One of the reasons it is important to use the available crop wastes, weeds and other organic matter as mulch instead of making compost is because of the long term benefits of mulch for earthworms. Compost gives only short term benefits to earthworms because most of the carbohydrates have been burned off. People tend to take earthworms for granted and discount their importance. But, they are vitally important for healthy soil that is rich in carbon and in beneficial microbes.
Earthworms must stay moist in order for oxygen to pass into their epithelial cells. So, they need cool moist soil to survive. Those conditions are only possible if cropping soils remain covered. Mulch shades the soil and prevents compaction and erosion. It also provides worms with nutritious food. They can survive on the organisms they obtain by consuming soil. But, organic matter provides them with high energy polysaccharides and cellulose.
Tilled soils are low in earthworm numbers. Tillage kills some worms outright, causes surface drying which kills others and enables predators to eat yet others. No-till fields with some crop wastes left on the surface have at least twice as many worms as tilled fields. (approximately 500,000 compared to 30,000 in maize fields and 200,000 in soybean fields) Well managed pastures have populations of at least a million earthworms and often two million or more. As grass leaves die, they build up a mulch layer over the soil providing energy rich food for the worms.
A lip like extension over the earthworm’s mouth pushes food inside, where the muscular pharynx sucks it in, coats it with saliva and forces it through the esophagus into the crop. It is stored there while enzymes in the saliva begin digestion. Then it moves into the gizzard, where it is ground up with bits of minerals before flowing into the intestine. There it is broken down further by enzymatic activity. Nutrients pass into the body and the rest passes out the anus as nutrient rich worm castings.
The burrowing activity of earthworms turns the soil once or twice a year, depending on climate, and incorporates their rich castings. This creates tunnels which speed up water infiltration and permit gas exchange between the roots and the atmosphere. The increased porosity also aids root growth.
There are three general groups of earthworms that pass most of their time in different habitats.
1. Litter dwellers (The epigeic species) live in the surface residues (mulch) of forests, pastures, prairies, agro-forests and no till fields where the crop wastes are left on the surface. They cannot survive in tilled fields. They live off of soil microbes, manure and organic matter that has been partially decomposed by microbes.
2. Topsoil dwellers (The endogeic species) live in the top few inches of soil and are the reason carbon rich topsoil exists. They live off of partly decomposed organic matter and soil microbes. They are also mostly responsible for the increased porosity of soils. In fields recovered from the sea in Holland, these earthworms had to be introduced in order to create topsoil.
3. Subsoil dwellers (The anecic species) live mainly in permanent vertical burrows that are up to six feet deep. They are the ones that deposit their manure in piles on the soil surface. Like the litter dwellers they need mulch to survive. But, they can consume organic matter that is larger and that is in an earlier state of decomposition than the litter dwellers. They pull their food into their burrows for later consumption and close the openings with organic matter plugs.
Although the majority of the atmospheric carbon that is captured and sequestered in soils is there thanks to the mycelia of mycorrhizal fungi, earthworms do their fair share of sequestering carbon in the soil. All they need from us in our agricultural soils is to be provided with cool/moist/shady conditions and food in the form of decomposing organic matter.
Although petrochemical fertilizers and herbicides do not affect earthworms the way they do mycorrhizal fungi, some types of pesticides have a highly toxic effect and drastically reduce their numbers. Earthworms are a great ally in the effort to reduce atmospheric carbon levels; capturing the carbohydrates produced by cropping plants and burying them in the soil.