Unless we are gardening, farming, or digging a hole, we don’t think much about the dirt beneath our feet. Yet it teams with life, and within it are complex food webs and a host of interesting creatures. Here nutrients that were once part of living plants or animals become part of the soil again, eaten and digested by a multitude of organisms. As they eat, grow, or tunnel through the earth, the many inhabitants of the soil have an important role in the making of soil and the ongoing life of terrestrial ecosystems, from the richest prairie to the rockiest northern forest.
Most people use the words “soil” and “dirt” interchangeably, meaning bits of earth we have to sweep up or wash off. But to a scientist, soil is much more than that. It is a complex ecosystem that takes eons to form. All soils have four main ingredients: particles of eroded rock, water, air, and organic matter – the remains of plants and animals that fall on the ground and are gradually decomposed. All four are necessary for the living things that inhabit or depend on the soil.
Above ground, the main players in food webs are plants, herbivores, and carnivores, but below the ground, the decomposers, organisms that feed on dead plants and animals, are at the base and first step of the food pyramid. The breakdown of organic matter enriches the soil, increases its ability to hold water, creates air spaces, and supplies essential nutrients to the plants that are rooted in it.
Soil organisms – from bacteria to beetles, algae to ants – are incredibly numerous. Many are too small to see without a microscope, but what they lack in size, they make up for in numbers. A teaspoon of soil might contain a hundred nematodes (tiny unsegmented worms), a thousand protozoa (single-celled organisms), yards of fungal threads, and a billion bacteria! In addition, many kinds of arthropods – animals with hard outer skeletons and jointed legs – make their homes in the soil, among them insects and spiders, centipedes and millipedes, springtails that hop like fleas, and pillbugs that hide under rocks. One animal we’re almost sure to find in a shovelful of dirt is a decomposer familiar to us all – the earthworm.
Earthworms are efficient decomposers with the simplest of body plans: a tube with a mouth at one end and a long, flexible body containing a digestive system. A worm has no teeth, eyes, or ears, but it does have a very small brain, five tiny hearts in a row, and a mouth that pulls food into the digestive tract. There, in the gizzard, gritty particles help to grind up food. The worm adds enzymes and calcium carbonate along the way, so its castings, little piles of worm scat, enrich the soil, raise the pH, and make mineral nutrients available to plants. As it goes about its life, pulling material underground, tunneling downward to escape heat, cold, or dryness and up again for more food, it mixes the layers, bringing minerals up and organic material down.
Worms need both air and water in their surroundings because they breathe through their skin. They move by using two sets of muscles – one set goes around their bodies and the other operates lengthwise. They alternately scrunch themselves up (tightening the lengthwise muscles) and stretch out (tightening the crosswise muscles) to move along. They hold onto the soil with tiny, retractable bristles called setae on their undersides, which makes them hard to pull out.
Near the head end of mature worms is a band, called a clitellum, that has a role in reproduction. All worms are hermaphroditic, having both male and female sex organs. To mate, they hitch themselves together, head to tail, and fertilize each other’s eggs. Both will produce egg cases, tough pea-sized packets, and leave them in the soil. Baby earthworms, looking like miniature adults, feed on the tiniest bits of decaying matter in your garden. All our earthworms are nonnative, brought here by European settlers. While they are helpful in our gardens, we should avoid bringing them into our forests where they are having a detrimental effect.
Besides the decomposers, the soil food web also includes herbivores. Some feed on tiny algae while others feed on the roots of plants. Cicadas are plant-eating insects that live part of their lives above ground and part in the soil. In summer, adult cicadas can be heard singing from tree branches, their loud mechanical songs filling the air. Females lay their eggs in twigs, and the young nymphs hatch out and drop to the ground. They tunnel down into the soil where they feed on sap from plant roots. Cicadas may spend from one to 17 years in this underground stage before tunneling to the surface again to molt into winged adults.
At the top of the soil food pyramid are predators like moles that feed on worms and insects as well as other soil-dwelling critters. A mole eats about half its weight each day and is active in summer and winter. Moles are well-built for their subterranean existence. Blind or nearly so, they use their noses and whiskers to find their prey. Short, velvety fur lets them slide easily through their tunnels, and tiny sharp teeth are good for grabbing hard-shelled prey. Their muscular forelegs and large, curved claws are perfect earth-moving tools, and we can often see the results of their work as ribbons of excavated dirt above ground, making them unpopular with keepers of tidy yards.
Plants are an integral part of the soil ecosystem. Anchored in the earth by their roots, they capture the sun’s energy and provide food and shelter for many other organisms. By shading the soil, plants help it to retain water. Their roots protect it from being washed or blown away and also help in the formation of new soil by fracturing or weathering rocks. When plants die, they provide organic matter, and their roots leave spaces in the soil for air and water to get in and new roots to grow. Compacted soil, which lacks spaces between the particles, cannot support a healthy soil community.
Plants get water and the essential nutrients that are dissolved in it from the soil, but there are some elements they cannot get by themselves. Many plants form partnerships with bacteria or fungi. A close-up look at the roots of clover reveals numerous tiny round nodules. These contain beneficial bacteria that provide the plant with essential nitrogen while the plant provides sugars for the bacteria, one more example of the myriad links in the soil ecosystem.
The soil under our feet holds and filters water and air, recycles organic material, and provides homes for a host of tiny animals and a place for plants to grow. Though generally unnoticed and underappreciated, this thin crust of earth is a rich ecosystem that supports life above as well as below the ground.
Kohnke, Helmut, and D. P. Franzmeier. Soil Science Simplified. 4th edition. Long Grove, IL: Waveland, 1995.
Nardi, James B. Life in the Soil. Chicago: U of Chicago, 2007.