Dirt Factory

For most of my life I assumed worms were simply good—good for gardens, good for compost, good for the planet. They seemed like harmless helpers, ploughing soil invisibly and giving quiet life to whatever grew above them.

The reality, like the soil itself, is knottier. In northern hardwood forests, European earthworms brought in fishing bait or plant material have stripped away the thick leaf-litter blanket that native plants need, altering entire understories within a few human generations. Farther south and west, hammerhead flatworms—shovel-headed predators that regenerate when cut—prowl flowerbeds, liquefying native earthworms and other soil fauna. Helpers in one place can be demolishers in another.

Darwin could not have known this darker half of the story. To him, earthworms were pure benefactors: unnoticed, tireless labourers building the fertile skin of the Earth. At Down House in Kent, he spent decades devising experiments that unfolded over years—sometimes decades—often with help from his young engineer-in-training son, Horace.

Darwin’s first experiments were deceptively simple. In one, he and Horace placed flat stones and broken tiles directly onto undisturbed turf. The soil beneath was left untouched, allowing natural processes to play out. They then waited. After several years, they returned to excavate carefully around the stones. What they found contradicted the common assumption that such objects “settled” into the ground. The stones had not sunk—the ground had risen. Worms, through the steady production of surface castings, had raised the soil level around and above the stones. In one recorded case, a stone originally flush with the surface was found buried beneath 1.75 inches of new soil. As Darwin wrote, “A stone laid on the surface becomes buried by the accumulation of castings, and not by sinking through loose mould.”

To move from observation to quantification, Darwin designed a second experiment. He buried sheets of fine wire netting horizontally just beneath the surface of pasture ground. These acted as fixed reference planes. Weighed down with stones to remain stable, they allowed him to return years later and measure precisely how much soil had accumulated above them. In some locations, he recorded nearly half an inch of fresh mould per year—a rate of soil formation driven entirely by worm activity. There was no slumping, no subsidence—only surface gain. “It is a marvellous reflection,” Darwin wrote, “that the whole of the superficial mould over any such expanse has passed, and will again pass, every few years, through the bodies of worms.”

Darwin also turned his attention to worm behavior. He scattered leaves of oak, hazel, and ivy across the turf, then observed how worms interacted with them. The results were striking: in one series of observations, 80 percent of ivy leaves that had been pulled into burrows had been grasped by the tip. This wasn’t random. Though blind, worms demonstrated a consistent tactile strategy—grasping and drawing in parts of the leaf that allowed easier transport into their burrows. This not only showed a kind of primitive selection but also accelerated the decomposition of leaf litter, speeding the return of nutrients to the soil. Modern ethologists would later recognize this as one of the earliest quantitative studies of invertebrate behavior.

Throughout, Horace Darwin was more than a helping hand. He designed a simple but effective “worm-stone gauge” for measuring how deep stones had been buried. He recorded measurements when his father’s hands could not. He brought precision, patience, and repeatability. Their experiments became a shared habit of attention, folded into the rhythms of life at Down House and the Kentish landscape itself.

Darwin’s earthworm studies ended not with triumph but humility. Change, he showed, does not always arrive with force. It can creep, mouthful by mouthful, unseen. The same process that slowly builds fertile soil can, in new contexts, erase ecological stability in a matter of decades. There is no contradiction—only context.

Today, whether in a compost pile under a kitchen sink or on a trembling forest floor, worms still labour patiently, raising new soil and unsettling old ground, reminding us that the smallest agents can remake the world.