By Jerry Brunetti
I use the word eternal to describe the earthworm because of the never ending work in which this remarkable creature of the earth is engaged. Even though this book has showcased much of the microbial soil food web, I wanted to devote some specific space to earthworms because they can readily be observed and counted, yet most lovers of the soil actually know little of them.
Charles Darwin’s seminal work The Formation of Vegetable Mould, Through the Action of Worms, with Observations on Their Habits, published in 1881, was perhaps as important a journal of nature as his account of his global expedition on the HMS Beagle. This book, finally published one year before Darwin’s death, summarizes more than thirty-nine years of observations. No scientist prior to Darwin had taken such an interest in this creature. In fact, many believed earthworms to be vegetable pests that attacked plant roots as do parasitic nematodes. Darwin, on the other hand, was fascinated by the behavior and beneficial effects of the unjustly maligned creature. In chapter 4, Darwin discusses how he placed stones on pastures and observed how those stones eventually “sank” into the soil. What actually occurred, as Darwin discovered, was that the earthworm castings, or feces, in the pasture increased the layer of topsoil by seven inches over twenty-nine years, or a quarter of an inch per year, and buried the stones. Darwin learned that worms are also calculating engineers, as they know that dragging pine needles or fallen leaves into their burrows by their base prevents them from getting lodged in the burrow. The fact that a creature so small can literally move the earth, aerate it, and fertilize it, thus creating habitat for microbes, channels for water to percolate, and conduits for roots to extend themselves, makes this creature a wonder of the visible world.
I have thought of earthworms as kind of a tiny hybrid between a chicken and a cow. Not having teeth, like birds, they use a gizzard, or a pre-stomach filled with grit, to grind up lignified material such as leaves. Then, like a ruminant, they regurgitate this digest and slime the walls of their burrows with it, allowing temperature and time to work its magic on this petri dish of nutrients. The burrow is then colonized by bacteria and yeasts rich in protein not found in the original plant debris, which are grazed by the earthworm. This process is very similar to a ruminant growing a large mass of bacteria, protozoa, and fungi in its rumen and synthesizing large amounts of microbial protein from an indigestible source of fiber such as cellulose or lignin.
Other times, I have thought of earthworms as the whales of the soil; they are certainly one of the largest creatures swimming in the deeps of our earth, ingesting untold quantities of bacteria, protozoa, nematodes, and fungi, just as our mammalian behemoths inhale tons of zoo plankton, like krill.
Earthworms can also be seen as massive herds of ungulates, like antelope or bison, tiny herbivores that continually graze the plankton of the earth, aerifying the soil so that plants and their microbial symbionts can thrive, creating more soil carbon (humus/topsoil) to serve as a sponge to hold more water and cleanse the waters that fill the aquifers and our streams.
Earthworm gut bacteria number more than four dozen species. Protozoa, predators of bacteria, are important fare for earthworms, and some species survive digestion by earthworms, thus enriching the worm castings. And though earthworms literally dwell in a soup of microbes internally and externally, their health is apparently not affected by the legion of bacteria, fungi, protozoa, nematodes, springtails, mites, and the other denizens of topsoil. Meanwhile, earthworms have provided some evidence of being able to “clean” an infested soil of such undesirable plant pathogens as take all fungus (Gaeumannomyces graminis) by spreading its antagonist Pseudomonas corrugata around, especially if the latter bacteria was first added to animal dung then spread on a wheat field where earthworms could ingest it. Apple scab, too, appears to be less of a challenge if the orchard is populated with high amounts of night crawlers able to devour the fallen infected leaves.
The dark side of nonnative worms, those that came here from Europe during the colonial settlement period (especially in the eighteenth and nineteenth centuries), is that deciduous forests are at risk of being radically altered by the deep-burrowing anecic varieties, like night crawlers (Lumbricus terrestris). Anecic worms rapidly convert leaf mold, or duff (leaves at various stages of decomposition), into bare soil, which discourages the survival of native plants dependent on the duff. In many cases native plants decline at a rate of 80-90 percent without the duff necessary for their growth. Even some deciduous trees dependent on duff for seed germination can’t hold on without it. Native plants are thus replaced by invasive species. The microbial landscape also changes, going form a fungal-dominated ecosystem to a bacterial one because fungi grow on forest litter better than on soil (fungal-dominated ecosystems are more conducive to certain native species such as wild orchid). Voles and shrews, which prey on insects more prevalent in forest litter than in soil alone, are replaced by mice. Plants that survive this ecological tectonic plate shift are more vulnerable to being eaten by the deer population.
But another report, in the USDA Yearbook of Agriculture for 1930, found that “mulls,” granular mixes of organic matter and minerals created by earthworms, are a forest’s best friend. Additionally Svend Heiberg, associate professor of silviculture of the New York State College of Forestry, wrote an article on the importance of mull production by earthworms as a critical factor for tree productivity. Could it be that invasive anecic worm species occupying our woodlands today far outnumber the native species prevalent a century ago? As I’ve stated above, the species suspected of being mostly responsible for the decline of native plants are the deep-burrowing anecic worms, like Lumbricus terrestris. Apparently, it is the horizontal burrowers like the “red worm” that manufacture those fertile “mulls” beneficial to trees up to a point. Because if all three functional groups of worms are in the forest, it will have a major impact on converting fallen leaf duff into soil. Earthworms are thus labeled ecosystem engineers because their burrowing not only churns the soil but also modifies the habitat of microbes, plants, and animals, for better or worse…
Jim Kinsella, a no-till farmer in Lexington, Illinois, has stated that “ten large worm holes and a hundred small holes per square foot means over 330 miles per acres of pencil thickness holes and over 800 miles per acres of small holes.” That’s saying a lot for water infiltration (four times higher than comparable fields without worms); soil aeration; hardpan penetration; reduced soil compaction; surface residue digested into the soil; release of crop growth stimulants, like auxins and cytokinins; minerals transferred up from the subsoil; neutralization of soil pH due to the worm’s calciferous glands; a rapid growth of soil microbes, especially because earthworm dung is so rich in them; improved soil tilth from the exudates, polysaccharides, and enzymes; better parasitic nematode control (because earthworms eat them); and a higher bioavailability of trace elements to crops.
Now do some math: Having twenty-five earthworms per cubic foot of soil (12″x12″x12′) means one tone of worms per acre, which some estimate equals 1 million worms per acre. That acre will receive thirty to one hundred tons of casting per year! That’s an additional plant-available source of nitrate, phosphorous, potash, magnesium, and five hundred pounds of actual, elemental calcium, which is three-fourths of a ton of limestone. Additionally, once earthworms die, their bodies provide another 50-00- pounds of nitrogen per acre. Clearly, pastures harbor the mother lode of earthworm population.
About the Author
JERRY BRUNETTI, 1950-2014, worked as a soil and crop consultant, primarily for livestock farms and ranches, and improved crop quality and livestock performance and health on certified organic farms. In 1979, he founded Agri-Dynamics Inc., and confounded Earthworks in 1990. He spoke widely on the topics of human, animal and farm health.