THE LIVING SOIL: Earthworms, called the "intestines of the earth" by Aristotle.
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 terms of biomass and overall activity, earthworms dominate the world of soil invertebrates, including arthropods.
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. Earthworms can generate 120 tons of casting per acre per year and this material contains valuable nutrients and other soil based microbial life that promote healthy trees-resistant trees and shrubs.
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 residue is gradually buried by cast material deposited on the surface and as earthworms pull surface residue into their burrows.
Earthworms are generally found in the top 12" to 18" of the soil because this is where food is most abundant. 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.
BENEFITS AND INTERACTIONS OF EARTHWORMS WITH OTHER MEMBERS OF THE FOOD WEB
||As earthworms tunnel through the soil, they ingest the soil and digest any organic matter in it. Organic matter is dragged into their burrows and is broken down. Although they are the most numerous in the top 6 inches, they also work in the subsoil, bringing mineral rich soil from below to the surface. This adds to the supply of nutrients available to the plants. Research shows that a soil that is well managed, rich in humus may easily support 25 worms per cubic foot, which translates into at least 175 lbs of fertilizer per year for the same 200 sq ft garden.
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.
Besides incorporating organic matter to your soil, earthworms are
good manufacturers of fertilizer. Castings have a nutrient level and organic matter level much higher than that of the surrounding soil. Each day they produce nitrogen, phosphorous, potassium and many micro nutrients in a form that all plants can use. Not only do they produce this fertilizer but spread it thoroughly within the top 12 inches of soil.
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.They may also incorporate it as far down as 6 ft.
This means that your landscape can be supplied with far more fertilizer and that of superior quality than a dry or granular fast-acting chemical fertilizer of 10-20 lbs. In fact, these fertilizers may even repel the earthworms that are present. As the fertilizers become soluble, they may leach down into the soil and force the earthworms to seek refuge elsewhere.
Worms make other contributions, such as adding calcium, a compound which helps moderate soil pH. Overtime earthworms can help change acid or alkaline soils toward a more neutral pH.
Earthworm tunnels help to aerate and loosen the soil. This allows more oxygen in, which not only helps the plant directly, but also improves conditions for certain beneficial soil bacteria. Finally, the tunneling of the earthworms provide an access to deeper soil levels for the numerous smaller organisms that contribute to the health of the soil.
Earthworms tunnels are coated with a sticky mucous that preserves the passageway longer than those created by aeration tines. It is estimated that these tunnels can stay intact for as long as a year. Their mucous is rich in nutrients and thought to contain hormones that stimulate plant growth. Plant and tree roots often occupy these tunnels and can reach greater soil depth using less energy.
In summary, earthworm activity in your soil is beneficial and should be encouraged. They help incorporate organic matter, improve the soil structure, improve water movement through the soil, improve plant root growth and minimize thatch build up in your landscape.
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.
Pesticides applied to control landscape diseases or insect pests may severely affect earthworms. Products commonly used on landscapes areas vary greatly in their toxicity to earthworms. Some pesticides can cause severe and long term reductions in earthworm numbers. Most of the common earthworm species grow slowly, live for several years, and have low reproductive rates. Consequently, repopulation of poisoned soil is slow.