by Clive A. Edwards, The Ohio State University (Reprinted from USDA website)
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.
To learn how to increase soil health and earthworm population click here.
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
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.
|Earthworms incorporate large amounts of organic matter into the soil.
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.
| Casts at the soil surface are evidence that earthworms are shredding, mixing, and burying surface r
To increase soil health and biodiversity click here.
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.
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.
To learn more about soil biology click here