The players in our hidden biosphere
grubs, termites and weevils, for example – but others are beloved or at least beguiling.
Dung beetles convert animal waste into humus, a service we take for granted. Ants are the most abundant soil insect. Although some species are pests or nurture pests such as aphids, ants with their highly organised colonies are essential members of the soil biosphere. They assist in the conversion of litter to humus, move and mix large quantities of soil, and spread the seed of bulbs and other desirable plants. crustaceans that like soft plant debris and make quick work of green plant material and newly fallen leaves.
One of the most abundant, but barely visible, arthropods in the soil are springtails. They are named for a tail-like structure that allows them to jump when threatened. As many as a billion can live in an acre of soil. Depending on species, they cycle plant debris or feed on fungi, algae or other springtails.
Mites are generally regarded by gardeners as pests, and some are – sucking sap from plants and spreading disease. But the soil houses an immense community of non-pest species that are essential to the cycle of life. Half the known species of mites live in the soil, where they feed on decaying plant litter. Nardi writes that they “set the stage for smaller decomposers like bacteria and fungi to free most of the energy and nutrients stored in those leaves”.
Some mites are predatory and attack nematodes and other small creatures.
Nematodes are tiny wormlike creatures traditionally viewed in agriculture as serious pests that harm plants by feeding on their roots. More recently, the view of nematodes has become more nuanced because some species are now commonly used (and purchased) as predators of garden pests. Experts believe there may be close to a million species, of which only a fraction have been described scientifically.
Some nematodes eat soil bacteria and fungi, while others prefer to consume other soil arthropods and protozoa. Their value to the garden is in converting nitrogen into a form that plants can use.
Protozoa are microscopic creatures that live in vast numbers in the film of water between soil particles. The most well-known is the amoeba, but these microbes come in several forms, including species that move with a single flagellum or with hairlike cilia.
They are the major predator of bacteria, and in consuming them they release nitrogen and other nutrients to plants.
Protozoa, in turn, are eaten by nematodes and other small arthropods.
Historically, bacteria have been associated with germs. Some of the nastiest human diseases – anthrax, typhoid, tuberculosis and syphilis, for example – are the result of bacterial infections. But we have come to know too that our guts are full of beneficial bacteria and essential to our health.
The soil is the same way. The bad actors are outnumbered and usually outwitted by the good ones. Healthy soil is loaded with bacteria, and because they’re not very mobile, they tend to hang out in vast numbers on and around the roots of plants, a zone known as the rhizosphere. There can be as much as 100 times more bacteria around plant roots than elsewhere in the soil, and with good reason. The plants feed them carbon sugars. The microbes give back nitrogen.
Fungi break down organic matter, which is why you will see mycelium strands in compost piles and under leaf litter. Two basic forms of fungi form a symbiotic relationship with plants. One exists in proximity to root tips and associates with hardwood trees and conifers, the other penetrates the cell wall of the roots and is found in plants of the domestic landscape of flowers, shrubs, grasses and vegetables.
The fungi grow tiny, fragile strands called hyphae. They are a tenth the thickness of human hair, but there are so many of them that they form a vast network, effectively extending the reach and efficiency of plant roots. In her book The Soil Will Save Us, science writer Kristin Ohlson says there can be as much as 515km of hyphae in a cubic foot of soil. At least 80% of the plants on Earth connect to these fungal partners. “Gardeners need to know this stuff,” Lowenfels said. “A thinking gardener is a better gardener.”
• Adrian Higgins has been writing about the intersection of gardening and life for more than 25 years, and joined the Washington Post in 1994.
He is the author of several books, including the Washington Post Garden Book and Chanticleer, a Pleasure Garden.