The burrowing and feeding activity of earthworms have numerous beneficial effects on overall soil quality for crop production.
Why are Earthworms Important
Only a few decades ago, the predominating thought was that earthworms were not very important for agriculture. Emphasis was placed on physical and chemical aspects of plant growth while biological aspects were neglected. We are now realizing how interactions between crops, climate, soil, and living organisms play important roles in sustaining our agriculture. Earthworms are among the most visible of soil organisms and have received considerable attention. They play a pivotal role in maintaining the productivity of our soils. This makes understanding these lowly animals and finding ways to make them thrive very important.
Benefits of Earthworms
The burrowing and feeding activity of earthworms have numerous beneficial effects on overall soil quality for crop production. The following soil properties can be improved by earthworms.
Some earthworm species create vertical burrows, whereas other species live in horizontal burrows in the soil. The vertical burrows are typically open, although the worms cap the top with residue and excrement. The vertical burrows are very important points of entry for quick water infiltration into the soil, especially in no-till systems.
Air-filled porosity is critical in helping plant roots to thrive. Roots need oxygen for their growth, whereas they produce carbon dioxide that needs to leave the soil. Because earthworms improve soil porosity, they improve the exchange of these gases with the atmosphere. Earthworms increase porosity by two mechanisms: (1) by creating permanent burrows, and (2) by improving soil aggregation. Aggregation is improved by the mixing of soil and organic matter in the earthworms’ guts. These highly stable aggregates are deposited by some earthworms in their burrows, and by others at the surface of the soil. In one pasture study, earthworms consumed between 20 and 40 tons of soil per acre per year. In another study, earthworms were estimated to consume 4 to 10 percent of the top 6 inches of the soil annually. This only goes to show the enormous amounts of soil that can be processed by earthworms.
Soil compaction reduces the porosity of the soil. Because earthworms increase porosity, they reduce the effects of compaction. In addition, the excrement of earthworms has a very stable structure, which makes the soil more resistant to compaction and improves its tilth.
Typical earthworm populations can easily consume 2 tons of dry matter per acre per year, partly digesting and mixing it with soil. The importance of earthworms to mix surface residue with soil becomes very clear in soils that do not have any earthworms. Most of our Pennsylvania soils have at least some earthworms and the effect of their complete absence therefore cannot be noted. In the Netherlands, some soils reclaimed from the sea at first did not have any earthworms. In these soils the formation of topsoil with reasonable organic matter content did not take place, resulting in poor crop growth. Once the cause was established, the government of the Netherlands started a campaign to introduce earthworms. After the introduction of the earthworms, a dark topsoil layer was formed, and crop growth increased substantially.
Earthworm casts have higher available nitrogen, phosphorus, potassium, and calcium contents than surrounding soil, as well as a higher cation-exchange capacity. Some micronutrients, such as zinc and boron, are more available in the excrement of earthworms through chelation of the micronutrients.
Soil passed through the gut of earthworms has a neutral pH. This is probably due to the pH buffering action of organic molecules produced in the gut of worms.
Earthworms excrete material that has high concen- trations of beneficial microbes that help decompose crop residue.
Some earthworms eat harmful nematodes, thus decreasing the concentration of these harmful organisms in soil.
Types of Earthworms
There are more than 1,000 different earthworm species, but they can be placed into one of three groups (Fig. 1):
- Litter dwellers or epigeic species live in crop or forest litter. They are not common in most agricultural soils. These species do not ingest large amounts of soil. The manure or red worm, Eisenia foetida. is an example of a litter dweller.
- Topsoil dwellers or endogeic species live in the upper 2 to 3 inches of the soil. They live primarily from partially decomposed organic matter that is already incorporated in the soil. They eat their way through the soil, creating horizontal burrows that they fill with their excrement. These species ingest large amounts of soil that they mix with digested crop residue in their guts.
- Subsoil dwellers or anecic species live in permanent vertical burrows that can be 5 or 6 feet deep. These earthworms need surface crop residue to live. Their burrows remain open, although they cap the top with crop residue that they pull to the entrance. These species ingest substantial amounts of soil that they mix with digested residue in their guts. Their excrement is primarily deposited at the surface of the soil. The nightcrawler Lumbricus terrestris is the most prominent member of this group.
Figure 1. Three major groups of earthworms are: litter dwellers, topsoil dwellers, and subsoil dwellers.
Earthworms are invertebrates composed of many segments (Fig. 1). They don’t have bones and move by contracting and relaxing the body segments in sequence. They also have little bristlelike organs that help them cling to slippery surfaces. Most earthworms have both male and female organs. Typically, however, they still need a partner to reproduce. When earthworms mate, they lay side by side in reverse position. At that moment they exchange sperm. The sperm of the partner is stored in little chambers called spermathecal apertures. These are positioned in front of the egg-producing organs. After mating, the swollen external gland, called clitellum, produces egg cases called cocoons. A cocoon case slides slowly forward, picking up eggs and sperm as it moves over the head of the earthworm. From 3 to 1,000 cocoons can be produced per year, depending on species and environmental conditions. Typically, an earthworm will produce 20 to 30 cocoons per year, with each cocoon containing 1 to 10 eggs. Peak cocoon production is in the spring or early summer. The eggs in the cocoons hatch when conditions are right. Under ideal conditions, it may take from 1 to 5 months for the eggs to hatch. It may then take from 3 to 12 months before these worms are sexually mature. Worms typically live only a few months because of the many environmental threats they face. They have been observed to live for 10 years in a protected environment. In a favorable environment previously without earthworms, earthworm populations increased 80-fold in 4 years after introduction.
Environmental Effects on Earthworms
The burrowing and feeding activity of earthworms, as well as their overall population, are affected by the soil environment in which they live. Some of the properties important to earthworms include the following.
Earthworms absorb and lose moisture through their skin. If soils are dry, earthworms may move to deeper soil layers, die, or revert to a hibernation condition called diapause. Earthworms in diapause are tied up in a knot in a little hole that is lined with a slimy substance to avoid moisture loss. Eggs in cocoons survive prolonged drought, allowing earthworm populations to survive drought periods. Because of dry conditions, earthworm populations in Pennsylvania are low in the summer. Earthworms can live under submerged conditions if the oxygen content of the water is high enough. In most cases, however, earthworms will die when exposed to excessive waterlogging. They move to the surface when the soil is saturated to avoid suffocation.
Most earthworms don’t tolerate temperatures below freezing, nor do they tolerate high temperatures. Prolonged exposure to temperatures above 95°F kills them. They can move down into the soil to escape these adverse temperatures. Optimum temperatures are between 50 and 60°F.
Earthworms commonly found in agricultural fields thrive at neutral pH, but can tolerate a pH from 5.0 to 8.0.
Earthworms prefer soils with loamy texture. Coarse sand can be a negative factor either because the abrasive action of sand grains damages their skin, or because these soils dry out more easily. In some studies, clay soils had fewer earthworms than lighter-textured soils. The reason for this phenomenon is not clear.
Quantity, quality, and placement of food influence earthworm populations. Earthworms eat organic residue that needs to be present in sufficient quantity. The quality of residue is also important. Residue with a high carbon to nitrogen (C:N) ratio is not very palatable for earthworms. Manure can help make it more palatable. Packed manure is an excellent food source for earthworms. In some cases, residue has to undergo some weathering before earthworms are able to digest it. Topsoil dwellers need smaller particles than subsoil dwellers, which can use large leaves, for example. The placement of food becomes a critical issue for some earthworm species. Topsoil dwellers prefer small organic residue particles incorporated into the soil, but subsurface dwellers need residue at the surface of the soil.
Management Effects on Earthworms
Soil and crop management practices can influence many soil properties that affect earthworms (Fig. 2). Some practices can have immediate effects on earthworms, whereas others may gradually change earthworm population and activities over several years. So when you are considering management options, you should also think about their effects on earthworms. Some of the more important management effects on earthworms are described below.
Figure 2. Management practices favoring/hurting earthworms.
Soil tillage greatly affects earthworms (Table 1). The burial of crop residue and manure by tillage favors topsoil dwellers over subsoil dwellers. In long-term clean-tilled fields, nightcrawlers are scarce. Tillage also stimulates drying the surface soil and wide day/night temperature fluctuations. This is another reason earthworms are less numerous in clean-tilled fields compared to no-tilled fields. Tillage brings earthworms to the surface where they are subject to predators such as birds. Total earthworm populations in long-term no-tilled fields are typically at least twice those of clean-tilled fields.
Table 1. Earthworm Populations as Affected by Management