation tilled fields (Coleman and Crossley, tillage such as chisel plowing and ridge 1996). tillage can increase populations of both types of earthworms (Edwards and Bohlen, Table 1. Earthworm Populations 2 1996) (Table 1). (No./yd ) as influenced by amount of surface residues at Langdon Research Although a single tillage event will not and Extension Center, ND (Deibert and drastically reduce earthworm populations, Utter, 1994). repeated tillage over time will cause a 40 - 45% 80 – 90% decline in earthworm populations. Residue Residue Earthworms 71 106 Cocoons 204 514 Total 275 620

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Crop Rotations and Cover Crops

Tillage affects decomposition and rootworms and anhydrous ammonia availability of surface residue, while choice fertilizer (Kladivko, 1993). of crop determines the quantity and quality of the residue as a food source for Crop rotations with pasture or hay greatly earthworms. Earthworm populations will increase earthworm numbers. There is a decrease to very low numbers under an strong correlation between earthworm exhaustive cropping system of plowing, numbers and years in grass and legumes. crop residue removal, and no additions of For example, a crop every third year in manure or other organic inputs. grass rotations will have greater earthworm numbers than a crop every two years or There is a strong correlation between annual cropping. Dick Thompson (Boone, earthworm numbers and the amount and Iowa) followed a 6-year rotation of corn, quality of residue returned to the soil. soybeans, oats, and 3 years of pasture (Table 3.) Generally, cereal crops such as (alfalfa, red clover, grasses, and other wheat (especially if straw is returned to the forages). He reduced tillage and used soil) encourage earthworms more than livestock manure. Researchers from the crops which leave less residue such as Agricultural Research Service National soybean. Studies in the 1940’s showed the Soil Tilth Laboratory (Ames, Iowa) found following ranking in order of earthworm more earthworms in Thompson’s fields population (all are conventionally tilled compared to an adjacent neighbor’s cropping systems): pasture = small grains conventionally tilled field in corn-soybean followed with legume hay grown in the rotation (Ernst, 1995). The larger summer > small grains with summer fallow earthworm populations were attributed to > drilled soybeans for grain > soybeans for more food from grass-legume hay crops, hay > corn (Hopp and Hopkins, 1946). At manure, and reduced tillage. the Rothamsted Experimental Station in England where crops have been studied Alfalfa and clover in rotations benefit since 1843, the largest earthworm earthworm numbers because of the absence populations occur under continuous of tillage and the high protein content in cereals, were lower under root crops such their residues. Rotations with alfalfa and as turnip, and were the lowest under fallow grass contain more earthworms than (Edwards and Bohlen, 1996). lespedeza and grass, and orchard grass contains more earthworm numbers than Despite its high residue production, timothy grass alone (Hopp and Hopkins, continuous corn supports fewer 1946). earthworms than when in rotation with soybeans, whether under no-till or Using cover crops helps to increase conventional tillage. Earthworms seem to earthworm populations by increasing their prefer legumes (Table 2). Although food supply (organic residue) and by probably less important, other factors that giving them a longer season to eat and may discourage earthworms in corn could reproduce. Cover crops insulate worms be soil application of insecticides to control from cold weather in the fall and from warm weather in southern climates. The

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extra food and ground cover provided by Wisconsin has reported residue cover being cover crops are especially important where reduced from 30% to 15% by earthworms earthworms are removing a high at planting time in no-till fields (Ernst, percentage of crop residue. University of 1995).

Table 2. Earthworm populations (No./yd2) under different no-till rotations at Brecker Farm, Havana, ND (Deibert and Utter, 1994).

Wheat-Corn Corn-Soybeans Wheat-Soybeans Earthworms 257 346 443 Cocoons 27 71 35 Total 284 417 478

Table 3. Earthworm populations affected by crop and tillage (Kladivko, 1993).

Crop Management Earthworms/m2 Continuous corn Plow 10 Continuous corn No-till 20 Continuous soybean Plow 60 Continuous soybean No-till 140 Bluegrass-Clover Alleyway 400 Dairy pasture Manure 340 Dairy pasture Heavy manure 1300

Fertilizers

Nearly all organic fertilizers benefit Edwards et al., 1995). Hendrix et al. (1992) earthworms. The addition of animal reported that earthworm numbers in manure, sewage wastes, and spent malt meadows receiving inorganic fertilizer from breweries, paper pulp, or potato averaged nearly twice the earthworms in processing waste all showed a positive unfertilized meadows on the Georgia effect on earthworm numbers (Edwards et piedmont. al., 1995). Additions of organic material can double or triple earthworm numbers in Ammonia and ammonia-based fertilizers a single year. The ammonia and salt can adversely affect earthworms. Annual content of some liquid manure can have an use of ammonium sulfate, anhydrous adverse effect on earthworms, but ammonia, and sulfur-coated urea has been populations usually recover quickly and shown to decrease earthworm populations henceforth increase (Edwards and Bohlen, (Edwards et al., 1995). Research at Park 1996). Grass (Rothamsted) since 1856 showed that after extremely long exposure to Normally, the use of inorganic fertilizers several levels of ammonium sulfate (0, 48, also has a positive impact on earthworm 97, and 145 kg/ha), the populations of numbers. This is probably an indirect effect earthworms were inversely proportional to of the increased crop biomass production the dose of nitrogen applied (Edwards and and consequent increases in organic Lofty, 1977). This is probably due to the residues (Edwards and Bohlen, 1996; effect these fertilizers have on lowering 5

soil pH. Direct exposure to anhydrous Lime seems to benefit earthworm ammonia during application will kill up to populations in otherwise acid soils because 10% of the population. However, farmers most species of earthworms favor neutral report increased numbers in the long run pH levels and require calcium for growth. due to higher yields and more food for Lime may indirectly benefit earthworms by earthworms to feed upon (Ernst, 1995). increasing plant growth and therefore plant Still, some farmers have switched from residues. A study in New Zealand showed anhydrous ammonia to 28% nitrogen to a 50% increase in surface feeding avoid killing earthworms during nitrogen earthworm species by adding one ton of application. Others have converted to using lime per acre (Edwards et al., 1995). manures in order to protect and increase earthworms (Ernst, 1995).

Pesticides

In general, most herbicides are harmless to propoxur (Baygon), and thiofanox. earthworms. The triazine class of Generally, insecticides in the herbicides has a moderate impact on organophosphate class are less toxic to earthworm numbers. Herbicides used prior earthworms. However, organophosphate to World War II, including lead arsenate insecticides that are extremely or highly and copper sulfate, are moderately toxic to toxic are phorate (Thimet), chloropyrifos earthworms. The main threat of toxicity to (Dursban, Equity, Tenure, etc.), earthworms is from long-term buildup of ethoprophos (Mocap), ethyl-parathion, and these compounds in the soil (Edwards and isazophos. Aromatic organochlorine Bohlen, 1996). insecticides (used predominantly in the 1950’s-1970’s) are generally not very The majority of the carbamate class of toxic. Exceptions are chlordane, endrin, insecticides are toxic to earthworms. The heptachlor, and izobenzan. Carbamate toxic effects of carbofuran (Furadan) have fungicides (carbendazim and benomyl) been studied extensively. Other insecticides have shown toxic effects to earthworms. in the carbamate class that have proved Other broad-spectrum fumigants highly toxic to earthworms are aldicarb (fungicides and nematicides) are very toxic (Temik), aminocarb, bufencarb, carbaryl to earthworms. (Ernst, 1995; Edwards and (Sevin), methiocarb (Measural), methomyl Bohlen, 1996.) (Lannate), oxamyl (Vydate), promecarb,

Irrigation and Drainage

Irrigated soil can support high levels of earthworms and their cocoons may act as a earthworm activity where moisture levels source of inoculum for certain species would otherwise be too dry. Irrigation also (Edwards et al., 1995). Draining poorly increases crop production, resulting in drained soils will potentially provide a more food and increased earthworm more favorable environment for earthworm populations. Irrigation waters that carry activity by aerating the soil.

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Seeding Worms

Shallow dwelling earthworms are generally particular site (Kladivko, 1993). Farmers in present in agricultural fields, so their Indiana and Illinois have seeded 10,000 – populations dramatically increase within 100,000 night crawlers to their farms at a one to two years of switching to earthworm cost of four to five cents per worm from friendly practices. However, the deep local bait shops and Canadian sources dwelling night crawlers take longer to (Ernst, 1995). If seeding is an option, drop increase. Even when favorable conditions 4-5 under mulch every 30-40 feet, have been established, night crawlers must preferably on a cloudy wet cool day. In one move into unoccupied areas by slow case, a farmer used an earth auger attached overland migration. Night crawlers are also to a cordless drill and put a handful of slow to breed. If left to their own devices, it worms per hole every 30 feet (Ernst, 1995). may take seven to eight years, or longer, Even with seeding, however, populations for populations to grow (Ernst, 1995). will still take five years or more to grow significantly, if they survive at all Seeding earthworms or their eggs may be (Kladivko, personal communication.) an option to increase populations in favorable environments. Although some The Netherlands has reported the addition soils, such as extremely coarse sands or of earthworms on once flooded soils that heavy clays with a high water table, will have been drained (polder soils). Natural not support night crawlers due to inherently earthworm densities have increased over 26 adverse soil properties. If a farmer wants to years following reclamation. However, try seeding night crawlers, it is much higher population densities have recommended to begin with a low cost, been found where earthworms were seeded small-scale trial to be sure they survive at a (Edwards and Bohlen, 1996).

Summary

Earthworms benefit soil quality by numbers. Generally, fertilizers increase shredding residues stimulating microbial earthworm numbers by increasing crop decomposition, improving soil fertility, and residues, especially when pH is maintained improving soil physical properties such as near neutral. Herbicides are generally soil aggregation and infiltration. Food harmless to earthworms. However, some availability is the major factor limiting insecticides, nematicides, fungicides are earthworm numbers. Producing food very toxic to earthworms. In some through crop residues and cover crops and situations, earthworm inoculation may be leaving them on the soil surface through desirable to introduce certain species to an the use of conservation tillage practices area once earthworm friendly practices are provides food to increase earthworm in place.

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References

Coleman, D.C., and D.A. Crossley, Jr. 1996. Fundamentals of soil ecology. Academic Press, San Diego, CA.

Deibert, E.J. and R.A. Utter. 1994. Earthworm populations related to soil and fertilizer management practices. Better Crops 78:9-11.

Edwards, C.A. and R. Lofty. 1977. The biology of earthworms. 2nd ed. Chapman & Hall, London, UK.

Edwards, C.A., P.J. Bohlen, D.R. Linden, and S. Subler. 1995. Earthworms in agroecosystems. P. 185-206. In P.F. Hendrix (ed.) Earthworm ecology and biogeography. Lewis, Boca Raton, FL.

Edwards, C.A. and P.J. Bohlen. 1996. Biology and ecology of earthworms. 3rd ed. Chapman & Hall, London, UK.

Ernst, D. 1995. The farmer’s earthworm handbook: Managing your underground money- makers. Lessiter Publications, Brookfield, WI.

Farenhorst, A., E. Topp, B.T. Bowman, and A.D. Tomlin. 2000. Earthworm burrowing and feeding activity and the potential for atrazine transport by preferential flow. Soil Biology & Biochemistry 32:479-488.

Hendrix, P.F., B.R. Muller, R.R. Bruce, G.W. Langdale, and R.W. Parmelee. 1992. Abundance and distribution of earthworms in relation to landscape factors on the Georgia piedmont, U.S.A. Soil Biology & Biochemistry 24:1357-1361.

Hopp, H. and H.T. Hopkins. 1946. The effect of cropping systems on the winter population of earthworms. J. Soil Water Conserv. 1:85-88.

Kladivko, E.J. 1993. Earthworms and crop management. Agronomy 279. Purdue Univ. Extension Service. http://www.agcom.purdue.edu/AgCom/Pubs/AY/AY-279.html

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