The Zoo Beneath Our Feet
The gardener has a long and touchy‐feely relationship with the soil. As every good cultivator knows, you assess the earth by holding it. Is it dark and crumbly, is there an earthworm or beetle in there, is it moist, and when you smell it, are you getting that pleasant earthy aroma?
All these signs are reassuring, and have been through the ages, but they are mere indicators of something much greater and infinitely mysterious: a hidden universe beneath our feet.
This cosmos is only now revealing itself as a result of scientific discoveries based on better microscopic imaging and DNA analysis. There is still much to learn, but it boils down to this: Plants nurture a whole world of creatures in the soil that in return feed and protect the plants, especially trees. It is a subterranean community that includes worms, insects, mites, other arthropods that you have never heard of ‐ amoebas, and fellow protozoa. The dominant organisms are bacteria and fungi. All these players work together, sometimes by eating one another.
The awareness of this biosphere should change the way gardeners think about cultivating plants and heighten everyone’s understanding of the natural world. In other words, don’t ever call it dirt again.
The sheer vitality of it is mind-bending: A teaspoon of good loam may contain a billion bacteria, yards of fungal strands, several thousand protozoa and a few dozen nematodes.
Basically, this is how it works: Plants manufacture carbohydrates through photosynthesis, but not just for themselves. They release some of their carbon sugars into the soil, which causes the bacteria and fungi to appear. The bacteria crowd around the root zone and the fungi form vast networks of interlocking strands that often link one plant to another. The bacteria convert nitrogen and other nutrients into forms the plants can use, often by getting devoured by other microbes.
The fungal strands, the mycelium, effectively increase the root mass of its host plant by as much as a thousand times and transport a bevy of goodies to the host plants, including phosphorus, copper, calcium and zinc. There is also evidence that trees use this network to send signals to one another if for example, leaf‐eating pests have arrived.
Although some plant and human diseases are caused by soil‐borne fungi and bacteria, most of these microbes are beneficial and keep the bad ones in check. The organisms assist in other ways, by increasing the size of soil particles, which improves the ability of the soil to hold water and air. Even in the middle of a city, the subterranean world is thriving.
Among the unexpected findings: The microbial species were the same as those found in parts of the world with dramatically different flora and climates from New York’s, including Antarctic cold deserts, tropical forests and grasslands.
There was a strong association between the diverse organisms in each sample. “Unravelling these relationships will be critical to building a more integrated understanding of below-ground ecology’, the researchers wrote in a paper published by the journal for the British Royal Society. “Our work highlights that most of the diversity found in soil remains undescribed.”
Enough is known, however, to create a 21st century subset of farming known as regenerative agriculture. The farmers have discovered that if you foster this biosphere, you do not need expensive fertilizers because the microbes repay the plants with nutrients. They also, for obvious reasons, avoid pesticides that would kill this soil life.
Advocates of this low‐impact farming say it can restore soil carbon lost by the historic conversion of forest and prairie to farmland and help to mitigate greenhouse gases. In the 1990s, Sara F. Wright, an Agricultural Research Service scientist in Beltsville, discovered a sticky coating to fungal threads named glomalin that is a major reservoir for carbon.
In the Autumn, mixing horse manure with fallen leaves, shreds the mixture and applies the mulch to growing beds. The organic gardener's mantra has never seemed more appropriate. Feed the soil, not the plant.
Worms provide critical assistance to smaller organisms by breaking down and incorporating leaves into the soil, so all may eat. Worm castings are rich in nutrients, including calcium, nitrogen, phosphorus and potassium.
Thousands of insects and spiders live in a patch of soil. Some are considered pests by humans. For example, Japanese beetle grubs, termites and weevils, but others are beloved or at least beguiling and include the larvae of lightning bugs and cicadas. 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.
The more conspicuous of these include millipedes and centipedes, as well as woodlice. Millipedes feed on plant debris and microbes. Centipedes eat other arthropods. Woodlice, or sow‐bugs, are 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 is springtails. They are named for a tail‐like structure that allows them to jump when threatened. As many as a billion or more 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.”
Nematodes are tiny wormlike creatures that have traditionally been 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 as predators of garden pests such as slugs, vine weevils and white grubs, to name a few. In truth, the world of nematodes is much greater and can only be imagined. 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. They are the major predator of bacteria, and in consuming them they release nitrogen and other nutrients to plants.
Historically, bacteria have been associated with germs. Some of the nastiest human diseases such as anthrax, typhoid, tuberculosis and syphilis are the result of bacterial infections. But we have also realised that our guts are full of beneficial bacteria and essential to our health.
The soil is the same and the bad actors are outnumbered and usually outwitted by the good ones. Healthy soil is loaded with bacteria, and because they are 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 one hundred 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; flowers, shrubs, grasses and vegetables. There can be as much as 320 miles of hyphae in a cubic foot of soil and at least 80 percent of the plants on Earth connect to these fungal partners.