Soil Biology
The Soil Biology Primer
Chapter 8: EARTHWORMS
by Clive A. Edwards, The Ohio State University
THE LIVING SOIL: EARTHWORMS
Of all the
members of the soil food web, earthworms need the least introduction. Most
people become familiar with these soft, slimy, invertebrates at a young age.
Earthworms are hermaphrodites, meaning that they exhibit both male and female
characteristics.
They are major
decomposers of dead and decomposing organic matter, and derive their nutrition
from the bacteria and fungi that grow upon these materials. They fragment
organic matter and make major contributions to recycling the nutrients it
contains.
Earthworms
occur in most temperate soils and many tropical soils. They are divided into 23
families, more than 700 genera, and more than 7,000 species. They range from an
inch to two yards in length and are found seasonally at all depths in the soil.
In ter ms of
biomass and overall activity, earthworms dominate the world of soil
invertebrates, including arthropods.
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Figure 1:
Earthworms generate tons of casts per acre each year, dramatically
altering soil structure.
Credit: Clive A. Edwards, The Ohio State University, Columbus. |
Figure 2: A corn
leaf pulled into a night crawler burrow.
Credit: Soil and Water Management Research Unit,
USDA-Agricultural Research Service, St. Paul, Minnesota. |
WHAT DO EARTHWORMS DO?
Earthworms
dramatically alter soil structure, water movement, nutrient dynamics, and plant
growth. They are not essential to all healthy soil systems, but their presence
is usually an indicator of a healthy system. Earthworms perform several
beneficial functions.
Stimulate microbial activity. Although
earthworms derive their nutrition from microorganisms, many more microorganisms
are present in their feces or casts than in the organic matter that they
consume. As organic matter passes through their intestines, it is fragmented and
inoculated with microorganisms. Increased microbial activity facilitates the
cycling of nutrients from organic matter and their conversion into forms readily
taken up by plants.
Mix and aggregate soil. As
they consume organic matter and mineral particles, earthworms excrete wastes in
the form of casts, a type of soil aggregate. Charles Darwin calculated that
earthworms can move large amounts of soil from the lower strata to the surface
and also carry organic matter down into deeper soil layers. A large proportion
of soil passes through the guts of earthworms, and they can turn over the top
six inches (15 cm) of soil in ten to twenty years.
Increase infiltration. Earthworms
enhance porosity as they move through the soil. Some species make permanent
burrows deep into the soil. These burrows can persist long after the inhabitant
has died, and can be a major conduit for soil drainage, particularly under heavy
rainfall. At the same time, the burrows minimize surface water erosion. The
horizontal burrowing of other species in the top several inches of soil
increases overall porosity and drainage.
Improve water-holding capacity.
By
fragmenting organic matter, and increasing soil porosity and aggregation,
earthworms can significantly increase the water-holding capacity of soils.
Provide channels for root
growth. The
channels made by deep-burrowing earthworms are lined with readily available
nutrients and make it easier for roots to penetrate deep into the soil.
Bury and shred plant residue. Plant
and crop residue are gradually buried by cast material deposited on the surface
and as earthworms pull surface residue into their burrows.
WHERE ARE EARTHWORMS?
Different
species of earthworms inhabit different parts of the soil and have distinct
feeding strategies. They can be separated into three major ecological groups
based on their feeding and burrowing habits. All three groups are common and
important to soil structure.
Surface soil and litter species
– Epigeic species. These
species live in or near surface plant litter. They are typically small and are
adapted to the highly variable moisture and temperature conditions at the soil
surface. The worms found in compost piles are epigeic and are unlikely to
survive in the low organic matter environment of soil.
Upper soil species – Endogeic
species. Some
species move and live in the upper soil strata and feed primarily on soil and
associated organic matter (geophages). They do not have permanent burrows, and
their temporary channels become filled with cast material as they move through
the soil, progressively passing it through their intestines.
Deep-burrowing species –
Anecic species. These
earthworms, which are typified by the “night crawler,” Lumbricus terrestris, inhabit more or less permanent burrow systems
that may extend several meters into the soil. They feed mainly on surface litter
that they pull into their burrows. They may leave plugs, organic matter, or cast
(excreted soil and mineral particles) blocking the mouth of their burrows.
LOOKING FOR EARTHWORMS?
It is easy to
determine whether you have an adequate population of earthworms in your soil.
Look for their casts in the forms of little piles of soil, mineral particles, or
organic matter at the soil surface. They can be seen moving over the soil
surface or even breeding, particularly on warm, damp nights. Dump a spadeful of
moist soil into a bucket or onto a sheet of plastic, and sort through for
earthworms. Can you identify different species? To find the deep burrowing species, pour a dilute mustard
solution onto the soil. Many will quickly come to the soil surface in response
to this irritant.
ABUNDANCE AND DISTRIBUTION OF EARTHWORMS
The majority
of temperate and many tropical soils support significant earthworm populations.
A square yard of cropland in the United States can contain from 50-300
earthworms, or even larger populations in highly organic soils. A similar area
of grassland or temperate woodlands will have from 100-500 earthworms. Based on
their total biomass, earthworms are the predominant group of soil invertebrates
in most soils.
The family of
earthworms that is most important in enhancing agricultural soil is Lumbricidae,
which includes the genuses Lumbricus,
Aporrectodea, and several others. Lumbricids
originated in Europe and have been transported by human activities to many parts
of the world. The United States has only one or two known native species of
lumbricids. Others were brought to this country by settlers (probably in potted
plants from Europe), and were distributed down the waterways.
Generally,
lumbricids are much more common in the north and east than in the drier south
and west of the United States. They tend to be more abundant in loam and clay
loam and even in silty soil, than in sandy soil and heavy clay. Populations also
build up in irrigated soil. Earthworm populations tend to increase with soil
organic matter levels and decrease with soil disturbances, such as tillage and
potentially harmful chemicals.
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Figure 5: Casts
at the soil surface are evidence that earthworms are shredding, mixing,
and burying surface residue.
Credit: Soil and Water Management Research Unit,
USDA-Agricultural Research Service, St. Paul, Minnesota. |
Figure 6: This
earthworm burrow is an opening in an otherwise crusted soil surface.
Credit: Clive A. Edwards, The Ohio State University, Columbus. |
INTERACTIONS OF EARTHWORMS WITH OTHER MEMBERS OF THE FOOD WEB
The lives of
earthworms and microbes are closely intertwined. Earthworms derive their
nutrition from fungi, bacteria, and possibly protozoa and nematodes, and they
promote the activity of these organisms by shredding and increasing the surface
area of organic matter and making it more available to small organisms.
Earthworms
also influence other soil-inhabiting invertebrates by changing the amount and
distribution of organic matter and microbial populations. There is good evidence
that earthworm activity affects the spatial distribution of soil microarthropod
communities in the soil.
Earthworms have few invertebrate enemies, other than
flatworms and a species of parasitic fly. Their main predators are a wide range
of birds and mammals that prey upon them at the soil surface.
EARTHWORMS AND WATER QUALITY
Earthworms
improve water infiltration and water holding capacity because their shredding,
mixing, and defecating enhances soil structure. In addition, burrows provide
quick entry for water into and through soil. High infiltration rates help
prevent pollution by minimizing runoff, erosion, and chemical transport to
surface waters.
There is
concern that burrows may increase the transport of pollutants, such as nitrates
or pesticides, into groundwater. However, the movement of potential pollutants
through soil is not a straightforward process and it is not clear when earthworm
activity will or will not have a negative impact on groundwater quality.
Whether
pollutants reach groundwater depends on a number of factors, including the
location of pollutants on the surface or within soil, the quantity and intensity
of rain, how well water moves into and through other parts of the soil, and
characteristics of the burrows. The horizontal burrows of endogeic earthworms
(such as Aporrectodea tuberculata,
which are common in Midwestern fields) do not transport water and solutes as
deeply as the vertical burrows of night crawlers (L.
terrestris) and other anecic species. Even vertical burrows, however, are
not direct channels for water movement. They have bends and turns and are lined
with organic matter that adsorbs many potential pollutants from the water.
Although there
is much more to learn about how earthworms affect water movement through soil,
they clearly help minimize pollution of surface waters by improving infiltration
rates and decreasing runoff.
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Figure 7: A mound
of organic matter was moved aside to expose the entrance to a burrow. L.
terrestris will quickly replug its burrow if its mound is removed.
Credit: North Appalachian Experimental Watershed,
USDA-Agricultural Research Service, Coshocton, Ohio. |
Figure 8: L.
terrestris mating, and earthworm cocoons. Earthworms mate
periodically throughout the year, except when environmental conditions
are unfavorable. The worms form slime tubes to help adhere to each other
during copulation which may take as long as an hour. After the worms
separate, they each produce a cocoon. One or two worms will hatch from a
cocoon after several weeks. L. terrestris cocoons are about a
quarter inch long.
Credit: Clive A. Edwards, The Ohio State University, Columbus. |
BUG BIOGRAPHY: Night Crawlers and Tillage
The substitution of conventional tillage by
no-till or conservation tillage is increasingly common and widely adopted in the
United States and elsewhere. In these situations, earthworms, particularly the
“night crawler,” Lumbricus terrestris
L., are especially important. Earthworms become the main agent for incorporating
crop residue into the soil by pulling some into their burrows and by slowly
burying the remainder under casts laid on the soil surface.
In reduced
tillage systems, surface residue builds up and triggers growth in earthworm
populations. Earthworms need the food and habitat provided by surface residue,
and they eat the fungi that become more common in no-till soils. As earthworm
populations increase, they pull more and more residue into their burrows,
helping to mix organic matter into the soil, improving soil structure and water
infiltration.
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