I spent whole afternoons in the dirt, making my patch of ground flawless. I even cleared the worms away, before I found out that all the tunnels they make give air, and probably other molecules I don’t know about yet, to the plants.
—JANE HAMILTON, The Book of Ruth, 1988
I HAVE KEPT WORMS on the back porch in a worm composting bin I bought on impulse seven years ago. At the time, I didn’t know much about earthworms, having only the vaguest idea that there was more than one species, and harboring nothing more than a dim notion about the value of their castings to my garden.
The bin itself—a recycled rubber contraption called a Can-O-Worms—is both simple and marvelously well designed at the same time, not unlike the earthworm itself. Three round trays stack on top of one another. Each one has a hole on the bottom to allow the worms to move up through the trays. They all nestle into a base, which sits atop three plastic legs and contains a spigot to drain liquid. The worms start out in the bottom tray, where they consume vegetable trimmings, coffee grounds, and newspaper. Once they’ve eaten their way through the bottom tray and filled it with about three inches of castings, the second tray is added and they move up into it, seeking food. The process is repeated for the third tray. The castings in the bottom tray can be emptied out into the garden, and then returned to the bin, serving as the new top tray. The process repeats itself, the worms moving through the rotation of three trays.
When I bought the worm composter, I was so excited that I could not wait even a few days to order from a worm farm, so I selected my worms from among the Styrofoam containers in the cooler at the bait stand. The red wigglers I purchased turned out to be sturdy, hardworking creatures that, after being spared the fate of the bait hook, were more than willing to spend their days eating my garbage. I probably bought twenty cartons, which works out to roughly one thousand worms. Now, seven years later, they have multiplied again and again. I don’t know how many I have—five thousand? Ten thousand? I added a second bin a couple years ago; perhaps between the two I have almost twenty thousand worms living just a few feet from the kitchen table where I drink my morning coffee. By the standards of most worm farmers, this a small operation, just large enough to keep up with the food scraps that one household produces.
When the worms came out of their bait stand cartons, they were less than an inch long and as thin and limp as a strand of spaghetti. They only had to survive long enough to make it onto the end of a fishing hook; no one expected these worms to go to work making compost.
But once I got them settled into the bin, it did not take long for them to fatten up and start reproducing. The next generation was substantially more healthy than the last: they were easily as long as my finger, measured twice as big around as the first batch, and moved vigorously through the bin, churning through potato peelings and old tea bags as if this was the work they’d been born to do—and, in fact, it was.
THESE RED WIGGLERS I’d purchased don’t live in the soil. They thrive in manure piles and layers of rotting leaves, where they feed off the bacteria that does the real work of breaking down organic matter. On the other hand, if I had dug up worms from my garden and put them in my new bin, they would have found the environment completely inhospitable. As a keeper of worms, one of the first things I had to learn was that not all worms behave the same way in the soil. Scientists find it useful to classify worms by function, and once I’d learned the system, I could pick them out in my own garden. Small yellowish-brown worms—endogeic worms—turn up only in the roots of plants; I never saw them in an empty vegetable bed. Large burrowing nightcrawlers, called anecic worms, live deep in the soil. Unless the weather had been damp for several days, they were hard to find near the surface. Epigeic worms, like the red wigglers that occupied my worm bin, would make a home in rotten muck on the surface of the ground but never deep in the soil.
My red wigglers didn’t rate a mention by Darwin but are among the most studied worms today, showing enormous promise as composters of food and industrial waste. Epigeic worms have a relatively short life span, living just a few years, and are known for their ability to reproduce quickly to match the available food source. (The exact reproductive rates are a source of some controversy. Many budding entrepreneurs have been lured into a kind of earthworm pyramid scheme that promises that a healthy worm population will double its size every sixty days, a growth rate that conjures up images of wildly escalating profits unmatched by any other livestock enterprise.) The fact is that reproduction rates are influenced by climate, environment, and food source. Epigeic worms have been known to lay anywhere from a dozen to a few hundred cocoons in a year. Many of them may never hatch if conditions are not right; others may hatch two or three young earthworms apiece.
Two of the best-known epigeic worms can probably be found at a bait stand in a carton labeled “red wiggler” (Eisenia fetida) or “redworm” (Lumbricus rubellus), or in a grade-school science classroom, where they often provide the basis for ecology and recycling projects. Both can live in a bin and both are favorites among worm composting enthusiasts. Some people do try to raise them to add to their garden soil, but the fact that they only live in rich, decomposing organic matter, and not underground, makes them best suited for worm bins, outdoor compost piles, or very heavily mulched soil. Most worms in this category are under three inches long and dark red or almost brown, and many have pale stripes between their segments.
The castings that epigeic worms produce are a rich soil amendment. These worms are uniquely well designed for their role in the detritusphere, or leaf litter layer of the soil, where they deposit nutrients that help plants germinate and grow. Here is just one example of an interaction between an epigeic worm and the plant life it supports: the epigeic worm is endowed with an active calciferous gland that helps it process calcium in its diet and excrete the excess in its castings. Calcium is critical to plant growth because it allows plants to take up nitrogen, which promotes leaf growth, and it assists with protein synthesis and other vital plant functions. Anyone who has ever picked a ripe tomato from the garden only to find a mushy brown spot on the bottom—an affliction known as blossom end rot—has discovered what a lack of calcium can mean. Fortunately, blossom end rot is specific to each individual fruit: a plant that has produced one or two spoiled tomatoes may go on to produce healthy ones later in the season. This means that a gardener like me will run to the nursery at the first sign of blossom end rot, hoping to find a calcium supplement that will ensure blemish-free tomatoes later in the summer. Dolomite lime, bone meal, and gypsum are all sold as calcium supplements for home gardeners and farmers alike. Although they’ve been proven to work against problems like blossom end rot, any off-the-shelf supplement has its own set of drawbacks. Many nutrients leach away during heavy rains, decompose too quickly to be of use to plants, or exist in a form that they can’t readily absorb. That’s where earthworms, particularly epigeic worms, play such a critical role. They address calcium deficiencies on several fronts, producing calcium in their calciferous glands during digestion, adding it to the soil through their castings, or even transforming it, as it moves through the intestine, into a form that is easier for plants to absorb.
DARWIN FOCUSED ON an anecic worm called Lumbricus terrestris—the nightcrawler—in his book. This is the worm that builds permanent vertical burrows in the soil, leaving little mounds of castings alongside the opening. It can burrow as deep as eight feet, rising to the surface at night in search of food. Unlike epigeic worms, anecic worms like nightcrawlers do ingest some soil along with dead leaves and other decaying matter. They are large, powerful worms well suited to the work of tilling the earth. A typical nightcrawler has a fairly long life cycle —up to six years—and reproduces more slowly than an epigeic worm might. Because it burrows deep into the earth, it can wait out drought cycles. It is considered a somewhat sluggish creature in comparison to its red wiggler cousin, but it can move quickly when it is trying to withdraw into its burrow to escape a predator.
Nightcrawlers are typical of most anecic worms in terms of size and coloring. They are dark red along their dorsal side, or backside, and are nearly translucent on their ventral, or underside. Their rear ends are flattened in what scientists call a spoon-shaped posterior. The clitellum—a thickened band of skin about a third of the way down their body—is quite pronounced, and with some magnification, one can even see the tiny bristle-like hairs called setae that worms use to anchor themselves in their burrow or to hold onto one another when they mate. Nightcrawlers are, in some ways, the archetypal worm, instantly recognizable to anyone who has ever picked them out of the lawn for bait or watched a robin pull one from its burrow, stretching it out of the ground while its spoon-shaped posterior clung to the burrow with every seta it possessed.
I first encountered Lumbricus terrestris in my own backyard. It turned up on the end of my shovel, six inches long, perfectly pink and clean in spite of the fact that it had lived its entire life in the dirt. I knew that the nightcrawler was a common enough worm in most parts of the country, but I never saw one in my first garden back in Santa Cruz. It had its share of earthworms, but none as large as the nightcrawler.
Anecic worms are valued not just for the castings they produce, but also for the ways in which they move soil around. Darwin claimed that earthworm burrows benefit the earth by letting in air and rainwater. “They allow the air to penetrate deeply into the ground,” he wrote. “They also greatly facilitate the downward passage of roots of moderate size; and these will be nourished by the humus with which the burrows are lined.” He was, as usual, quite prescient in his description. Oligochaetologists working decades after Darwin’s death have discovered that the walls of those worm burrows, called the drilosphere, are rich in bacterial and fungal growth, thanks to the mixture of the particular mucus that the worms excrete and the casts left behind as they move through the soil.
Try pressing your finger into compacted clay earth and see how far you get. Some of the dirt in my own garden is so hard that I can barely make a dent with one finger, and even a shovel has trouble penetrating except on very damp mornings. Now imagine a worm, that limp and spineless creature, working through the same soil. First it anchors its setae into the soil to brace itself, then it stiffens the muscles that run in segments around its body. It increases the pressure inside its coelomic cavity, a vessel that holds the mucus it excretes during locomotion, reproduction, or a time of stress. The increased pressure in this cavity propels the head of the worm forward. It eats a little soil as it moves. The tail contracts, pulling it in the direction its head was moving, and the process begins again. It can take a worm weeks to build a system of burrows in a laboratory study, where the soil may not be as compacted as those unyielding, undisturbed areas in my backyard. Although it is difficult to clock a worm’s speed in the wild, we do know that nightcrawlers only migrate a few yards per year. Lumbricus terrestris does not race through the soil; it meanders, sticking close to food sources and always seeking damp, cool ground.
OF THE THREE MAJOR categories of earthworms, endogeic earthworms are probably the least familiar to most people, for good reason: they rarely come to the surface. Many endogeic species inhabit the rhizosphere, the area immediately around plant roots, where they feed on soil that has been enriched by decaying roots, bacteria, and fungi. Aporrectodea caliginosa, often called a grey worm or a southern worm, is one of the most widespread endogeic species. The name actually refers to a cluster of closely related species that inhabit farmland and pastures. As its common name indicates, it is grey or slightly pink and about two or three inches long. I often turn up a few Aporrectodea caliginosa when I’m transplanting shrubs or pulling weeds. Worms in this category are mainly geophagous, meaning that they feed almost entirely on soil, although they do seek out earth that is relatively higher in organic matter. Because they stay below the surface, they are less likely to be harmed when agricultural fields are tilled or when the ground is disturbed during planting. That makes them a popular choice for farmers who want to inoculate their soil with earthworms. But because it is difficult to breed them in captivity, farmers are often advised to find an area of undisturbed pastureland and dig out a square of sod that seems well populated by grey worms. The sod can then be transplanted directly into agricultural fields, where the worms will, over a period of years, colonize the land.
These small, greyish worms aren’t the only endogeic worms out there. Much larger ones exist well below the surface of the earth; they are large pale creatures that almost never see the light of day. They can build tunnels ten feet belowground, where they may encounter tree roots but few other plant roots. These deep-burrowing worms are among the only living creatures that can survive at such a depth. Beyond that, only microscopic organisms like bacteria are found, and they disappear almost two miles below the earth’s surface, where temperatures reach 160 degrees and life in any form is difficult to support.
Deep-burrowing worms, then, inhabit a world that would seem stark and barren to us. For their size, they are surprisingly delicate. One such worm, Megascolides australis, can grow to several feet in length but its skin is so fragile that it could burst if it is handled too much. Its tunnels are so large and well lined with coelomic fluid that some Australian farmers can hear a gurgling sound coming from deep within the earth when the worm is on the move. Another giant worm in Oregon, Driloleirus macelfreshi, measures two or three feet long and is known for its coelomic fluid, which smells distinctly of lilies. Both of these species survive only in undisturbed habitats; both are nearing extinction due to the encroachment of cities and roads. They are sensitive to vibrations on the surface of the soil and can detect a bulldozer at work. They move quickly through their burrows to escape notice, making it nearly impossible for scientists to collect specimens or study them in the wild. Still, they can live for many years, possibly decades.
Recent efforts have been made to locate the giant Oregon worm, but none were found. Unless one turns up by accident, scientists have no way of knowing if they have managed to adapt to the industry, noise, and pollution created by the encroachment of humans, or if they are, by now, entirely extinct.
IT HAS TAKEN over a hundred years for scientists to put together this portrait of earthworm’s life. Now what emerges about life underground is an image like a map of a city in which the inhabitants build roads as they go, choose neighborhoods based on the abundance of food and availability of damp, dark quarters, and carry microorganisms in their guts like passengers on a bus.
Picture, once more, that drawing of the apple tree with its roots extending twelve feet into the soil. How many earthworms are at work in the roots of that tree? Dozens? Hundreds? An apple tree can live for decades. What difference could a population of earthworms mean to its health and longevity? I am starting to believe that it could make all the difference in the world. In an age of ecological uncertainty, when natural habitats are disappearing and creatures like the giant Oregon earthworm are becoming extinct before they have even been properly identified, when farmland is being paved over in favor of neighborhoods and shopping centers, and farmers are turning to genetic engineering and biological pesticides to solve their agricultural problems, in this complex age, the earthworm may emerge as a kind of unsung hero, one whose potential we are only just beginning to understand.